[greenspree]

Another heating season (almost) done with!

As an introduction I thought I would talk about my passive solar strawbale home built in PEI, Canada.



This will be our third winter in the home.

Stats:
2 story hybrid stick-frame/dtrawbale infill
21'x29' per floor interior dimensions
shallow frost protected slab on grade
wood stove and 12000BTU mini split air source heat pump (region V)
electric hot water storage heater with drain water heat recovery
chest fridge
CFL and high efficiency halogen lighting

Results this year so far (to end of March):

1111KWH’s above our base rate of 18KWH/day in the non-heating season. This is the consumption of our 12000BTU air source heat pump. We pay $0.1265/KWH including all taxes for $140.54.

We also burnt 1/2 cord of hardwood in our small airtight stove. We bought this wood when we were building at about $100/cord in 8’ lengths. So about $50 for the wood and a high estimate of $10 for gas and oil for the chainsaw to cut it up and the electricity to run the electric hydraulic splitter to split it.

So a fairly accurate estimate of costs to heat our house this year of $200 for 5 months of heating, or $40/month.

April may have a little extra heating costs, but I can’t see it being more than another $20 or so.

Built for about $100/sf (Canadian) owner built (subcontractors for plumbing, electrical, well, septic, sitework and concrete placing/finishing.

Additional info at http://greenspree.ca

Video tour: http://youtu.be/WDkR-6pEJvs

[davep]

What do you use for internal thermal mass? Do you have south-facing glazing for passive solar energy collection?

[greenspree]

The first floor slab is a black coloured concrete which is 8" thick, coupled with the high thermal mass of the exterior walls themselves.

Yes, we do have most of our windows facing south and very little facing north.

[davep]

The first floor slab is a black coloured concrete which is 8" thick, coupled with the high thermal mass of the exterior walls themselves.

Yes, we do have most of our windows facing south and very little facing north.

What do you have on the interior of the straw bale insulation for the exterior walls for thermal mass? Thermal mass is generally the key for passive solar design (along with the south facing windows of course).

[greenspree]

An average thickness of about 3.5" of claw/sand/lime stucco.

For our house this is about 35,000lbs of stucco, and 77,000lbs of concrete in the floor.

Thermal mass is important, but not the only considerations, insulation and air tightness are in my opinion equally important.

[davep]

An average thickness of about 3.5" of claw/sand/lime stucco.

For our house this is about 35,000lbs of stucco, and 77,000lbs of concrete in the floor.

Thermal mass is important, but not the only considerations, insulation and air tightness are in my opinion equally important.

Insulation should be a given for a well-built straw bale house! Air tightness is of course important, but people should never install vapour barriers in this kind of house, as clay/straw/lime allows water vapour to naturally move through the walls. Adding a vapour barrier will mean the straw bales' lifetime is greatly reduced (this is also why you shouldn't use cement in these walls as well as being careful about paints).

I'm not sure you can call your place passive solar, but it is definitely nearly passive and sounds like a good healthy environment to live in. I've been looking at a structural straw-bale cum passive solar hybrid for a few years now. It's a great concept. My biggest barrier is my other half

BTW, what do you mean by "claw"? From what I've read, a better internal covering for the bales is clay due to its very high thermal mass (with a lime overcoat). This can be used externally too, but with a thinner layer. It's a very cheap option and makes the whole wall recyclable too.

[greenspree]

Typo, I meant clay.

Why do you think I can't call it passive solar?!

[greenspree]

Also, I didn't use a vapour barrier, air tightness doesn't = vapour barrier required.

Lastly, don't combine clay layers of stucco and lime on the exterior in climates with freezing temps in winter, the two material expand and contract at different rates and the bond between them will fail.

Have you ever built a passive solar home?
Have you ever built a strawbale home?
Have you ever built anything?

You sound like you have read a lot of websites.

[davep]

I made plans to build one but my wife vetoed the build. I used the following book a a reference, and it does go into the engineering side quite heavily: http://www.amazon.com/Design-Straw-Bale-Buildings-State/dp/0976491117/ref=sr_1_14?ie=UTF8&qid=1335940927&sr=8-14. I also met the architect who designed a double international award-winning passive solar home in Darmstadt, Germany.

I wasn't having a go at you, just asking for details as it is a subject which obviously interests me. Also, I was wondering why you consider it passive solar when the majority of the heat appears to come from wood rather than solar energy.

[greenspree]

I have 20 years of construction/architecture/energy efficiency design and implementation experience.

Passive solar design =/= passivhaus standards from Europe for example.

Passive solar design means you are designing your house to take full advantage of solar gains, something difficult to rely on solely in a latitude where you have extreme cold and very limited hours of low angle solar energy per day during the heating season (For PEI -7degC average winter temperature, frequently dropping below -20degC during the coldest parts and a minimum of 8.5 hours between sunrise and sunset)

The average heating load for new homes in my area is 85MMBTu's and my house requires only 21MMBTu, so no, most of my heat doesn't come from wood.

[davep]

Passive solar design =/= passivhaus standards from Europe for example.

I agree.

Passive solar design means you are designing your house to take full advantage of solar gains, something difficult to rely on solely in a latitude where you have extreme cold and very limited hours of low angle solar energy per day during the heating season (For PEI -7degC average winter temperature, frequently dropping below -20degC during the coldest parts and a minimum of 8.5 hours between sunrise and sunset)

The average heating load for new homes in my area is 85MMBTu's and my house requires only 21MMBTu, so no, most of my heat doesn't come from wood.

But are you taking full advantage of solar gains? It seems most of your heating load decrease is to do with the great insulation rather than the passive solar side.

The reason why I'm being a bit finicky is just because I was also looking specifically at a hybrid straw-bale / passive solar design using more extensive south-facing solar collection (also with a woodstove backup as we can go a long time without sun here in winter). Don't get me wrong, I think your place is great. I guess I was a deflated after expecting to see something done exactly as I had also planned.

[greenspree]

But are you taking full advantage of solar gains? It seems most of your heating load decrease is to do with the great insulation rather than the passive solar side.

And you're basing this observation on what exactly?

[davep]

But are you taking full advantage of solar gains? It seems most of your heating load decrease is to do with the great insulation rather than the passive solar side.

And you're basing this observation on what exactly?

The fact that you aren't using a large southern area for solar capture relative to the area available on the wall (and in this scenario you'd want overhangs too, to stop overheating in summer). That's why I questioned the "passive solar" appellation. I'm probably being a bit over-nerdy though.

[greenspree]

Who says I'm not using a large southern area? I already said we have a large percentage of windows on the southern face. They do have overhangs as well, the windows are set in to the wall and this provides substantial shading in the summer months, espicially on the shorter windows. We don't have a 100% glazed south wall I concede, but this is a design flaw many who attempt to design passive solar homes fall into. Glass allows heat to escape as well as solar radiation to enter and most professionals will tell you to not exceed 30% WWR (window to wall ratio) unless you are incorporating a tromb wall or something similar.

I am just confused as to how you are able to critique the design and operation of my house based on the limited details I have posted so far.

Like I have said, my years of experience in the field leaves me no qualms in stating that my house incorporates passive solar design, and heating a home for $200 a year when most people are spending between $2000-4000 a year speaks for itself, and it's not just because of insulation.

[davep]

My critique is based on seeing the frontage on youtube. I like your design and think it's cool, because at the end of the day you are massively reducing your energy bills using mainly sustainable materials (which is what it's all about). I guess we'll just have to agree to disagree on what constitutes passive solar (unless you have some anecdotal evidence of how the house can be heated over a period of time in winter during sunny periods without recourse to the stove).

[greenspree]

Certianly, my heat pump never comes on during sunny days and the wood stove is never lit. I only light the stove in the evenings after work on cold days for a maximum burn of about 4 hours.

[davep]

Certianly, my heat pump never comes on during sunny days and the wood stove is never lit. I only light the stove in the evenings after work on cold days for a maximum burn of about 4 hours.

But I guess the question there is whether that's due to the insolation or the thermal mass and decent insulation after the fire of the previous day (it's no doubt a bit of both, but in what proportions?). I may be misreading your post though as I'm not sure if you're saying you use the stove in the evenings of cold sunny days or not.