Insulating a tight crawl space in an old house under a large room
Hi Folks. My first time here! I think our application is novel, and we’ve received conflicting advice, so I’d love to hear your thoughts.
We have a 24′ x 15′ room with a 16″ unheated crawlspace below it. The crawlspace has brick walls on all four sides and dirt underneath. The room above gets very cold in the winter, and when in the basement we can feel cold air flowing out of a hole (former window) into the adjacent heated section under the house. We have plastic covering the dirt and fiberglass between joists, but it doesn’t completely fill the gap (old house has odd sizing).
We’ve received varying advice, including:
o Put high-R foam board against the bricks along the interior crawlspace “walls.” (We got 1/3 through with doing this.)
o Do the same, but use expanding foam spray instead of foam board.
o Use expanding spray at the margin between the bricks and the wood sills.
o Put larger sized fiberglass in the joists (bend concave to fit).
o Put high-R foam board on the crawlspace “ceiling,” in essence sealing in the current poorly-fitting fiberglass.
I guess the question is where is the coldness coming from – the ground or the sides? And for the latter, from the brick or the margin?
Thanks again,
Matt
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Replies
Insulate the foundation walls with foam insulation, condition the crawl space. You will have to address the rim joist area with insulation and an air barrier. Make sure the poly covering the dirt floor is sealed to the foundation wall to prevent moisture from entering the space.
Matt,
You don't indicate where you are (what climate zone), nor whether there are any moisture problems associated with the crawlspace.
Since the crawlspace communicates with the conditioned basement, it makes sense to condition the crawlspace, which will also help keep it dry. I would recommend installing the minimum insulation level required for your zone on the crawlspace walls and removing the poorly-fitted fiberglass from the joists, which will reduce the likelihood of moisture damage to the framing.
Oops! Sorry about that, Robert. I'm in Western Massachusetts, which Wikipedia says is "a humid continental climate."
And thanks, both Doug and Robert, for replying!
Re: Moisture problems, I don't know of any. No mold and no water collecting on the plastic that's on the dirt. The dirt underneath the plastic is moist. QUESTION: What else should we look for?
Re: removing the poorly-fitted fiberglass from the joists: QUESTION: I'm confused because the room above the crawlspace is heated, so I'd think insulating that room from the crawlspace would be important. Otherwise, the room is heating the crawlspace itself, which has a cold dirt floor, right? I think that's what Doug was getting at with installing insulation and an air barrier.
Re: conditioning crawlspaces, I did a little research and found advice that says to basically seal it up tight with no ventilation. Here's one company's take (http://www.basementsystems.com/crawl-space/crawl-space-ventilation.html): "Seal the crawl space with a vapor barrier, seal the crawl space vents with vent covers, seal any gaps or holes to the outside, and seal the crawl space door". QUESTION: Is something like their product necessary, or will the thick plastic we put down work OK? (I don't recall mils.)
I'm getting up to speed on this, and I'd love to hear your answers.
As far as moisture forensics, look for white powdery efflorescence on the bricks, indicating that they are wicking up ground moisture, in which case try to eliminate outside water sources such as reverse grading or absent gutters.
Any plastic ground vapor barrier is OK unless you find yourself tearing it up while crawling around down there, in which case a tear-resistant material like Tu-Tuf would be preferable. As Doug said, make sure seams are taped and the plastic is caulked to the foundation walls.
Since you already started placing foam board on the brick walls, finish that and then seal the band joists with cans of spray foam.
The point of removing the fiberglass is to allow the framing to breathe and to semi-condition the crawl space by the heat loss from above, combined with the open window from the conditioned basement. If the crawlspace walls and band joists are sealed and insulated, most of the heat loss is controlled. The dirt floor will always be warmer than the winter air.