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Old 11-08-2014, 08:05 PM   #1
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Adding insulation to knee wall


Fairly new homeowner and definitely a newbie for DIY home improvement.....

Currently my knee walls in my bonus room above the garage have spray foam insulation on the rafters and faced insulation in between the studs up against the backside of the drywall for the room. The fiberglass insulation is held up by a clear plastic covering (not TyVek). The knee wall spaces are noticeably colder than the interior of the room.

We are planning on using the room for our new born in a few months, but the room is much colder than the rest of the house. When the thermostat downstairs is at 69, the bonus room only gets up to 64ish. I'm wondering if adding some kind of foam board in the knee wall space up against that plastic covering will give me any improvement. I also don't know if that will create a double vapor barrier and whether or not that would be problematic.

There is also about a 3/4 inch gap between the bottom of the knee wall doors and the carpet. I've thought about getting some door sweeps to add to the bottom to prevent any warm air escaping the interior room. Would that cause any ventilation problems?

Any help would be appreciated! Thanks.
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Old 11-08-2014, 08:37 PM   #2
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In the common living areas one will notice that heating ducts are located on exterior walls. The reason is this : The heat heats up the cold exterior walls. Then after they heat up the heat radiates towards the interior. In your bonus room it sounds like you don't have the heating ducts located on those exterior walls
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Old 11-08-2014, 08:46 PM   #3
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Quote: "Currently my knee walls in my bonus room above the garage have spray foam insulation on the rafters and faced insulation in between the studs up against the backside of the drywall for the room"

Do you know the R value of the spray foam insulation on the rafters ? Insulation of any type on ceiling should be a minimum of R31
And insulation in walls should a min value of R19
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Old 11-08-2014, 09:01 PM   #4
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The only air duct that goes to the living space is above the ceiling (in the attic).

I don't know what the R value is of the spray foam. I did find a couple of batting rolls that had the facing turned away from the drywall for some reason, but I don't think those alone are causing the majority of the problem. However, because these batts are turned around I can see that they are R-13.
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Old 11-08-2014, 09:20 PM   #5
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R-13 is to low. R-19 for the walls is the minimum. R-19 fits "comfortably" in between 3.5" studs. R-13 sits "comfortably" in say 2" studs. Herein, "comfortably" means that the insulation works the best when it is NOT compressed

Last edited by curtd; 11-08-2014 at 09:27 PM.
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Old 11-08-2014, 10:06 PM   #6
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So back to my original questions. Knowing that my R value is too low I could add foam board to cover up the batting and clear plastic sheet, but then am I creating a double vapor barrier and is that really as problematic as some people make it out to be?
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Old 11-08-2014, 10:28 PM   #7
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Double vapor barrier's means you will trapping moisture...have at least one side to breathe....
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Old 11-09-2014, 11:27 AM   #8
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Not supremely problematic if the wall is airtight but if you aren't sure, don't chance it.

You can cut the plastic out.
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