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Insulating an outbuilding in the rainforest

tall_weeds | Posted in General Questions on

I am in need of some second opinions. I am in the process of building a 10’x14′ shop space with close to %100 salvaged materials I have been acquiring over the last several months. The shop is going to have a metal shed roof, 14′ sloping to 12′ and there will be a sleeping loft in half the space for occasional guests. I will be heating with a woodstove, most likely every day of the winter. There is no electricity, bathroom, or running water inside the shop. The location is Olympia, WA which gets over 50″ of rain a year and is a hotbed for mold and mildew.

Now, my questions have to do with moisture and insulation because in every article I find, the space is a larger home with kitchen, water, bath, multiple rooms, etc and my project is not that. On my 2×6 studs I am planning on putting OSB, Tyvek house wrap, then 1″ cedar board and batton. To insulate I recently acquired a large amount of 3″ extruded polystyrene in 2′ wide pieces and I am having a hard time deciding if using it between the studs is a good idea, especially if i seal it in with spray foam around the edges. The interior will be T&G pine. I will have 4 double-pane windows that I can open and am planning on installing 2 small vents.

If everything is done right, is this sheathing and insulation combo a good idea in a climate where its raining for months at a time? Is there a better way to do it or better insulation style? I am hoping the inside will stay dry with a fire and some ventilation but will the thick insulation prevent the outside from receiving any heat if it gets damp? Are there any things I should be wary of if I insulate the ceiling and floor with the extruded polystyrene as well?
(the base of the shop is framed with treated lumber off the ground on cinder blocks and I am planning on insulting that as well). Any tips for small buildings in a wet climate are helpful. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    In the Pacific Northwest, it's possible for siding to develop mold, especially on the north side of the house. There's not much you can do about this except to occasionally wash off the mold; it's climate-related.

    There is no reason to expect any mold or moisture problems in the interior of your house during the winter if you have a fire in your wood stove every day. During the summer, of course, it's always possible for the interior of your home to get a little humid. However, the humidity won't be as bad as it would be in a house without air conditioning in Houston or Miami. Don't worry -- this humidity is natural and harmless.

    If you want to use rigid foam to insulate your walls and roof, then the insulation should be installed as a continuous layer on the exterior side of your wall sheathing and roof sheathing. You don't want to cut the rigid foam into narrow strips and insert the strips between the studs. (If you did, you would have thermal bridging through the studs. A continuous layer of installation on the exterior side of the sheathing addresses thermal bridging.)

    For more information, see these three articles:

    How to Install Rigid Foam Sheathing

    How to Install Rigid Foam On Top of Roof Sheathing

    Getting Insulation Out of Your Walls and Ceilings

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