Foundation Walls

The MCNZH foundation. The rectangle that's jutting out is the cold room, located under the front steps/landing.

A frequently asked question regarding the Mill Creek NetZero Home (MCNZH) is: "Why didn't you use Insulated Concrete Forms (ICFs) for the foundation walls?".  The short answer is: "using traditional concrete forms and innovative insulating techniques, we can achieve a much higher R-value for less money".

ICFs are concrete forms made of styrofoam that you set up, pour concrete into, and leave in place to act as insulation. Here's a picture of someone setting them up:


The advantages of ICFs include decent insulation value, ease of learning for the do-it-yourselfer, and ease of setup (foam is a lot lighter than plywood).

There is a myth that ICFs provide walls with an "equivalent energy loss performance" to an R 50 wall. The argument goes that the mass inside of the concrete walls acts as heat storage, thereby buffering heat as it goes in and out of the building. The counter-argument is quite complex, but suffice to say that I've heard very knowledgeable people say that ICFs performing like an R 50 wall is complete bunk. The argument makes sense, too. When it's minus 20 outside, the relatively small amount of thermal energy stored in a concrete wall will have little effect. R value is R value. ICFs provide about an R 22 wall, with no thermal bridging. That's pretty decent, but it's not R50.

The foundation walls for the MCNZH will be insulated to about R 49 - actually R 49, no caveats or stars beside that term. We had the walls poured conventionally - with plywood forms - in August (see picture at the top), and before we poured the basement slab Peter Amerongen's crew glued 5 inches of ozone-friendly foam against the inside of the foundation walls. That will provide R 22. Then, we had interior walls (called frost walls) framed along the foundation walls, 7 inches away from the foam. When the cavity between the foam and the frost wall is filled with cellulose fiber insulation, at R 3.74 per inch, it will add 26.88, for a total R value of 48.88 for the foundation walls, with no thermal bridging. Not too shabby, considering that conventional builders will insulate to R 12, with a full thermal break at each stud. Here are some pictures:

 The door opening in the MCNZH's basement. The foam is glued directly to the concrete (only 2.5 inches here, we ran out of foam, but we'll add more later), and the frost wall is framed out 7 inches away from the foam.

The window opening in the MCNZH's basement bedroom (opening still filled with plywood). Here you can see the full 5 inches of foam, with the frost wall framed 7 inches away.

Another view of the frost wall.

This foundation will provide very high energy-saving performance, and it was fairly economical. It should be a very comfortable place for someone to live.

(cross posted at

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I have 2 questions:

1. I thought it was important to have an airspace between the foam and the concrete foundation wall, but you don't have that (to allow condensation to drip down the wall, if there is any). Why?

2. You used sand under the basement foundation slab rather than raked gravel (which is all I ever see here under foundation slabs in Toronto, Ontario). Why?

One other thing I noticed that is unusual is that you have foundation waterproofing on both the outside and inside of the foundation wall. Why also on the inside?


I'm probably not as knowledgeable as you about building details, but I don't understand why an airspace between the foam and the concrete would help - foam is waterproof. Plus, I think that water would drip down anyway, even if there's not a big space.

We used sand because it's cheaper than gravel.

The interior foundation waterproofing is just to add another layer, I think.

I'm not sure I understand the way thermal mass helps store energy for the house.

If some of the insulation was on the outside of the wall wouldn't you benefit from the thermal mass of the concrete in the foundation?

With all your insulation on the inside isn't your foundation effectively "outside" and therefore not relevant to thermal mass calculations?

Thanks for the comment.

Thermal mass is only useful if the house has more than 7% of south facing windows, as a percentage of floor area. So, if a house has 1000 square feet of floor space, and more than 70 square feet of south facing windows, thermal mass becomes useful.

Thermal mass comes into play when the sun is shining through the south windows. If there was no thermal mass, the house would overheat to, say, 30 degrees. Because this overheating would make the difference in temperature between the inside and outside more extreme, the house would lose a lot of that heat (a house that is 30 degrees will lose heat faster to the outside than one that is 20 degrees).

If there is extra thermal mass, it absorbs the extra heat, so the house doesn't heat up as much. So the house would only heat up to, say, 23 degrees instead of 30. The extra heat would go into the thermal mass. The house loses heat more slowly to the outside, because the differential between the inside and outside temperatures is less. Then, the thermal mass slowly releases its heat to the house once the sun has gone up. Heat that would have overheated the house had the mass not been there is stored, and released when it is needed later in the evening.

You are correct, the mass in our foundation walls is outside of the thermal envelope. We will be adding mass to the house by pouring concrete floors on the main and second floors. I talked about that here.

Could you please identify the 2.5 inch foam insulation used on the inside of the basement walls?

HI conrad! being in the foundation business myself it is always interesting to see people that are trying different technique to make things better, fortunately for me , a friend of mine actually developped a panel that is poured in concrete forms,so no air space is between the concrete and the type 2 expanded polystyrene panel , very clever. and very efficient.thanks. and keep informing people to do things better.

Was the foam glued to the foundation walls expanded or extruded polystyrene or some other type?

Bill: it was expanded polystyrene. Sorry that took me so long.

Adding insulation at the edge & perimeter (my preference is wing insulation) is good. I don't think adding insulation at the center of the slab is good. If you can't do some form of AGS then sub-slab insulation at the center may be desirable for comfort. I don't think it makes much difference for energy efficiency.

Plus, insulating on the inside of the foundation walls instead of the outside eliminates most of the thermal mass of all that concrete.

UMN has done research on foundation design, temperature modeling, and optimum insulation levels.

So the center of your slab is going to be an uninsulated heat sink to the ground below? I don't see how that makes any sense...

Also, thermal mass is really only a benefit if you can expose it to solar heat. Our basement is already over-massed with just the slabe, given the small amount of solar energy that reaches it.

Conrad, I think you're forgetting the R-value of the earth beneath your house. After a number of years, it is impossible to lose heat to the ground beneath your house; the heat can only go lateral and then up (to the cooler ground around your house).

The Passive Annual Heat Storage concept has been around for over 20 years.

I ran a line of Pex under the strip footing so I can pump hot water from my solar collector under the slab when my domestic hot water tank needs no more heat.

Drake Landing uses the ground as a thermal store; AGS (annualized geo-storage) puts the storage under the house which is cheaper than a borehole field.

Okay, that I can get behind. As long as the heat is from solar.

But I don't buy the concept that without extra solar heat the ground would stop accepting more heat. It would suck heat from your house every single year, because it transfers heat, and it's definitely colder than room temperature.

According to my research and measurements, the average ground temperature under a full basement slab approaches 17-18C after ~5yrs.

I agree that it will suck heat from your house every single year. With insulation under your slab, you'll get almost no free cooling effect in the summer, and the average ground temp under the slab will be measurably less than one without insulation (I'd estimate ~15C).

I don't know the answer of how much (if any) insulation beneath the slab is best. I don't think you do either. Here's another discussion thread on the subject that explains more of the issues.

One thing I learned about energy efficiency when I built my house is that standards and "best practices" are often mistaken or over-simplified. I though making my house more airtight than R2000 is a wasted effort. I got 1.48ACH@50 on my first blower door test (1.5yrs ago), and my latest was 1.19 (1 month ago), yet air infiltration accounts for almost 1/3 of my heat loss in January & Feb. So now when some "expert" tells me what is the best way to build, I scientifically analyze their idea to determine as best I can if it is correct. In the process of learning and understanding the science I develop more ideas myself.


I'm glad to see that Drake Landing made a mention here!

Read his stuff and you'll see that the ground must be dry in order to hold thermal mass. Water in the ground will suck heat away very quickly - so any assumptions about heat lost to the ground depend upon keeping that ground under the home dry.

Heating up the ground around the home is the Earthship idea. All measurements, heck all data, needs to be qualified. I'm betting that soil heating to 18C depends upon:
1) no slab insulated
2) heated basement

The fact that the soil hits 18C is a measure of the HUGE heat loss to the ground. Now "huge" is a relative term. When you insulate well then the majority of your heat loss will be via air infiltration. If you insulate the above ground portion well then you get cold feet in the basement because you'll have signif. heat loss to the ground. This is one place where Passive House got things right. Everything must be looked at in the context of the whole.

I would have kept the insulation outside the foundation walls - to keep them warmer and have the thermal mass as cooling in the summer (but where I live we have hot muggy summers). On the other hand I would never build a basement because of moisture, mold and lack-of-solar-light problems. Use the ground with 100 tons of sand as a heat storage and/or use a Kachadorian Solar Slab. Basements need light - and there is no reason to go to the expense of digging out the basement as shallow frost footings can be used.

Hello Conrad. A couple of comments. Researchers at the Oak Ridges National Laboratory in the U.S. did a series of tests to determine the most effective placement of insulation in a concrete wall.
The options were: all the insulation inside the wall; all the insulation outside of the wall; insulation between two poured concrete walls; and a layer of insulation inside with an equal amount outside (as in an ICF). They concluded that the most effective placement for insulation value and utilization of the heat storage potential and temperature modulating effects of the thermal mass was to place all the insulation on the exterior.
In your blog, you mentioned that cellulose was used to fill the frost wall. Did you install some poly at the bottom of the cavity to reduce the possibility of moisture wicking into the cellulose from the slab? Thanks Jim.


How then would one avoid a thermal bridge between the bottom of the wall and the footing? I believe that extra mass is only useful if there is an excess of solar energy.

I believe that we did indeed add the poly at the bottom.

Thanks for the comments.


Conrad, Previously you mentioned Dow's high compression SM. I would insulate under and around the perimeter of the footings. Jim

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