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Designing for Wood Heat With Backup

mountainbanjo | Posted in General Questions on

Hey there,
I’m in the first stages of designing a home that I’m hoping will be off-grid in the mountains. I’ve got good southern exposure for solar. The issue is heat – I will usually be using wood, but would like to have some backup. I don’t want to overbuild the solar PV, but my thought was, if I do a very tight envelope (SIPs and ICF) with an HRV, would that be sufficient perhaps with some good passive solar to keep things at a non-freezing temperature? Not a huge house, probably 1,800 sqft or so.

Also, does anyone know of a good source of green build house plans I could build from?


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  1. Expert Member
    NICK KEENAN | | #1

    Here's where I would start:

    First, gather information for your site: what is the heating degree-days by month, solar insolation by month, 99th percentile temperatures.

    Then I would come up with a very rough sketch -- how many floors, the footprint, how many rooms, how many windows and doors, the pitch and orientation of the roof. Set some goals for R-values for the walls and ceilings. With that you can do a preliminary Manual J and get your heating needs.

    With the info in the first paragraph and the second paragraph you can calculate how much solar you have to capture to meet your heating needs in the winter months. Figure out how you're going to capture that -- either through windows and doors or a collector on the roof.

    Then revise your assumptions based on the trouble spots. Add more insulation, take away windows, change the pitch of your roof, whatever. Keep doing that until you strike a balance that works for you.

    It may take a while. And you may never come up with a plan that works, passive solar is hard and doesn't just work itself out. But it's much easier to work it out on paper than it is to build and adjust in the real world. And if it doesn't work on paper it's never going to work in the real world.

  2. Robert Opaluch | | #2

    We need to know your climate (approximate location) to really know what you can do in the winter. Regardless of type of heating, its best to get your wintertime heating load low. Then you may find that solar heat gains and daylighting provide most of what you need for space heating and daylighting for most winter days from the late morning until evening. (This would constitute a "solar tempered" building.). Wood could provide space heating for overnight and overcast days. This article provides detailed info on solar gain and the potential for your regional geography:

    When later you need detailed data on solar gains from windows for any location or window orientation, check out this site:

    True passive solar that provides almost all your heating needs only works in certain climates (e.g., Rocky Mountain front range) and not areas with overcast or far northern locations (e.g., Pacific Northwest, Alaska, North Dakota, upstate NY).

    The recommendations from DCContrarian are good too, that data is needed to compute your average January (coldest month) and "design" typical worst day heat losses.

    Try to use a spreadsheet on your computer (Microsoft Excel, Apple Numbers or free Google Sheets) to keep track of all your data. You can use it to compute heat losses, compare design or material alternatives, R-values, alternative room layouts, etc. (I also use a spreadsheet to design floor plans and elevations, as it can be used as a drawing tool.). Calculate total building losses and gains to start more simply. Calculate losses and gains per room or space later. Later use Cool Calc to compute your Manual J if you wish.

  3. GBA Editor
    Martin Holladay | | #3

    I'll echo others: we need to know your climate zone or geographic location.

    In general, though, a house built to the Passivhaus standard (0.6 ach50) anywhere in the Lower 48 or southern Canada, shouldn't have frozen pipes, even if left unheated.

    I suggest you read this article: "How to Design an Off-Grid House."

  4. mountainbanjo | | #4

    This is all REALLY helpful, I’ll get on it and get back with some data. I’m not that far north (central Colorado) but also at about 9000 feet so… thanks again for the recommendations!


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