Passive Solar Design

 PassiveSolar

(MCNZH concrete floor being bathed by sun through a 9’x6’ window)

The most important design considerations for cold climate building are insulation, building envelope, and passive solar design. Given our lofty goals for the Mill Creek NetZero Home (MCNZH), we pushed hard to maximize our return on every one of these fronts.

The MCNZH collects 54% of its annual space heat through passive solar design – that’s 8747 kWh or 31.5 Gigajoules. It does so by:

  1. having huge south windows that are specially manufactured to maximize solar heat gain
  2. containing a large amount of thermal mass to absorb the solar heat when the sun shines
  3. having movable solar awnings that allow 100% of the sunlight to hit the windows during the heating season (the awnings are strictly speaking not a passive part of the solution).

I’ll discuss the first two bullets on this list, given that I’ve already described the movable awnings at length.

Passive solar design is a very simple concept that is encapsulated well in this picture (source):

image

 

The idea is to add a lot of south-facing windows to your home, allow sun to shine through them during the heating season and capture the solar heat in some form of thermal mass. During the summer and fall, it’s imperative that you block most of the sunshine to avoid overheating.  The beauty of passive solar design is that you get cheap solar energy with a system that has no moving parts.

The MCNZH was designed for solar energy collection. With an east-west width of only 25 feet, we had to cover every square centimetre with windows, PV modules, or hot water collectors:

image

(MCNZH, as designed)

mcnzh-back

(MCNZH being built)

 

The site is wonderful for solar – there is almost complete solar exposure throughout the year.

Windows

To maximize passive solar production, we made the south-facing windows as big as possible. They main floor windows are 9’x6’ each and the second floor windows are 10.3’x6’ each. That’s a total of 232 square feet. The windows are triple-paned and they have fibreglass frames. They have insulating spaces where the panes of glass are separated along the frames, and the cavities between the panes are filled with Argon gas.

Most importantly from a passive solar standpoint, the Low-E coatings on the glass have a solar heat gain coefficient (SHGC) of about 0.57. That means that the windows allow more solar heat to pass through them than the east, west and north windows. It does so at the expense of insulating (R) value (the Centre of Glass R-value for the south windows is 5.7 vs. 8.3 for east, west, and north windows), but the extra heat loss is more than compensated by extra solar heat coming in when the sun is shining. One always needs to make a compromise when choosing south-facing windows. We could have had a higher SHGC with dual glazing and a lower R value and still come out ahead on an annual heat loss basis, but the house would have been less comfortable on winter nights and have more condensation on the windows in cold weather

Thermal Mass (Concrete Floors)

On a sunny day during the heating season a super-insulated house like the MCNZH will be entirely heated by the sun. However, in some instances it will be overheated. Those huge windows let more solar energy in than is needed. It would be great to store the excess and to reduce overheating, so we added thermal mass to the home. We did so by pouring concrete on the main and second floors of the MCNZH. The concrete weighs 10-12 metric tonnes on each floor, so we had to design a beam down the middle of the house to support the weight.

As I mentioned above, the passive solar design accounts for 54% of our heating needs (8747 kWh or 31.5 Gigajoules). However, if the design incorporated the big special windows alone, with no extra thermal mass, solar energy would only provide 41% of our heating needs (6583 kWh or 23.7 Gigajoules). So the concrete floors will provide a net benefit of 2164 kWh or 7.8 Gigajoules annually.

Some pictures:

IMG_0757

IMG_0762

(installing concrete floors - MCNZH)

Technology does not have to be complex. In fact, the simpler it is, the more resilient and cheaper it is. Passive solar design is as simple as it gets – as long as the MCNZH stands, it will collect almost eight Gigajoules of solar heat every year with nothing to break and no parts to replace.

The mantra for solar homes in the warmer United States used to be “mass and glass” (lots of south facing windows and thermal mass). Up in Canada we realized at some point that a performant home should be “light and tight” (insulate and seal a house, forgetting about passive solar design). Now we are proving that the optimal solution is “mass, glass, tight, and thick” – passive solar design, a very well sealed building envelope and super-insulated walls.

(cross posted at raisingspaces.com)

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"During the summer and fall, it’s imperative that you block most of the sunshine to avoid overheating. "

This contradicts your post on the movable solar awnings as well as my own observations.
For windows facing within 15 degrees of south, sizing overhangs to block most of the sunshine in the summer & fall leads to significant reductions in passive solar gains in March & April.

I built with 2' overhangs thinking summer shading was important; that was one of my big mistakes.

-Ralph

What window manufacturer did you use for MCNZH? I'm just in the process of costing out windows for The Shift Home (modern, green and affordable infill in Saskatoon) and am looking for options beyond All Weather and CWD. Last time I checked, Accurate Dorwin had excellent thermal performance but was quite a cost premium. Any other recommendations?

Also, did you model dual glazing for the south windows? I've heard controversy saying that the higher SHGC of dual glazing can net larger energy gains over the year but have not done the modeling myself.

-c-

So, Conrad... Anne and I always wondered what became of you after you moved on to NAIT. Looks like things have gone well, and I'm impressed by your NetZero project. One of these days I'll have to pick your brain for advice on the subject!

Keep up the blogging!
Leanne

Curtis,

LBNL Resfen is good for doing what-if scenarios with windows.
http://windows.lbl.gov/software/resfen/resfen.html
IMHO, the biggest reason to go for triples on the south side is condensation resistance.
If you can get a double-glazed lowE with a 16mm airspace & insulated frame I'd go for that vs a triple with 2 smaller airspaces (i.e. 2x11mm airspaces as some manufacturers use in triple-glazed).

-Ralph

Curtis,

We used Duxton windows out of Winnipeg (http://www.duxtonwindows.com/). I never did get around to modeling double-paned south windows, but we decided against them because of the greater temperature swings and condensation. It is simply not comfortable to sit in front of a double-paned window in -20C weather, marginal annual energy gain or not. Along those same lines, during the overheating month of October it would be more uncomfortable when the sun is blasting the house. Finally, I'm not even sure that we have enough thermal mass as it is (with triple-paned windows) to maintain comfort in the home.

Conrad

Curtis,

One of my mistakes was sizing overhangs to reduce late-summer solar gains. In a heating dominated climate I don't think it makes sense to give up heat gains for potentially cooler temps in the early fall.

Conrad is also leaving out important details about comfort. During the day it is comfortable sitting next to a double-paned lowE/argon window, even when it is cloudy, and even at -25C (this is my first experience). In a bedroom at night you will still get a downdraft if you don't use window film or some form of insulated window covering (shutters/curtains).

-Ralph

Curtis & Conrad,

I have questions about window manufacturers.

Curtis:
I'm curious why you're not inclined to go with All Weather or CWD. When I look at the material of various manufacturers, I can't tell much difference between them. If you're concerned about sharing your opinions in a public forum, please e-mail me at harec13@yahoo.ca.

Conrad:
I'm curious how you came to choose Duxton for your windows. I know the Riverdale net zero home used them, but since buying local is a green thing, what made Duxton from Winnipeg your choice over the numerous local manufacturers.

- Cory

Cory,

You're right, a local choice would have been greener. With the complexity and busyness that accompanies building a house, we ended up going with Duxton in part because of the established relationship that my builder has with them.

Duxton window frames are fiberglass, which from what I've read holds a (slight) edge over PVC or aluminum-clad wood frames in terms of both durability (they expand and contract at virtually the same rate as the panes of glass) and energy efficiency. There are no manufacturers of fiberglass windows in Edmonton that I know of.

However, the small advantage that the fiberglass offers over PVC or wood frames probably doesn't justify shipping them all the way from Winnipeg. If we had to build this house again, I would probably source the windows locally.

Conrad

If the frames are foam-filled fiberglass extrusion, they likely perform much better than the equivalent wood or PVC-frame windows.

-Ralph

p.s. I've tried filling the hollow cavities in my PVC-frame windows; it seems to reduce condensation on the frames.

Hello, and thank you for this topic. I'm very interested in this sort of thing, and just learned of this site. I'm about to start the process of building our new home, and I want to maximize passive solar in our design if I can afford to. I planned to add in floor radiant tubing as well, and our home is also a two story. We hoped to go radiant & passive on both floors as well.

So first, the eaves: what would a recommended 'eave' depth be here in Canada? This is something I don't want to botch up. Above, Ralph suggested that 2' overhangs was a bad decision. So I'm very concerned I may get it wrong also.

And second, how thick would you make the thermal mass floors? I've read the floors should be approx. 4" or more of gypsum concrete, but that's a huge amount of cement/weight/cost.

What would you all suggest? (oh, and yes, I'm going with super insulated and tight).

Pete

Hi Pete,

To get the overhang right, consult the book "Tap the Sun" by CMHC. It's worth the the money to buy it, and it includes software that lets you play with the overhang with regard to overheating.

It says in Tap the Sun that the first 4" of mass are the most effective. We poured a 2.5" topcoat in our house.

As for Ralph's comment, I'm speculating that his location in unsunny Nova Scotia (if I recall correctly) has some influence on his opinion. I talked to a very experienced solar designer/engineer in Edmonton, and he said that my house, with 13% (again, IIRC) south glass to floor area ration, would need AT LEAST three foot overhangs in order to not significantly overheat.

Conrad

Hi and thanks Conrad,

Thanks for the advice! I'll go by Chapter/Indigo today. I want to be well armed, so to speak, when it comes down to ordering roof trusses and floor joists. I hope to see an architect this week to go over our floor plan. We'll need to make sure we have enough support for the added weight of concrete.

I'll buzz through that book/cd so I have a decent understanding of the eaves. I'm guessing eave depth will make a difference, depending not only on the depth of the windows, but also the distance from the top of the window to the eaves themselves.

I actually haven't calculated the % south glass from the plan. But is looks high. I'm guessing around 10%. But I'll get that figured out. I didn't think it would matter as much because we were going full concrete with tile floors both upstairs & downstairs. But I'd hate to be wrong and trying to make expensive fixes afterwards... Yikes!

Thanks again,
Pete

Peter,

Where is your house? Sounds very interesting.

You can only buy "Tap the Sun" directly from CMHC. They have an office in Edmonton, but phone ahead to make sure that they have a copy in. Otherwise, you can order it online.

Conrad

Hi Conrad,

I just realized that about the book today. So I'll order it tonight. I called to check and locally CMHC only keep copies of their free books.

We're building in Bonshaw, Prince Edward Island. It's a hilly area, just inland from the south shore. It's well wooded, but has quite good south sun. The lot slopes every so slightly, down to the north, but there are no significant obstacles to block sun exposure, as most trees are maple & birch.

The house itself is 2000 sq.ft, and although not specifically a passive solar design, the garage is on the north side facing west, main entry also facing west, and the bulk of the windows are to the south. We're planning to change the roof somewhat to expose 2/3 bedrooms & a bathroom to direct winter sun (shed dormer on the south roof).

All in all we're very excited.

Question:
Will the 2.5 inch topcoat of cement be enough mass? And what type of cement did you use?

Hi Pete,

Sounds great. I did some rationalization/calculations and wrote about them in a little position paper here: http://greenedmonton.ca/mcnzh-insolate-and-add-mass. It includes references to a book by Daniel Chiras that is also excellent, and that offers some rules of thumb for determining thermal mass quantities.

We used "Agilia" concrete, which is a patented mixture offered by Lafarge here in Alberta. It settles really well. It has a high percentage of fly ash, but they won't reveal how much.

Conrad

Hi Conrad,

We lived in Calgary for 10 years. We are familiar with Lafarge. Is it the fly ash that makes the difference? I don't know enough about it. Are you confident only 2.5 inches of thermal mass flooring will be enough? I'm curious because I've ready that 4 inches is optimal. Or is that for a more southern climate with longer days? I think realistically, I'd be pleased to get direct sun on december 21st, from 10am - 3pm.

I'll eat up that book when it comes in. I'm eager to know about the length of overhangs for my latitude.

Also, We're considering foregoing a basement, and perhaps building on a slab.

Pete

Hello Peter and Conrad. I read this article because I was searching for articles about passive solar designs, but Thought I would mention to you, Peter, that I think I've actually been though your house with your builder in Bonshaw (maybe). My wife and I are currently in the process of building a passive solar design in PEI as well, in Belle River, and if your house is the one in Bonshaw that I think it is, then I'm the guy who asked for your permission to steal your plan for our build. Guess the Internet isn't that small after all.

In the case that you're NOT the person I'm thinking of here, Peter, I'll be hoping that your project is / will be going well - I'd be interested in hearing all about your experiences as you continued through your project. If you're on PEI and building Passive Solar, I'm sure we'll bump into one another somewhere along the line.

Thank you as well, Conrad for your interesting article (and discussion it generates for the benefit of the rest of us).

Shawn

Hi Shawn,

I'm not the same fellow you are thinking of, but it's good to know I'm not alone out there! I can say, that I haven't consulted much about the passive solar. I've read a lot, and in the end, my wife and I have chosen to customize a fairly traditional house style and make it as passive solar as we can. I'm really hoping we get things right... tight envelope, high r-value, about 9% south glass, cement/tile floors and deep overhangs.

I hope we do get to meet. I'd love to see what other people are doing, and how they turn out. We're terrified we're making a huge mistake, but if it works out... (crossing my fingers). It's just money, right? (sigh)

Good luck with your project.

Conrad: Sorry for hijacking your blog.

Pete

I have been told recently (but cannot verify) that Daniel D. Chiras, the author of one renowned book on passive solar house design, has lately changed his opinion on the concept of installing different windows on the south wall (see discussion above "the Centre of Glass R-value for the south windows is 5.7 vs. 8.3 for east, west, and north windows). Apparently, he now advocates using the highest R-value windows on the south wall as well (again, I cannot verify) I totally understand Conrad's logic above and have generally accepted it 100%. But I'm about to purchase triple glazed windows for our eco-build, and am nervous!! I can see some logic to using the higher R glass on all windows, since the higher R glass will be useful at night and during the day when the sun is not shining.

Has anyone else come across Mr. Chiras' revised opinion or any other information that could shed light on this issue? Perhaps the original comparisons that came to the conclusion about higher SHGC windows on the south took this into consideration? An actual case study would be useful ...

Mississippi John
Mississippi Mills, Ontario

John,

I could see that being a good strategy. When we have a month of cloud as we did in each of the past two winters, our south windows are nothing but big heat radiators (heating-wise). I wonder if you could even avoid some of the extra mass with a low SHGC but high R-value on south windows.

Chiras seems to still support the strategy that I took in this undated interview (0.2 U-value is R5):

"If you intend to capture solar energy for heating, you will need to install low-e, high solar heat gain coefficient windows (SHGC of 0.6 or higher) on the south-facing edifice. You should install a window with a U-factor of 0.3 or lower, preferably 0.1 to 0.2.

You are in luck. There's an amazing window company in Canada called Thermotech which sells a product that will work for you.

Throughout the rest of the home, you will need to install low-e windows (with a U factor 0.1 and 0.2, if at all possible) for best performance. Solar heat gain coefficient for these windows is not as important as the south-facing windows. I'd shoot for a SHGC of around 0.35 for these windows."

I've sent an email to Peter Amerongen to see if he's heard anything different.

Conrad

Hello John,

I cannot remember where I read this, it might have been in a posting by Stephen Thwaites (Thermotech Fenestration) on the Green Building Advisor website. Whoever the author was, he said that, in a well designed and well insulated home with the appropriate windows, the high solar gain windows will have a net gain effect on the heating load.

Conrad mentioned the downside of windows under periods of prolonged cloud. This is true, but, usually, there is more sun than cloud in the winter.

Perhaps Mr. Chiras is changing his stance due to the concerns about climate change and global warming. If that is the case, then overhangs will reduce the summer overheating effect.

In one post, you mentioned that Ross Elliott is advising you in the design of the house. Ross is extremely knowledgeable. What is his advice about your concern? - Jim

Hi Conrad -

Thanks again for your input. We'll likely go with Thermotech. They are slightly more than certain competitors, but they have been terrific to deal with and have suggested several ways to reduce the costs from my original specs. They are also located about 20kms from where we live, so they are local and will charge no shipping fee.

We're aiming to have this project framed by snow fall. So much to do, so little time to do it ... Thanks again for your blog - we have used so much of the information provided!!

Mississippi John
Mississippi Mills, Ontario

Peter Amerongen had this to say:

"It all depends on the modelling. The right answer will be dependent on the situation. If there is significant shading of the south facing windows by trees, adjacent buildings or even jogs in the building itself, or dirt on the glass it may indeed be better to go for the higher R value and accept the lower heat gain coefficient.
The Passive House software forces you to reduce the heat gain according to the factors above. Even if the reduction factors have some guesswork, the result will likely be closer than H2K which assumes that the house is getting 100% of the theoretically available solar radiation at the spot in that orientation ( except for the shading of any overhang immediately above the window).

On an unobstructed site, I would trust H2K, for any others I would try to model int the PHPP software and then do what it says.

There is also new glazing available that will give an R value of about 7.5 with a SGHC of .5."

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