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Community and Q&A

Raised floor over slab, musty insulation problem

Kevin Hardy | Posted in Green Building Techniques on

About a year ago I built a small apartment inside part of a detached garage. I have been noticing a strong musty smell lately. The space is built on a raised floor about 8″ above the garage slab with a 6 mil vapor barrier over the slab and inside of the stem wall. I’ve tracked down the smell to the fiberglass batts I used to insulated the space between the slab and raised floor. I opened up part of the floor to investigate and haven’t found any visible mold, water or condensation. However the insulation smells really bad.

My plan is to tear up a strip of the flooring for access and pull out the smelly batts. I’m trying to avoid tearing up the entire finished floor. My idea was to just leave the floor uninsulated after removing the batts and use mechanical venting to bring conditioned air from the living space down through the mini crawl space and then exhaust it outside. My question is will this keep the uninsulated space under the floor dry and mold free or will I end up with condensation on top side of the poly barrier that is over the slab? Could I use a small HRV as the mechanical venting (the apartments is built very tight and gets a bit stuffy and humid as well)? That is have the have the HRV exhaust duct pull air through the under floor space and solve both the under floor issue and indoor air quality at the same time and minimize heat loss?

I’m on the border between climate zone 4C and 5B. Warm to hot dry summers and cold-ish wet winters. Heating is simple electric wall unit.

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  1. Kevin Hardy | | #1

    Any ideas here? I'd like to start on this before it gets worse.

  2. Charlie Sullivan | | #2

    If you send interior air into that cavity and the interior air is warm and humid, and the slab is cool, you will get condensation. In my climate, that hazard is worst in the hot humid summers, but for you it might be worst in the winter, particularly as you say it is humid and stuffy inside. Do you know your slab temperature, and indoor temperature and humidity in the winter? A zip code might also help as I don't know your climate.

    The right solution is to remove the floor, install foam (e.g. EPS) against the slab, and then reinstall the floor.

    But you might be able to vent it with outside air, depending on the climate, and have success with that.

    If necessary, it might be possible to seal off that cavity and mount a dehumidifier in it, or ducted to it (to and from it) and use that to keep it dry in there. If the cavity is well sealed, the dehumidifier wouldn't need to run much.

    Just to check, it's clear that the smell is from something going on above the vapor barrier, not under it?

  3. Kevin Hardy | | #3

    Thanks for the reply. My zipcode is 97031, Hood River, Oregon. I just got a humidity gauge for inside and it's been reading 50% RH at 70F interior temperature. RH outside was 96% at 45F. Pretty typical winter day today. Summer temps are generally 70 to 95F and ~40 to 50% RH with a lot of wind.

    I definitely thought about install EPS against the slab, I could fit about 2" under the joists and then spray foaming the rim area. I just hate to tear up the entire nearly new wood floor though. Makes me sick to think about. Unfortunately, I just followed the local energy code and stuff R-30 batts in there.

    Pretty sure the smell is coming from something above the barrier as the insulation seems to be the source of the smell. Below is just bare concrete so not sure what could be going on down there? Will pull up a bit of the vapor barrier to see for sure though.

    Reading this , I'd guess that I'm ended up with condensation on the bottom of the batts.

    It also seems to me if I'm bringing conditioned air into the floor space the surface of the slab will gain some heat (or cooling) from the conditioned air and prevent condensation on the top of the poly vapor barrier (or on the underside of the floor)?

  4. Charlie Sullivan | | #4

    Your average temperature over the whole year is about 50 F, so with the slab well insulated, it would sit at 50. You are right that if you blow warm air in there you could warm it up, but I think of that as somewhat of a losing proposition, as much of the heat gets conducted away. And your indoor air now has a dew point of 50 F as well, so you are right on the threshold of condensation.

    If you use an HRV to lower the indoor humidity, you could reach a point where that air would be safe to send down there. But you wouldn't need to do that actively in the winter--there's be no source of moisture to need to vent out.

    Summer is trickier though, unless you have air conditioning to keep the dew point of your indoor air low.

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