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basement ceiling insulation

3 years ago

Our 1850 house has minimal insulation. Some of it is over inaccessible crawlspace, but we also have a large shed/unfinished basement area right under our living room that we can access. We can feel cold air from there blowing right into the living room - that area is freezing. We considered spray foam but don't want to use it. I want to insulate the basement ceiling/living room floor with either batts or rigid foam. Do I really need to staple poly onto the floor side (before batts?), cause that seems like a pain. I was thinking either using foam (vapor barrier, but expensive), or rockwool with the poly stapled under it, OR, the poly on the floor (but worried it will tear). Our joists are 26" so no matter what this is gonna be a hassle and imperfect. But I think something will definitely be much better than nothing. We don't want to do the walls because I know the space will still be freezign if we do that (in Vermont). Also considering denim or sheeps wool, but want to start this like tmw and don't have time to wait for those to ship. (I know the 2nd pic has a barrier, but much of shed doesn't).

Featured Answer

res2architect

3 years ago

last modified: 3 years ago

A vapor retarder would be needed if there were a ceiling and the basement/crawl space was expected to be below freezing in which case condensation would freeze like in a wall cavity. [assuming the basement was dry]

Filling the joist cavities with rock wool or similar insulation requires the basement/crawlspace to be dry or mold will form on the joists if it hasn't already happened. The first step is to create a dry space.

The crawl space should be accessible and vented to a dry basement space if possible.

Usually the best solution is to seal and insulate the foundation walls if that's feasible.

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Comments (7)

Jenny Kessler
Original Author
3 years ago
The basement will definitely get below freezing. So if I put the vapor barrier on the underside of the insulation (like ceiling, then rockwool, then plastic). Then also maybe plastic on the floor cause I'm terrified of mold, should that work? I see most things say vapor barrier on the warm side of the insulation, but then the water will get stuck in the insulation and possibly rot joists?
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res2architect
3 years ago
last modified: 3 years ago
The warm side vapor retarder rule assumes a warm humid environment on one side and a dry cold environment on the other side (like an exterior wall) where moisture would naturally move toward the dry side and condense on the cold surface of the sheathing and possibly freeze. In your situation the moisture is on the cold side (basement)..

The simplest solution is to put a dehumidifier in the basement connected to a drain or pump and making sure outside water is directed away from the foundation. Then fill the joist cavities with insulation that is exposed to the basement.

I'm surprised the basement gets below freezing since it should be receiving heat from the floor above and from the earth. If that's really true you need to see if the footings are deep enough.

Where is the house located?
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ptreckel
3 years ago
You need to consult with someone who has a knowledge of old houses. Those as old as yours are VERY difficult to insulate because their foundations are stone, not brick, and have no waterproofing. Please. Consult with people in Old House forums on the proper way to do this.
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HU-867564120
3 years ago
last modified: 3 years ago
You need to address the issue of air leakage before insulating. Even a 1/4" hole in an imperfect poly sheathing job can undo a whole lot of insulation value. The description and pictures of your crawlspace looks to me to be a nightmare to get a perfect poly job. IMO, spray foam is your best option for getting a complete air seal, and especially so when the site conditions are tough to work with.
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party_music50
3 years ago
This site says to use unfaced insulation for an unconditioned basement. http://www.energydepot.com/RPUres/library/bcinsulation.asp
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carringtown
3 years ago
I just moved into an 1827 stone house—previous owners stapled unfazed insulation between joists directly to subfloor. But there are also 3 sump pumps and a Nu-Air ventilation system which controls for moisture/prevents mold.

I’m new to old houses though so I’m down there every few days and def am checking for signs of mold growth.
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