Designing a Solar Heated Structure (Part Two)

On February 12, 2008 in Solar Cabin

Alright, I know it has been a while, but here is the second installment of the “Designing a Solar Heated Structure” series I promised I’d write.

As I’ve said before, I am a rank amatuer, and am not claiming to be certified, highly experienced, or anything else. I am merely offering my opinions, and carefully organizing my research so that it can be understood by a more broad range of people.

I try to reference my sources as much as possible but if you see something that sounds out of whack and I don’t provide a source, ask me about it. I don’t work on the site while drunk so there shouldn’t be any made-up facts in here unless they are from someone that seemed a credible source at the time.

All ranting aside, let’s get down to designing our solar heated structure.

The first thing you will need to do is talk to a contractor or licensed professional because I am just some guy on the internet and why are you listening to me, anyway? After that is over with you should have an idea of the soil conditions in your building site, and you should already have a basic plan of what you want from Part One of this whole ordeal. Hopefully the contractor helped you form your plan to work well in the area you will be building. You will want to choose the right foundation/building type for your area. Consult with a contractor on this as I only have experience in the deep woods of Northern New York. Different soil types will need different preparation/construction methods.

Places like Florida (an Ode to John, a commenter from the last post, and our friend Matt from diyrobotics.info) have hurricanes and termites and year round humidity and you can see that most buildings are made out of anything but wood (concrete blocks, formed concrete, brick, adobe or whatever). Up here in the north, wood is the primary building material. Apparently Europeans don’t use wood much anymore either, although I can only assume this is due to it being in short supply with the overcrowding and all of that.

All of these are perfectly good reasons for you to not listen to some guy on the internet about what foundation and/or building type to use. Consult with a contractor. Consult the local building codes and Code Enforcement Officer. Observe what others in the area are using, and check prices on the various materials.

Since this is a DIY website, I am kind of targetting this article at people that are going to build themselves a small retreat in the wilderness, not people building a solar heated warehouse or gigantic residential building. With that in mind, I apologize if this article doesn’t fill in all the gaps for anybody trying to figure out how to build a skyscraper by reading something on the internet.

Hopefully once you have taken all of this into account, done your research, ripped out your hair, done more research, sat down at the drawing board, had a few beers, and actually gotten to work, you will end up with something like this (or thereabouts):

Don’t worry about the whole window thing for now, as we haven’t really gotten to that point. Also don’t worry if your plan doesn’t look anything like the one here, this is for a cabin that would be sitting on rock. There was no foundation to be dug. Concrete piers would be poured on the rock and then the beams anchored to the piers.

Hopefully though, you have taken into consideration the “airlock” or “mudroom” I mentioned earlier, and at least have an idea of where your windows will go, if not how big they will be. Also make sure the building is going to be oriented properly with regards to the sun, and that the site you have selected has a good southern exposure that will receive light for as long as physically possible during the day.

I will leave the building materials to you, as that depends on your design. Some will go with shingles for the roof, others will go with corrugated steel or something else. Some will use wood for the walls, others concrete and others straw bales or cordwood. Either way, make sure there are not going to be lots of air leaks and that the structure will be heavily insulated to reduce the chance of losing that precious heat we are trying to gain.

After the site has been selected, the sun’s path verified, the building codes checked, the design of the building roughed out, and the building materials chosen, we need to focus on window and thermal mass sizing/placement.

It is EXTREMELY important to make the windows large enough to gain the amount of heat you want, and equally important to have enough thermal mass to store that heat that came in. You also don’t want to go overboard on either one. As with everything there is a point of diminishing returns.

There is an excellent article at renewableenergyaccess.com called “Balancing Solar Gain and Thermal Storage” that I have linked in the Heating section of the site. The author checked with Ron Judkoff, Director of the Buildings & Thermal Systems Center at the National Renewable Energy Laboratory regarding a question from some readers.

He listed some “Rules of Thumb” for Passive Solar design:

* Place mass inside the insulated envelope of the building.

* Place mass where it will receive as much direct gain radiation as possible and make sure it is at least in the same room/space as where the sun enters.

* Mass for direct gain systems is only effective for the first 2 to 3 inches, so 8″ concrete block is overkill (diminishing returns) unless you are doing a trombe wall system in which case the trombe wall should be 12 to 16″ thick.

* Surface area is much more important than thickness. So the same weight of mass widely distributed in a thin layer is much more effective than a thick layer less widely distributed.

* The mass (except for trombe walls) does not have to be in exterior walls. It can be in interior walls, or floors. In fact a 4″ brick partition wall separating two rooms with each room having South facing windows is a very effective design because you utilize the first two inches of depth from each surface of the mass wall.

* If the floor is used for mass, do not cover in carpet or some other non-conductive material. Use tile or masonry interior floor finishes. Also insulate under the entire floor area (not just perimeter insulation) being used for thermal mass.

* The mass surfaces do not have to be dark in color (except for trombe walls), but it would be bad to have the non-mass surfaces dark and the mass surfaces light. If both the mass and non-mass surfaces are light in color, then there will be enough reflections to get the energy to the mass.

* Be careful with Low E windows. Work with your window dealer to specify high solar transmissivity low e on the south facing windows (not easy to get) (one product is Heat Mirror 88). If you can’t get a high solar transmissivity low-E window then specify triple clear with a krypton fill for the south windows. Windows on all other orientations can be the usual low-E products.

But unfortunately you can’t just go about this all willy-nilly. Different parts of the world receive different amounts of sunlight, and that is where Part Three of the “Designing a Solar Heated Structure” series will come in.

In the meantime, you can check out this Solar Radiation Data page I have linked in the “Charts and References” section of the site. This page has annual BTU’s per square meter for specific latitudes/longitudes. Check out the data for your closest major city and start paying attention to the sun in your area, this knowledge will come in helpful soon.

You might also want to have a look at this Heating Degree Day Map, and the wikipedia article on heating degree days.

Thanks for stopping by, I’ll add some more later.

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