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Accidentally "built" a poor wall?

Have read through many of the assembly techniques here in the past, including the guidelines on insulating with rigid foam to avoid condensation / moisture accumulation and filed it all away as "stuff to worry about if / when we make changes to the house".

Fast forward to earlier this week... I went up to my son's room and went to straighten up the sheets on his bed, only to find that there was a large amount of water underneath the side of the bed closest to the wall. The floor underneath the wet side was freezing to the touch (barely warmer than the outdoor temperature).

The room is partway over an attached garage -- based on the insulation in the rest of the garage, my guess is that the current layering as-constructed is:

1. Hardwood floor
2. OSB
3. R-19 fiberglass batts
4. Drywall
5. Garage

Now we get to the part where I think I accidentally "built" a poor wall -- since my son is only 2 and we were concerned about the possibility of him falling off a taller bed, sitting on top of that assembly I put a 12" high density foam mattress wrapped in a waterproof cover, which makes the final assembly:
1. Vapor barrier
2. 12" foam
3. Vapor barrier
4. Hardwood floor
5. OSB
6. R-19 fiberglass batts
7. Drywall
8. Garage

Cleaned up the water and moved the bed away from the wall and no more moisture is showing up on the floor. Floor over the garage is a little colder than the rest of the room, but much closer to the indoor temperature than it was before.

Does my "poor wall" hypothesis sound correct? If so, does that mean the only real option for having a bed along that wall will be to have it raised with some provision for heated air to circulate underneath?

Asked by Timothy Tucker
Posted Dec 28, 2017 3:15 PM ET


4 Answers

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You didn't tell us where you live, or what your outdoor temperature is right now. (I live in Vermont, and my thermometer read -23°F this morning.)

When outdoor temperatures get cold, pillows and mattresses can insulate wall surfaces and floor surfaces from the indoor air temperature. I have pillows in my living room that lean up against the drywall on the wall -- and when I reach behind the pillows in mid-winter, the drywall is quite cold.

Condensation happens when warm, humid air contacts a cold surface. To reduce the chance of condensation, you have two options: (a) Lower the indoor relative humidity, or (b) Raise the temperature of the condensing surfaces.

Does your house have a humidifier? If so, turn it off.

If you think the floor is the condensing surface, then raising the mattress above the floor will keep the floor warmer. If you think the wall is the condensing surface, then moving the mattress away from the wall will keep the drywall warmer.

Answered by Martin Holladay
Posted Dec 28, 2017 3:23 PM ET
Edited Dec 28, 2017 3:28 PM ET.


The floor insulation between the bedroom and the garage is part of the problem -- especially if the fiberglass batts have fallen away from the subfloor, and if there are air leaks moving through the gap between the top of the fiberglass batts and the subfloor.

For more information, see How to Insulate a Cold Floor.

Answered by Martin Holladay
Posted Dec 28, 2017 3:30 PM ET


In SE Michigan, so it's been in the 10-20F range. The floor definitely appears to be the condensing surface.

The whole house is what I would deem an "average mid-90's fiberglass job". Lots of problems that need to be addressed, but not always the time / budget / working temperatures to get to them. It gets a little overwhelming trying to prioritize all the different potential projects.

At this point, the path of least effort will probably just be to move the bed to one of the inside walls for the winter.

Answered by Timothy Tucker
Posted Dec 28, 2017 4:00 PM ET


It's the insulating effect of the mattress that is making the floor too cold. Raising it even an inch or two of inches to allow easy air flow would pretty much prevent that issue. (Say, a sheet of 1/2" plywood or MDF supported by a series of blocks of cut up 2x4s.)

Answered by Dana Dorsett
Posted Dec 29, 2017 10:42 AM ET

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