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Old 07-30-2020, 09:32 AM   #1
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Insulating knee wall


Im reworking a 50+ Yo attic space. Kneewalls were 4" added 2x2 i have 6 "walls installed 6" insulation.On the exterior attic side Installed the old paneling to the walls I stapled a sheet of plastic to wall continued across top of 8" rafters with blown in insulation.On top of the plastic i ran 2" foam board. Im wanting to install Radiant barrier over the plastic . Dose this make any sense. Do I need a air gap on the barrier.
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Old 07-30-2020, 09:38 AM   #2
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Re: Insulating knee wall


"I stapled a sheet of plastic to wall continued across top of 8" rafters with blown in insulation."

Not sure I have this right. It sounds like plastic to the bottom of the rafters with blown-in insulation filling the space above? Is there any provision for air flow for ventilation?

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Old 07-30-2020, 09:57 AM   #3
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Re: Insulating knee wall


No The Rafters are open. I ment the 8" floor joists are full of blown in. Plastic on top of joists then rigid foam. There are sofit vents and ridge vent. Im wanting to install radiant barrier over the knee wall . there is 1/2" drywall on 2" x 6"s - 6"insulation paneling on back side then plastic. I want to add the barrier over the plastic. Do I need to add a air space .
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Old 07-30-2020, 10:09 AM   #4
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Re: Insulating knee wall


You haven't mentioned your climate, heating, cooling, or both?
With a few exceptions the plastic is not needed. Wherever you have installed the rigid foam board that is functioning as a vapor retarder as it will allow a very small amount of drying through it.

With kneewalls it is important to block the joist cavity directly below to prevent air circulation under that floor.

As for the radiant barrier in the attic side of the kneewall should be fine, but not certain where you are mentioning an air gap.

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Old 07-30-2020, 10:22 AM   #5
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Re: Insulating knee wall


Im refereeing to an air gap between the radiant barrier and knee wall. I read that this barrier needs a 1/2" gap to work properly.
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Old 07-30-2020, 10:51 AM   #6
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Re: Insulating knee wall


A radiant barrier works in both direction, reflectivity and emissivity. If your RB is foil faced on both sides and you want the benefit of both then yes you would strap the kneewall to provide an air gap behind the RB. But the benefits are minimal as the exposed surface will be doing most of the radiant work. I'll try to put some numbers to that.

Assuming a hot attic and cool room on the other side the exposed RB will be rejecting 98% of the radiant energy leaving 2% to heat up the interior RB. But the interior RB will also work in your favor as it will only radiate 2% of the heat getting through. Reflectivity and emissivity are always complementary adding up to 100%. Net benefit would be a second 98% reduction of heat transfer, but there is only 2% left. That would mean the heat coming through would be down to 2% of the 2% that came through the first RB.

Clear as mud, right. Bottom line in dollars would be pennies and difficult to justify any cost and effort involved. Putting that cost and effort into air sealing would yield a much greater benefit.

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Old 07-30-2020, 11:42 AM   #7
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Re: Insulating knee wall


The cealing Joists are filled Completely between 1st floor and attic space with blown in insulation.I put plastic on room exterior wall and continued across top of rafter to stop airflow.installed 2' FOAM ON TOP wouldn't this be enough to do this. The House is in Illinois. The attic temp gets to 120+. Im trying to get a barrier on the knee wall to stop radiant heat migrating into a HVAC room.
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Old 07-30-2020, 12:09 PM   #8
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Re: Insulating knee wall


"The cealing Joists are filled Completely between 1st floor and attic space with blown in insulation."

If I'm reading that right, there is no air gap above that insulation and no air flow?
You mentioned soffit vents, where doe that air go?
There should be an air path from soffit running below the bottom of the roof all the way to a ridge vent or other high vents.

This is what I was asking before #3 so probably a terminology issue.

Here are some pictures
https://www.finehomebuilding.com/201...ttic-kneewalls

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Old 07-30-2020, 01:02 PM   #9
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Re: Insulating knee wall


I said Celing Joists are completely filled under attic floor. Rafters Are 6" 4" of insulation air baffles between insulation and sheeting. ridge vent 30' on top. in the unheated side rafters are open.soffit vents are installed.
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Old 07-30-2020, 01:14 PM   #10
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Re: Insulating knee wall


Slap, slap, now I'm awake .

Ok, any more questions. I like RBs just be sure to limit air leakage. Often kneewalls are used to pass plumbing vents through to the roof above and not sealed well. I've seen some whoppers while doing audits.

With 6" of insulation and the RB you should be good.

You mentioned 125 up there and depending upon where you are that isn't really bad. Passive ventilation can never make the inside equal to outside as it is the difference in temperature that moves the air.

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Bud
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