Remove crawl space joist insulation – yay or nay?
Hey all, I am the proud new owner of a 1943 1500 sqft house in East TN, climate zone 4a. The crawl spaces (one is from a 350 sqft addition and there is some air connection between them) are vented. I’ve read Martin’s articles on attics and crawl spaces, and consequently I plan on sealing the crawl spaces, putting down a good vapor barrier, insulating the foundation walls on the inside, etc.
However, I don’t plan on sending an HVAC return or supply down there. I figure there is enough air communication that it’s unnecessary – I’ve read some papers indicating 60% of makeup air filters thru the crawlspace in old homes. There is a dehumidifier down there that I will retain.
To the question. The crawl spaces currently have single-faced fiberglass batts insulating between the floor joists. The paper faces up. My question is – baring no apparent mold problem on the paper between the batts and the subfloor, should I leave them or remove them?
I guess the argument for leaving them would be that they would offer some residual “defense-in-depth” insulating value, since the crawl space temp will probably lag somewhat behind the interior temp in terms of conditioning, depending on season. Also, leaving them up is easier.
On the other hand, there is the characteristic funky musty order in the crawl space. I’m not sure exactly where it’s from, but I suppose it’s not too out of the ordinary for a crawl space. Even so, I guess I can’t rule out completely that some of the batts don’t have mold behind them.
Just figured I would get some thoughts.
Thank you,
Eric
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Replies
I'm not one of the incredibly knowledgeable professionals who respond here, but it seems to me that the papers discussing makeup air sourced from the crawlspace are addressing *unsealed* crawl spaces. If you have a sealed crawlspace, how is the air to move?
And if you have a sealed but leaky crawl space... well, that's just not in line with what you're trying to achieve.
A sealed crawlspace is generally sealed off from the outside of the structure (no vents to outdoor air), so the crawlspace becomes part of the interior “conditioned space” in the home. Oftentimes there are enough leaks between the crawlspace and the home that the air can flow, but code is to provide 50CFM of forced ventilation if I remember correctly. Sometimes all that needs to be done is to add a vent from an air duct in the crawlspace to provide airflow when the furnace runs. The ventilation needed in a sealed crawlspace is between the crawlspace itself and the rest of the home, not to the outdoors.
If the crawlspace is connected to a basement and there are openings like a permanently open access hatch (door removed), then I think the argument can be made that the crawlspace communicates with the basement and no further ventilation is required.
The idea is to prevent the crawlspace air from becoming stagnant and saturated with moisture and causing problems for the structure, as well as growing nasty things.
If you properly seal and insulate the crawlspace, I personally would remove the old batts. The reason is that the old batts in floors above crawlspaces are frequently in bad shape since they tend to fall down over time. If yours are in good shape and you decide to leave them in place, you’ll have a little more insulation so a bit warmer feeling floor. If you want to leave them, I would spot check a few places just to be sure you don’t have any hidden problems like mouse nests or mold.
Bill
Your dehumidifier allows you to control humidity independent of airflow or heat from above. So your plan is OK.
Hi Sippy -
Air exchange with the full basement AND the action of the dehumidifier should give you enough moisture management and air exchange that you don't need a dedicated supply, so long as your building department is ok with this approach.
In terms of the 1st floor kraft-face batt insulation: interesting that it was installed UPSIDE DOWN - the code says that the vapor retarder needs to be installed to the "warm side in winter" side of the assembly. That would be the kraft paper up against the 1st floor structural sheathing. BUT, almost certainly a good thing that it was installed upside down because the real moisture threat was coming from the bare soil of the crawlspace. I have investigated more than one vented crawlspace where the floor insulation installed "incorrectly" (not per code) saved the 1st floor framing and sheathing from severe moisture exposure (bare dirt). And you are correct: I bet 9/10 it is installed this way because much easier to staple to the underside of the floor framing.
Peter
Peter, I think I’ve only ever seen insulation installed between floor joists installed correctly maybe 2-3 times. I think you’re right, it’s just easier to staple with the facing on the crawlspace side.
It’ll probably all fall out eventually anyway though :-)
Bill
Hey guys, really appreciate all of your responses, some excellent points made by all. I think at this point I'll spot check above 5-10% of the batts to look for anything nasty.
Regarding the batt install, my preliminary rumaging has not revealed any staples used. They're held up by small gauge steel wire that is affixed beneath the batts and between the joists in small holes drilled in the joists.
Those are wire insulation supports. They are an acceptable method for holding up insulation. You sometimes see them used in walls too to keep unfaced batts from falling out since there is no paper facing to staple with those.
Bill