Insulating under house
I have been trying to do some upgrades to my home and the next big project will be to insulate the underside of the house, currently there is no insulation and it is ventilated. I have done some research and it appears the best solution will be to seal the vents seal everything with a minimum of a 6 mil plastic and insulate the foundation walls. Here is where i feel like my house may have some special circumstances. The house is setting on quite a hill and part of it has been dug out. At one end there is a 4′ to 5′ crawl space that is about 1/3 of the length of the house. However the other 2/3’s of the crawlspace (not really a crawl space) ranges from about 10′ to 12′ and it is currently a make shift shop but it still has dirt floors. This part of the house also has a standard exterior metal door and a set of plywood double doors allowing entrance to under the house. This is where the my concerns start, from what i have researched, if sealing and insulating the foundation walls the space needs to be completely sealed which I will not be able to do with the doors, as I would like to continue to use a shop until I am able to build a larger dedicated building. Also with the amount of cubic feet that will be added and converted to conditioned space seems very large. My initial thoughts is to split this into two spaces by building a demising wall. I could seal the the shorter space (1/3) and insulate the foundation and the demising wall. But the larger (2/3) of the space would be insulated between the floor joists and stay as a ventilated space? My main concern with this is I have read so many negatives about insulating the floor joists and that the insulation ends up collecting moisture. At this point this part of the space would remain dirt floors as the plastic will get holes in it very quickly and adding concrete to the floor is out of budget. Looking for some advise, is insulating the floor joist really that bad of solution? Would help to add house wrap to the underside of the joist? Would reduce air flow but still allow moisture to escape the joists and insulation? If this space was not being utilized would it be best to seal and convert to conditioned space although it is so large?
Thanks in advance, Mike
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Mike,
To summarize: your house has a partial basement. The basement has a dirt floor. The rest of the foundation consists of a crawl space.
The best way to improve your situation would be to pour a concrete floor in the basement area of your foundation. You should then make sure that the walls of this basement (or workshop) are airtight, are built of durable materials, and are insulated.
You don't want to that, because it's not in the budget. So you are asking about Plan B.
If you want, you can insulate between your floor joists. If you do it correctly, you won't have moisture problems. I advise you to install some type of insulation between your floor joists -- fiberglass batts, cellulose, or spray foam. If you choose fiberglass batts or cellulose, you should also install a layer of rigid insulation -- I advise you to use foil-faced polyiso -- that is at least 2 inches thick on the underside of the floor joists. This foam should be carefully air sealed. Most building codes require the foam to be protected with gypsum wallboard for fire safety.
If you do this work on only half your foundation -- the basement half -- you can build a wall to separate the basement from the crawl space, and you can use a different approach for your crawl space.
For more information, see:
Building an Unvented Crawl Space
How to Insulate a Cold Floor