Insulated Basement Slab

The best two things a home builder/renovator can do in the name of energy efficiency is to insulate and seal a house. It's almost too bad, what with our culture's obsession with everything high tech. When people ask me about the Mill Creek NetZero Home (MCNZH), they're often expecting to hear about technology's magic answers. When I start talking about insulation levels, eyes start to glaze over, but the truth is that 75% of the difference between this house and a conventional one is that it's sealed as tight as a plastic bag and it's super-insulated.

We poured the basement slab a few weeks ago, and before that happened Peter Amerongen made sure that the slab would be in thermal isolation from the unheated components of the house. That meant laying 5" of ozone-friendly foam over the sand that we had spread over the exposed clay in the foundation hole (see picture above). Rated at R4.4 per inch, the foam will provide about an R22 insulating value. There is a special type of high-compression foam that we put over the interior footings (exposed in the pictures, under the posts), insulating the slab to about R11 from them.

So, the slab was poured onto a nice cozy blanket. This stands in sharp contrast to conventional building, where the accepted "wisdom" says that it's not worth insulating the slab from the ground AT ALL. Throughout the entire heating season, then, the dirt under the slab is constantly sucking heat from the house, wasting energy and making the floor uncomfortable. No thanks. I'll pay for a comfy bed of cush for my basement concrete, thank you very much.

[Edit]: Peter just ran the HOT2000 heat-loss simulation for the MCNZH without insulation under the slab, and it increased yearly heat loss by 20%! Incredible. We should never pour another basement slab in this country without insulating under it first.

(cross-posted at

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Hi Conrad. I'm really enjoying your posts about MCNZH. Could you please describe the construction/design of the basement walls. Thanks.

Thanks Brad! I'll try to write up a post about the basement walls this weekend.

What was the brand name of the special type of high-compression foam that we put over the interior footings?

Also, why did you use sand and not gravel (as is commonly done here in the City of Toronto) over the clay and under the foam?

Also, I am curious as to why you just didn't make the post foundation a bit lower so that you could use the same thickness of insulation everywhere?

Hi Leon, The brand of the foam is Dow High 40. As in 40 PSI. Here's a description.

From that website:

"STYROFOAM™ Highload extruded polystyrene insulation is a closed-cell foam insulation. Available in compressive strengths of 40, 60 or 100 psi, STYROFOAM Highload insulation features superior moisture resistance and R-value (RSI) retention. All three STYROFOAM Highload insulation products resist compressive creep and fatigue, delivering long-term compressive strength. Like all STYROFOAM insulation products, STYROFOAM Highload 40, 60 and 100 are durable, versatile and reusable — making them the preferred choices for a variety of high-load applications. CAN/ULC S701, Type 4."

We used sand because it's cheaper. Plus, it's quite a bit easier to level out.

There's not a really good reason that we didn't make the footings lower. It did save us quite a bit of sand.

I don't believe in this concept at all. I think the ground under the floor is your friend. Isn't it encouraging your basement to be at 60 degrees? Now, if you insist on heating the basement to a much warmer temperature, its different, such as if its a living area and you need it to be hot for some reason. I don't believe a blanket statement that all floors need to be insulated is correct.

In a heating climate it's absolutely correct. In Edmonton, the ground isn't 60 degrees F (15.6 C), it's 45 degrees F (6 degrees C). You're heating a room to 70 F (20 C), so the ground just a massive heat-wasting sink. Even if you're not living in the basement, you are living in an attached heated living space, so either way, the ground is sucking out valuable heat.

Given that you used Fahrenheit measurements and a very warm temperature for the ground, I suspect that you live in a cooling climate in the US. But we're talking cold climate homes here.

Like you say, if the ground is 45 degrees F (6 degrees C)in Canada, that does change things. I didn't notice that your blanket statement was for Canada. I live in New York State, northeastern US. Thank you for your response.

6C is really quite warm for Canada at Tuk NWT there is 1000 ft of permafrost below your basement floor and you do not want to lose heat to it or to melt it.

The basement heat loss modeling in Hot2000 appears to be the second-largest source of error after air infiltration.

Hi Brian,

Thanks for taking more time to look into it.
The math looks right, but the assumptions don't.

1) The main floor temperature will be much higher than 21C during the
summer months; an average of 23-25C is likely from June to September.
Therefore more heat will be transferred in the summer, increasing the
average basement slab temperature, and reducing the heat loss in the

2) RSI 1 (R5.7) between the main floor and the basement floor is too
low. Perhaps for upward heat movement it is reasonable, but not
downward. I have often measured temperature differences of 2-3C
between the slab and ceiling in the basement (i.e. slab 17C and
ceiling 19-20C). The stratification of the basement air reduces heat
I managed to find a reference to corroborate my assertion that RSI 1
is reasonable for upward heat flow:
RSI 2 likely approximates the downward heat transfer through 3/4"
hardwood flooring, 3/4" subfloor on 2x8 joists and 8' to the concrete

3) 16.6C is too low; my research and measurements in several houses
indicate a long-term average of ~18C. My house was under construction
during the winter of 2007, and the slab was exposed to freezing
temperatures. It was occupied in August of 2007, and the center of my
slab averaged 12-13C during the winter of 2008. This winter the
temperature at the center of the slab is 15-16C. Next year I expect
it to be 17-18C, and stabilize around 18C in subsequent years. The
ground temperature modeling done by DLSC corroborates this 3yr


On 2/4/09, XXX, Brian wrote:
> Ralph, I think the attached file should clear up any misunderstanding
> about the heat loss through the basement floor. You can download HOT2000
> and do the run yourself if you wish, the file is included.
> The txt file is a "Full Report" for your house file as modified by me to
> reduce the problem variables. In the attached file and report I have
> made the following changes:
> - File mode to "General", rather than EnerGuide for Houses which is
> meaningful for rating purposes. General mode simulates the house as per
> all of the user inputs without imposing EnerGuide rating conditions.
> - Close the basement door to prevent heat transfer via air movement
> through this opening.
> - eliminate the HRV and the heating system fan.
> - change to Metric units, easier for calculation.
> Now we have a relatively simple model of the basement.
> According to the outputs, the main floor temperature is 21 C, and the
> average annual basement temperature is 16.6 C. From this we would expect
> a heat gain in the basement of
> Q = A/R * (Delta T)
> = 152.15 / 1 * ( 21 - 16.6)
> = 669.5 Watts
> For the whole year, this would be
> Qa = 669.5 * 8760 * 3.6 / 1000 = 21112 MJ
> The report indicates a value of 21285 MJ, essentially the same obtained
> by the simple calculation.
> Brian

I have read thru everyone's comments eagerly as I am currently debating this very topic for my new construction.

I have a couple of variables that I need questions answered however -

1) My foundation is designed on a sloping lot, resulting in 15% of my basement walls being exposed and I also have designed a walk-out door. With this amount of exposure along the one wall and the insulative properties of an insulated slab...will I subject myself to the potential condensation problem on the interior walls where the foundation is more exposed?

2) I've read several other articles on this topic - some subscribe to the thought of using 6mm vapor barrier in addition to insulating the slab - question is, should I be laying the vaport barrier above or below the insulation before pouring my slab?

Your knowledge in this matter and comments are appreciated.
Halifax, NS

Hi Paul,

It seems you're not far from me; I built my house in Belnan (halfway between Halifax & Truro).

As per my previous post, I don't recommend full under-slab insulation. If you go for a poured foundation (instead of say ICF), I suggest exterior foam insulation (2" of white styrofoam), and 2' of wing insulation (1" thick foam board over the drainage tile).

Some form of vapor barrier is required under the slab by code; not for moisture but as a radon barrier. I recently paid <$60 for 1500sf of CSA approved 6-mil poly at Pierceys. At 4c per square foot it's an insignificant cost.



I appreciate your I see that this Thread is pretty thin on conversation.

I've already poured my foundation (last fall) and insulated the outside walls completely...and I have the 6 mil already to lay down...and I too got a roll at Piercy's.
My biggest concerns will still be the exposed (down slope) walls on the walk-out or daylight basement side of the structure. I will certainly be insulating the walls and buttoning it up air/water tight when I get around to framing the basement, but I can't help but imagine that there will nonetheless be some condensation forming on that wall due to temperature differential - especially in the winter months.

Again, I am grateful for your comments. I know that whatever path I do choose, I will live with whatever nature can throw at me. I can't wait to get this cottage/retirement home completed so we can enjoy retirement and some of our Nova Scotian summertime weather.


I have a 4' concrete pony wall on the south side for a basement walk out with the top 4' framed with 2x6 studs. This winter I insulated the inside of the concrete basement walls with R12 fiberglass (basically free when you consider the energuide grants). On the outside I have 2" of EPS to 6" below grade, then 1" EPS down to the footings. I put no vapor barrier or drywall over the fiberglass, and the only place I got condensation on the concrete walls was in the northwest corner of the basement where the foundation of my attached garage creates a bad thermal bridge with the foundation walls. So if you just insulate on the outside with at least 2" EPS you should have no worries about condensation.


Again your comments are not only appreciated, but helpful.

Your design sounds strikingly similar to mine (ie//pony wall, etc..) although my exposed basement wall faces East. I won't have an attached garage as in your case. So, going from your example and especially considering our shared climate zone, I shouldn't expect too much in the way of condensation at all. I've insulated all around the exterior foundation below grade with 2" and the interior insulation will come with time, after the floor is poured.

I have decided to forego insulating under the basement floor in view of all the comments I have received and documentation I have read. The net gain just doesn't appear to be there for my basement design and planned use for my structure. I think I stand to gain only when the interior is insulated fully along all exterior walls complete with vapor barrier.

Thanks again.

Hi Paul,

If I do it again I'd add 1" thick wing insulation going out 2' from the footings (laid over the drainage tile and then cover with a few inches of 1" clear gravel to protect the foam during backfill).

Ideally I'd have removable insulation over my basement slab during the late winter & early spring, but haven't found a practical way to do that. 2009 was our 2nd full winter in the house, and with the heat turned off for a few days last week with warmer weather, basement slab temperature was ~14C. Below the slab in my sump pit the temp was only 12C. So while initially (based on info from dlsc and PAHS info) I thought after 3 years the ground below my basement slab would come up to ~18C I now think that may take ~5yrs.



Sorry for the extended silence. I'm not knowledgeable about moisture problems, so I kept quiet. Insulation on the other hand...

If I may, let me reiterate my disagreement with Ralph with regard to insulating under the slab. A modest one-time cost that pays back for the life of the home and provides insurance against major energy shortfalls? What am I missing here? Ralph even admits that it may take 5 years for the ground under his slab to rise to 18C. In other words, five years of burning expensive fossil fuels to heat up the ground under his house, and then continuous burning every year to keep the ground "charged" with heat. Why would you expose your living space to an enormous heat sink? To save $1500? I don't get it.



I'm talking about PAHS; I have pex run under my strip footing so excess solar hot water in the summer warms up under my slab. I'm pretty sure I explained that in another post.

Even without PAHS, where's the science behind the sub-slab insulation. I've pointed out the material errors in BASESIMP, so citing a Hot2000 report doesn't prove anything.

Without insulation under the slab you get free cooling in the summer; in fact heat "lost" to the ground under the house outside the heating season is good. Hot2000 reports show gross heat loss in the below-grade foundation graphs and summary info, giving the false impression that insulation will always be a net benefit.

I've also posted in another thread the cost of heat from my geothermal heat pump. I'm not certain I'm right, and I've provided lots of data on why I think it is better to go without sub-slab insulation. You seem certain I'm wrong, and don't want to provide any data to support your premise. I made lots of mistakes when I built my house. I find I learn (and in turn help others learn) more from mistakes than what I did right (and there is little that I did perfectly right).


Well now, this has turned into the everlasting thread...ha!

I believe I mentioned that my building was primarily a cottage/retirement interest in moisture/condensation is of greater concern than heat at this point, due to the possibility of long periods when the structure will be closed up tight and only running heat in an "anti-freeze" state.
Moisture/condensation which may promote mould/odor is my main concern over the long haul...although I must admit, if this were going to be my "primary" year-round residence, I would be going the distance for maximum heat retention.
Despite all that has been said in this forum and other readings, I can't help but be concerned over the possibility of condensation building up....y'a just can't defeat the laws of chemistry...if something is warm/moist adjacent to something that is cold/dry...there is bound to be some form of condensation. Who knows, maybe I might have a Thunder & Lightning storm in my basement in January. Ha.



In winter, when unoccupied, the air will be VERY dry. The average outdoor dewpoint in Halifax is below -5C in Jan/Feb. Even if you build to PassivHaus air tightness standards and only keep the heat at 10C in the winter, you will get at least 1 full air change per day in Jan/Feb. Without occupants to add moisture to the air, and after the initial drying out period, that means the indoor dew point will be close to -5C. Therefore you would need a really cold surface (below -5C) to get any condensation (actually deposition of ice).


Ralph, I can't thank you enough for that last tidbit.

This info eases my mind about any potential condensation.

On the otherhand, the potential for freezing has always been a consideration for me, during unattended periods, with power failure being the primary cause for this. I have planned a plumbing system complete with "low point drainage" throughout the structure for just such reasons. Therefore, my only concerns will only be the remote possibility of "single point failures" should the temps fall below those you have mentioned above. By single point failure, I mean "S-traps", water pumps, etc... but I know there are ways to combat these as well.

I really don't expect to have too many problems down the road. Unless we get another White Juan....with days/weeks of endless cold and power outages.



All the more reason not to insulate under your basement slab. During a sustained power outage, heat from beneath your house will keep the inside from reaching freezing temperatures. If you insulate under the slab, the chances of freezing are much greater.

UMN has done lots of great work on foundation insulation, frost protection, etc.


Hey There Conrad,

Thanks for the great documentation. I'm getting ready to begin on a DIY NetZero just to the northwest of Edmonton. Just wondering the exact type of Foam Insulation used under the slab.

Hi All,

I am in the process of planning an energy efficient in the Edmonton area and was kicking around the idea of not having a basement but instead just insulating a concrete slab with an ICF home on top of the slab. As I can think of more pros than cons, I would invite anyone's opinion on the subject as it seems that as Canadians, we are obsessed with basements, but I see very little advantages of such. I look forward to any comments. Thanks.

One of the reasons that we have basements in Canada is that we want the footings of the house to be on stable ground. That means that they must be protected from freezing because freeze/thaw cycles cause the ground to shift. It's much easier to ensure that footings never freeze if they're below the frost line. If you want to have the foundation of your house at grade then you'll want lots of insulation around your slab to ensure that none of the ground anywhere near it will ever freeze.


ICF is good for basements, but significantly more expensive than a well-insulated stick-frame wall above grade.
Basements are good for utilities (hot water heater, furnace, etc) and temperature-controlled storage.
If the site is flat, a slab can be a very inexpensive foundation. If the site has a >10% slope I think a basement is a better choice.


I am building a house in SW Edmonton starting in the spring 2010 and have an idea for free cooling & a warm basement. I'm planning to install in floor heating pipes (deep) in the basement floor, covering with either sand or (if heat transfer properties are worth it), 2 inches of concrete, then cover that with 4" of insulation and pour the basement floor on top of that. I'll use the pipes in the ground through a fancoil for free cooling of the house in the summer. In the winter the basement would stay relatively warm while the ground gets re-cooled for use in the summer. I'm probably not the first to think of this or try this so just wondering if this has worked or has been tested and failed.
The house will be very well insulated so cooling load is smaller than a typical house.
If extra cooling is needed or for better removal of humidity in the summer, I could always use a GSHP instead of the fan coil.
Any opinions?

See for something similar to what you are suggesting.
I'd recommend much less than 4" of insulation, as you want to let the heat come up through the floor in the late fall & early winter. I'm using a GSHP and pumping the hot water under my basement slab. Here in the maritimes I found the humidity got too high (60-70%) before I hooked up the GSHP (I had put pipes under my slab before construction). With the GSHP I've been keeping the humidity at 40-50%; not only does my family like it, so do my wide-plank hardwood floors (which were starting to cup in the high humidity).

Hello everyone. This thread petered out about a year ago, so these may be a belated comments. For several years, I have been thinking about many of the issues and observations raised by Conrad, Ralph, Bob H. and others. I have concluded that building a superinsulated house, in Canada, and not insulating the slab and foundation to at least R 20 is like wearing a downfilled Goretex (TM) parka plus shorts and sandals. Heat loss through the basement is as important a consideration as heat loss through the walls and ceiling.
For many years, I worked as a news and sports photographer. I covered countless outdoor winter events. Even with big mukluks, I found that, after standing still for a period of time, my feet got cold. If my feet were cold, the chill spread up through my body. I learned that if I stood on a piece of 1/2 inch foamy, I was insulated from the snow and ice. I felt much warmer.
I have noticed that, in a house, if the floor is cold, the room feels cold, even when the thermostat is set at 20 degrees C or more. I have concluded, from these folksy observations, that when I build a superinsulated house, the slab and foundation will be well insulated. The concrete of the slab and walls will act as a thermal flywheel, as studied by researchers at the Oak Ridges National Laboratory in the U.S.
I'm siding with Conrad's approach, not Ralph's.
Conrad, Thanks for initiating this blog, and for all of the thoughtful observations and comments.

I just ran this scenario through Hot2000. The results surprised me! A typical ICF foundation without underslab insulation combined with other predeterminced factors yeilded a Energuide rating of 86. Once I added 2 inches of XPS IV (R8) to the underside of my slab my results decreased to an Energuide rating of 85. Interesting indeed - Ralph may be on to something. I`m a couple months away from building our dream home in Prince George.

I can speak anecdotablly about two house I am very familiar with - my house and my inlaws' house. Both are in Lanark County, Ontario about 60 kms west of Ottawa. Both have heated basements using wood stoves or gas stoves. Both were built before 1990. ONE always feels warm, the other always feels cold. Why?

My basement has more wall insulation as a result of a retrofit I did a few years ago - R7.5 exps on exterior, 8" concrete wall, R7.5 on inside of wall, 1" airspace, R13 2 x 4 wall filled with Roxul - that's atleast R33 with no bridging, but probably performs higher due to thermal mass in wall. You would think it wouyld be a very cozy room. However, I made the mistake of not insulating or isolating the concrete pad - I merely placed tile on top of it.

In contrast my inlaws' basement has a simple 2 x 4 wall with fibreglass and no exterior insulation on the foundation. My inlaws concrete pad, however, is insulated to R7.5.

My basement always FEELS cold because the uninsulated floor is very cold. Even if the thermostat reads 21C, it is an uncomfortable room to be in this time of the year - all the hot air is at the ceiling, and the lower 3 feet of the room is very cool. It also likely costs me more to heat. You CANNOT stay in that room without slippers on, and generally, you need to be under a blanket to watch a movie even though the thermostat reads 21C. The opposite is true of the other example, where the floor is insulated. It always feels warm and cosy and the pad is the same temperature as the rest of the room Interestingly, it is also nice and cool in the summer (as is my basement)...

Just anecdotal, but I vote for going R20 under the concrete.

Mississippi John

Let's run the reasonableness test on this:

I bring a "magic" block of concrete into my bedroom. It is always 6 degrees, and it can absorb a virtually limitless amount of energy.

Will my bedroom be warmer or colder?

I only makes sense that not insulating under your slab would reduce energy use if you have a cooling load. That is, if you are running air conditioning all summer, and having a cooler basement would reduce its usage. If that's the case, then I understand why HOT2000 is reporting what you say it is. If not, then there is a problem in your inputs or with HOT2000.

We are at the building permit stage for our super-insulated house being built this summer in Saskatoon. The city inspectors don't like the idea of having underslab rigid insulation right up to the footings because "typically we want some heat loss to the footing to keep it warm in winter. In a well-insulated house, less heat gets to the footing and there is risk of frost heave." The only compromise they have accepted thus far is for us to keep the rigid insulation 16" from the footings, meaning we will have a 16" uninsulated perimeter around our otherwise cosy basement floor. From reading through the above posts I understand that there are many opinions about the need for underslab insulation, but we have decided that this is important for us and we don't want to compromise on it by placing it away from the footings. Has anyone else come up against this problem? Any other ideas?

How deep are your footings below the grade level?
Can you lay rigid foam horizontally out four feet from the foundation walls to force the frost penetration away from the footings? Each inch of foam is about equivalent to one foot of soil. I believe the website has info on this.
If you are committed to building an energy efficient house, you could consult an "informed" engineer. An "Engineer's" stamp often eases any concerns of building officials.

The drawings show the bottom of our footings as either 8' or 7'8" below grade. Our inspector feels that the frost line here is somewhat greater than 8 feet. We may go the Engineer's stamp route depending on what is involved with that.

I don't think you have Dow high load 40 as it only comes in 3 inch thickness and is blue in color?

Anyone have suggestions for a crawl space floor that will be at roughly the same height as the surrounding terrain? For example, when building on solid rock? If we cannot dig down much due to the solid rock, will the floor loose heat in winter if not insulated? (This is a cottage). Moisture problems if not insulated? Would something like this Dow High Load 40 (or similar) be wise, to lay before pouring the concrete? Thanks!


Go to and
Enter "crawl space" into the Search box. There are several good articles about crawl spaces and the problems with moisture. If your space is not designed , insulated and sealed properly, the floor joists and living space floor will rot.

If your floor is the same height as the outside grade, you should have a shallow drainage swale to divert any surface water away from the house. It would be wise to install "GA" drainage stone, then pour the concrete for the slab. The floor height should be above the grade. The stone will to allow water to run away under the slab. Weeping tile and a sump pump would also help. However, in the fall, remove the sump pump to avoid freezing.

You should also study the drip line. Ensure roof runoff is either captured by an eavestrough or drained away.

My comment has to do with the ICF basement walls. Why wouldn't a waffle-type ICF wall with proper rebar and bond beams over openings be a no-brainer for exterior walls? You use less concrete and increase the R-rating of the wall system. an analogy- no one uses a solid block of lumber to build exterior walls- rather it is standard to use 2x6 stick framing on 16" centers. Similarly- waffle-type concrete would provide a similar savings on material and still has WAY more shear strength than a stick-framed wall.

Further- you should control heat loss and gain in your house and not reply on a slab to provide "free cooling" when there is no way to measure that value. Insulate the slab and if you want the cooling effect of the constant temp of the ground- use geothermal.

Thanks for the comment.

I disagree that you increase the R-rating of the wall by going ICF. We got an R50 wall for cheaper than an ICF wall. That was our motivation.

Using less concrete is an excellent argument for a waffle-type ICF wall.

As to your "further" comment, it's not either or: we control heat loss and gain AND we use the slab for free cooling. The only measured value that I need is the fact that our house has no cooling system at all (so a measured value of zero for cooling energy use), and it stays comfortable all summer long.

How did you get an R-50 wall?

Also- that may be a better idea anyway using your basement for cooling. By insulating slab you reduce heating costs but possibly increase cooling (electric) costs? of course you would need a circulating system to take air from basement and redistribute to first or second level. also a system for de-humidifying. so basically you are saying you run AC as normal in summer but by not insulating slab it runs less?

And not sure if my last comment came up but how do you get an R-50 on a basement concrete wall? if it's in another post, sorry- i didnt catch it.

As for the cooling, we have no air-conditioning at all. We open the windows at night, the cool air pulls heat off of the concrete in the house (we have concrete floors on the main and second floors), and the concrete that was cooled during the night keeps us cool during the day.

I believe that you could get a wall with R-50 with the way it is shown in the pictures from that link. However, I am wondering how that can be cheaper than ICF construction. If I can understand correctly- you formed, poured, and stripped the concrete wall, then glued foam to the wall, and then framed a wall in front of it? So all of that material and labor was cheaper than a comparable amount of ICF R-value wall?

What if you had urethane ICFs at 4" each side- basically R-50 of foam. And if the urethane had fastening strips in the forms you wouldnt need all of the extra labor to build 2 walls and glue the 5" of foam- you would just set the ICF blocks, pour concrete, and then fasten drywall?

I am in Northeast Ohio- not quite as cold as Canada but it can still get pretty brutal in terms of cold weather. In talking to an insulation contractor, he gave me the following formula to calculate the cost of heating for different R values of wall systems:

A (area of walls) x U (1/R-value) x HDD (heating degree days zone (6201)) X 24 (hours)/ 1 million x 10 (cost per MCF of natural gas)= cost per year to heat that space.

So in trying to pitch to builders and homeowners the benefit of using ICF walls I show them this calculation.

For a 40x50 rectangle basement with 9' high 8" thick concrete walls wall with an assumed R value of R3 would cost $803 per year to heat. For the same wall with R-38 insulation around it would cost $63/year to heat. That is a difference of $740.

The real sell is showing value in an investment. Obviously the ICF system will cost more up front but, like a good investment it will provide a return on investment through the lower yearly energy costs. To me, a good investment would return 10% per year. So to figure how much extra you should spend on an ICF system you would want to know what cash savings per year would return 10% on investment. making 10% per year doubles your money in 7 years. so $740 times 7 years is $5181.

Answer to the riddle- if your ICF quote is $5181 higher than your poured wall OR lower (in this case) then it is more than worth the up front cost.

In reply to the final visitor comment from Nov 13th: My understanding is that there are several companies that can be hired to build basement walls including supplying, installing and removing the forms following the poring of the basement walls. There are fewer ICF contractors and, as a result, they tend to charge a bit more. True you should be able to do an IFC basement on your own but very few people have the desire, or time, to do so. I know three families that have moved into brand new homes within the past 1.5 years and none of them went with ICF basements.

However, the real limitation for ICF walls in a very high efficiency home like Conrads is that they have 2 or 2.5 inches of foam insulation per side which, at R5/inch, gives R20 to R25. I attended a presentation by Peter Amerongen from Habitat Studio where he said they needed to get to R50 to hit net zero and that it was cheaper to build a conventional concrete wall and insulate to the desired level than it would be to build an IFC wall and add additional insulation to hit the needed level.

I did the math on ICF vs. conventional poured with either rigid eps or studs and fibreglass and it came out just about even with regards to material and labour costs with the catch that the conventional method offered much less insulation value (R20 +/- vs. R40 +/-). I think there are enough ICF contractors out there now to create a more competitive environment which is driving prices down (at least in the GTA). I used Ontario ICF out of Sunderland - great workmanship and flexible enough to work around a constantly changing start date due to permit issues (Oak Ridges Moraine + EP = big headaches!) I'm also insulating under the slab - nothing I've read convinces me it's not a good idea and, for $1500, not a huge risk.

The following link shows that ICF are available with up to 8" foam and R66 assembly. Now have 4 distributors in Edmonton area. I know nothing of price.

We are planning a super-insulated home in Quebec, including elements in its design to maximize solar gain and storage. One of these is the idea of an insulated area of sand and water under our slab that we would heat during the summer (not the winter) with passive solar methods. No insulation between the slab and the sand so that during the winter we will benefit from the heated sand. Any opinions on this?


Have you calculated how much energy the sand/water can store? Taking into account the amount of sand, it's potential to store thermal energy, and the temperature that you think you can heat it up to?

I once did the same calculation for a massive amount of water (20 cubic metres, I believe). It was superinsulated, and if heated to 100 degrees it was modelled to lose one degree a day to standing losses. Without even taking those losses into account, it still only stored something like 500 or 700 kWh of heat energy.

If you can somehow design a simple system with very large collection and storage capacities, it might work.

Consider, though, that my house can burn through 100 kWh in one day when the temperature is -30 outside. So you would need a very large system for it to be worthwhile, seems to me.



Good comments by Conrad. Seasonal storage of heat is difficult. If the slab is not insulated, then the heat from your storage area will transfer directly into the house as soon as you apply the heat to the storage. It will overheat your house through the summer and fall and there will be little heat left in the winter when you need it. In other words, the storage must be thermally decoupled from the house or it's not storage. Have a look at Drake Landing Solar Community

You talk about heating your storage area in the summer with passive solar methods. Passive solar implies having sun shine directly on your storage. How are you going to do that when it's under your slab "where the sun don't shine"?


Edith has a valid point, asymetrical seasonal solar storage works. MIT did it in Solar House #1 in 1936. Since then, it is routinely done in Austria, Germany, and many other countries. There are thousands of homes built over the past 30 years this way, and studied by Universities and PHDs in detail. I will be covering the studies plus demonstration homes built in BC to a Uvic audience in a couple weeks.

Edmonton has such a system in the Riverdale Netzero home, using 21m2 collectors and 17m3 storage, operating fine since 2008.

Concordia designed a 186m2 home in Montreal with CMHC in 2009 that used this principal. The result was they reduced the annual heat demand down to 2.3% of the home energy, at just 192 kWh/yr for space and DHW, using a 32m3 storage tank, and 51m2 of collectors. The design was overkill, it did not need to run a lot of the time, it was twice the size it could have been is what we learned.

A scientific study published this month showed that 15m3 is optimal for annual solar underground storage efficiency size, when they tested a range from 5 to 60m3.

Solar Heating works so well it should become competitive with incumbent systems, at a modest upgrade from comparable gas or electric, and half the cost of Geoexchange.

Bob, heat exergy transfers through the ground at pretty slow rates, typically 1/2" per day, or 16" over a month. Anyone that deals with Geo Exchange can look it up in a table.

When solar thermal energy is stored 4 feet under a slab, the peak temperature of 70-90C is reached in September, and the peak heat conduction will reach the slab 3 months later in December.

It is true that about 1/2 the solar energy could leave the storage over a season, that is why it is best to put it under the slab and extend the house envelope to incorporate the enthalpy as passive gains.

What could be better than having your home absorb the summer solar surplus?


Thanks for your comments. A couple of my own:
-I would dispute that assertion that the Riverdale system is working "fine". From what I hear, the performance is a bit underwhelming. I think that the Riverdale experiment is inconclusive at this point.
-Your points about putting heat into the ground are very compelling. It sounds like the technology has a lot of promise. However, I would suggest that there is very little expertise/knowledge in the area, so a person putting such a system in better do their research extremely well, and acknowledge that there is a fair amount of risk involved. Much more risk than installing a ground-source heat pump or just building a passive solar home, for example.


I have just finished building a basement with Polycore which blows ICF out of the field and no concrete involved except the footing of course. Also laying 4" of EPS under slab. Two 5" ducts from the forced air furnace heat 1600 feet.

Hi Gerry, what was your resolution regarding the insulation above the footing? I'm also in Saskatoon, and have a similar situation...

Hi Kurt,

wow, I had to go back and review my comments to see what issue it was - there have been so many issues since then! So, we wanted to put 5" of rigid insulation under our basement slab, right up to and level with the footing, but not on top. And the city would not permit this - they wanted the insulation back at least 18" from the flootings. Our designer spoke with them a few times and they would not change their minds. In the end, we got an engineer's stamp (Rempel Engineering, $300) and installed the insulation up to the footings. We never did submit the engineer's stamp to the city, but we have it in our files. To be honest, there is no inspector there when you pour the floor, so we could have done what we liked regardless. If you have any more questions, you can phone me at 242-2922. We probably have a lot of things we could discuss regarding trying to build something different in Saskatoon.

Hi Gerry, in the end I didn't have as much difficultly as you did. I was able to resolve it this week by finding a research paper on frost depth in relation to well insulated basements in Saskatoon that had been completed in the 80's. That, plus with my arguments as an engineer (I'm a Mech P. Eng. myself), I was able to pursuade them it would be okay. Hopefully they remember this precedent for the future. I will take you up on your offer of advice and give you a call soon. Thank you.


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