A design challenge: solar heating low income homes in N Mexico

[GaryGary]

Hi,
ClubRust is a volunteer outfit that that builds a few homes each year for low income families in northern Mexico. The homes are about 300 sf with no indoor plumbing, but the homes are well built and appear to be durable. Cost to build one is about $7000 -- all coming from donations. All the labor is volunteer.

I got an email from them asking about solar heat and insulation strategies they might be able to use to heat these homes on a very low budget. While it seems like the northern Mexio is pretty warm and would not need much space heating, they do get some winter weather and occasional nights below freezing, and would really like to provide some heating.

I've put down all the particulars here: http://www.builditsolar.com/Projects/SolarHomes/ClubRust/ClubRust.htm

Got any ideas for a good solution??

Gary

 

[woodgeek]

Nice project. I think they need to insulate the roof first, prob help cooling (OT) in the summer. OF course, cathedrals are v hard.

Have they looked at/budgeted recycled foam board bought by the truckload from the US? My understanding is that it is 70% discounted. http://www.insulationdepot.com/
Find a corporate sponsor to cover freight?

How to use it at the roofline....can't put it between the plywood and rafters (structural weakness). Between the rafters and drywall might be ok with longer drywall nails if not too thick? An 1" of foam with no bridging would make a HUGE difference. Could do the same foam under drywall on the walls for design simplicity? Or, a guy with a razor blade could cut 14.5" strips of 2" from the sheets and just friction fit them between the rafters or wall studs and leave the drywall detailing as it is. Thermal bridging would be worse, but the 2" cavity insulation might perform better.

Maybe the roofing guys have ideas about how to integrate asphalt shingles with foam over the plywood decking.

Getting the R-value up to something decent will make the solar job a lot easier.

Edit: Ok, read the post closer. No ceiling drywall. IMO foam is ok from an outgassing POV, but not for fire safety, when uncovered. Need some cheap sheetgood, or a way to put it over the plywood with shingles....

 

[woodgeek]

I think unfaced styrofoam would be cheapest, v low outgassing, a good choice for wall cavities behind drywall where there is room for a couple inches (lower R-value per inch).

Between (or under) the rafters, foil faced Polyiso might be ok firesafety wise (local folks can test/decide) but bored kids will fill it with dents/holes if they can reach it. Prob go for paper faced (cheaper) and min cost sheetgood (1/8" masonite or equivalent), all 'under' the rafters to avoid thermal bridging. Then could prob vent the cavity for better cooling/drying?

 

[GaryGary]

I agree that getting the roof insulated the key item and difficult to do.

I wonder if there is a way to get rigid foam board over the plywood and under the roofing paper and shinglles? It seems to me that I had a house once with a rigid roofing material in that position, but I think it was a tar and gravel roof.

Seems like cellulose might be a pretty good option in the walls, but, as you say, foam board might work well -- especially if they could get that low price.

Gary

 

[woodgeek]

Given the climate, we are talking about a few cold mornings and nights, and winter daytime highs are pretty moderate. Insulation/buffering would help a lot to even out the (desert?) temp swings.

On the solar side, seems like natural convection driven air heater would be your friend. At modest outdoor temps, gain could be ok with cheap plastic glazing. Working with the E/W vertical walls might be good, get some gain on a cold morning, and to 'charge' the heat store right before the sun goes down. Thinking those 'barn heaters' on your site with the lightweight plastic valves. Would need to 'plug' the openings in the spring, unplug them in fall. The W facing unit would get retired earlier in the spring, would be my guess, for cool sleeping.

Based on my (limited) travels down S, I have a high opinion of the savvy/motivation of the mexican crew. Smart and low budget rather than rich and clueless. Bodes well for the local folks to do the necessary working out of kinks IMO.

 

[begreen]

Simple passive solar would be the best. With their high altitude it gets cold at night, yet can be hot during the day. Adobe is the local material of choice for good reason. It helps even out the temp swings.They can build the walls out of earth if the budget is very tight. http://kentscabin.blogspot.com/2012/09/17th-posting-adobe-is-on-all-walls.html

Check out Michael Jantzen's small home plans.
http://www.tinyhousedesign.com/ebook/free-plans/
http://www.tinyhousedesign.com/wp-content/uploads/2009/04/8x16-tiny-solar-house-plans-v2.pdf

PS: ClubRust sounds like a colony out on the Pac NW coast. lol

 

[pdf27]

My inclination for heating would be some sort of air collector, with a fan to suck the air through some flexible pipe in the slab and then vented outside. Thermostat on the fan so it only runs when the slab is below a certain temperature and the collector is above. That satisfies the requirement for thermal mass and should have a fairly low capital cost. Water would normally be a better choice, but in this case the extra capital cost is probably a show-stopper.

If you're feeling clever, it might also be possible to use this for cooling (run it overnight in summer). In either case, you'll need to put a small amount of insulation under the slab to make it work, and the more you can fit in elsewhere the better.

 

[pdf27]

I wonder if there is a way to get rigid foam board over the plywood and under the roofing paper and shinglles? It seems to me that I had a house once with a rigid roofing material in that position, but I think it was a tar and gravel roof.

Pretty common over here, got to be a bit careful about condensation though which may put the cost up a little.
(broken link removed to http://www.buildbase.co.uk/buildbase_roofersguide/pages/warmroof.html)