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Adding more insulation to damp-spray cellulose?

I've got an old pier and beam house with stucco skirting and about 1-2" of damp spray cellulose over the subfloor and joists. I was thinking that rather than deal with the complexity of keeping the closed crawlspace and conditioning or dehumidifying, that I'd take it off and add more insulation. I can't figure whether I can spray more (damp) cellulose, or whether I should just put foil afced foam over the bottom of the joists, or remove (?) the cellulose and replace with closed cell spray foam. I'm in Austin, TX - hot & humid, zone 2A

Asked by Matt Desloge
Posted Jun 1, 2014 1:59 PM ET
Edited Jun 2, 2014 5:42 AM ET


7 Answers

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Just to be sure I understand you correctly: Are you saying that your house has a crawl space foundation, and that damp-spray cellulose was installed against the underside of your subfloor by an insulation contractor working from below?

If that's what you're saying, that sounds unusual. Are you sure that the insulation is really cellulose? Here in New England, I've never heard of cellulose being installed in this way. If you are accurately reporting the situation, that cellulose must have a lot of glue in the mix in order to stay put.

Answered by Martin Holladay
Posted Jun 2, 2014 5:46 AM ET


It's either cellulose or some sort of fiber - mixed gray and white, soft to the touch, and stuck to the subfloor and joists. I haven't tried scraping off a sample yet, but I'll go get some photographs. I don't know how well it is stuck to the joists, but I hope not too well since the thin coat is probably pretty open to water vapor I would think.

Answered by Matt Desloge
Posted Jun 2, 2014 6:30 AM ET


The bottom line is that in a hot, humid climate, the best approach is to seal the crawl space and insulate the crawl space walls. If you do that, I imagine that you can leave the fibrous insulation you describe right where it is.

For more information on crawl spaces, see Building an Unvented Crawl Space.

Answered by Martin Holladay
Posted Jun 2, 2014 6:48 AM ET


I was thinking of that, sealing the vents, rebuilding the roten access door, and then conditioning or dehumidifying of the space, when I wondered about taking off the skirting (it's just cement on mesh applied to vertically driven rebar around the perimeter) and having an open crawl space (height about 40"), and just insulating the underside of the joists (closed spray foam or thermax). My WAG is that the labor might be a bit less, with access to a relatively small footprint (30x36) on all sides rather than crawling around with closed skirting. In any case, if I was to go in that direction, could I leave the cellulose and just apply rigid foam to joists, or should I try and add more insulation to the joist bays first?

Answered by Matt Desloge
Posted Jun 2, 2014 6:58 AM ET


At Austin's low-70sF deep subsoil temperatures it is advantageous to thermally earth-couple the house to the thermal mass of that subsoil, so insulating and air-sealing the crawlspace walls is a higher-comfort higher performance approach than insulating under the floor.


Installing 2-3" thick foil-faced EPS on the interior side of the stucco skirting (foil facing the crawlspace, not the stucco, if the facer only on one side), and taping the seams works. If the soil is extremely well drained you could use polyiso instead of EPS. Tape the seams with 2" FSK tape (foil duct tape) and seal the top edge to the band joist with can-foam.

An EPDM or polyethylene ground vapor barrier would also be a good idea to limit soil gas diffusion into the crawlspace. Lap the vapor barrier a foot or so at the seams and seal with duct mastic or housewrap tape. The vapor barrier should also be sealed to the foil facer of the wall-foam about 6" up from the ground with tape or mastic, and to the piers that penetrate the vapor barrier.

(edited to add)

That may even end up being less rigid foam and less work than applying the foam to the underside of the joists. You can safely leave the fiber insulation on the subfloor & joists. If the band joist has at least 1.5" of fiber on it, leave it- just be sure to have continuity between the band-joist insulation and skirting insulation (even if it means a bit of cut'n'cobble rigid foam to fit, and a bit of can-foam work.)

Answered by Dana Dorsett
Posted Jun 2, 2014 11:22 AM ET
Edited Jun 2, 2014 11:29 AM ET.


Thanks for the help - you're right, there *would* be less foam used on the perimeter (440sf to cover) vs the underside (1080sf). One of my worries was the detailing of sealing around all the piers (at least 20) as well as the potential need for a dehumidifier (I don't want to condition the space), vs dealing with a few plumbing penetrations on the underside, and no soil cover or dehumidification needed. Good tip on soil temp, but I think the humidity is the big problem.

Answered by Matt Desloge
Posted Jun 8, 2014 5:46 AM ET


" Good tip on soil temp, but I think the humidity is the big problem."

Yes, humidity is the big problem, but the largest source of the humidity in Austin is the outdoor air! The mid-summer outdoor dew points hit the mid-70s and higher all the time, but as long as that air doesn't come into a crawlspace that's at a much lower temp it's not a problem.

Closing off that space from the outdoor air with insulation from the ground moisture with sheet vapor barriers means the air conditioning of the fully conditioned space above will dehumidify the crawlspace without increasing the energy use, even if you have to actively (but modestly) ventilate the crawlspace with conditioned space air, with a tiny amount of ducted AC or a small fan on a duty cycler timer exchanging crawlspace air with fully conditioned air. (Even one air exchange per day is probably going to be enough.)

The sealing of the vapor retarder at each pier doesn't have to be perfect- moisture transfer vapor diffusion is function of cross sectional area, so with 99% coverage of the ground area you've fixed 99% of the problem.

But the air-sealing of the perimeter skirting has to be as perfect as you can make it, since wind-driven and stack-effect driving moisture can easily move orders of magnitude more moisture than vapor diffusion through 6mil polyethylene. The dew point of 50% RH 75F conditioned space air is about 55F, much drier than your summertime outdoor air in Austin, so as long as you get at least SOME minimal amount of air exchange with the conditioned space, the crawlspace will run drier than it currently does by quite a bit.

By insulating the perimeter, earth-coupling to the subsoil temps will probably keep the crawlspace somewhere in the 75F range in summer- much cooler than your mid-day peak temps if ventilated with outdoor air. The effect on the sensible air conditioning load will be fairly modest, but it will lower both the sensible & latent cooling loads. It'll will likely have as-large or larger effects on reducing your heating-season loads, for year-round benefits.

Answered by Dana Dorsett
Posted Jun 9, 2014 4:51 PM ET

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