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I live in a house with a finished attic. There's a substantial upper attic (big enough to walk around) and there are also knee walls between the upstairs rooms and the lower attic. There is ventilation at the bottom of the lower attic that connects to the upper attic. I'm concerned that the ventilation is cooling the upper attic, causing a cold knee wall all around the upstairs.

lower attic doorlower attic
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Our attic seems properly insulated for the most part, but one thing is missing: hardboard along the rafters to block off the insulation. This would limit the amount of cold air circulating from the outside to the lower attic and it would also support the insulation.

insulate lower attic

Bringing any kind of hardboard into the lower attic will be tricky and tedious (the door is 3.5' tall and 2.5' wide). What would be easiest and most effective to use in a space like this?

Also, our floor is tongue-and-groove that also serves as the ceiling for the downstairs. There are holes in the surface (in the grooves or at knots) so warm air can pass from the downstairs directly into the lower attic. Does it make sense to add a barrier to airflow on the floor? What would I use?

Update: the lower attic area has 46 ft of wall and the rafters are 5' long in the lower attic.

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I own a cape with two finished bedrooms upstairs. We have knee-walls like you, as well as a cap space above the rooms (your upper attic).

We had both the crawl spaces (lower attic) and cap insulated. At the time, we were told there are two types of attic insulation: hot roof and cold roof. In a hot roof, the insulation is attached against the roof between the rafters and the air in the attic is considered warm. On a cold roof, the floor of the attic is insulated, making a warm ceiling, and ventilation and circulation occur in the attic itself (air comes in through the soffit and rises to a ridge vent or other ventilation device).

Ventilation is important for your attic and roof, so you'll want to be careful what you block up.

In my house, we used blow in insulation on the floor of the cap, and we used batting and foam board in the crawl space. You'll want to setup a ripping station outside, and you'll cut the foam board into lengths that fir through your crawlspace door. You carry them in in pieces, nail them to the rafters with special nails, and then use insulating/foil tape to seal the seams.

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To make sure I understand you correctly, you have bats along the rafters of your lower attic with a foam board sealing them in? Is it a problem if the bats already have a vapor barrier? –  Michael Schmitz Feb 21 '13 at 20:10
    
That's right: in the lower attic, the vapor barrier of the bats goes against the roof, insulation facing in. Then the foam board is nailed to the rafters, sealing the insulation in. As I understand it, the insulation you have is currently installed backwards, you'd generally put the vapor barrier against the outside. –  Jason Feb 22 '13 at 4:10
    
Thanks @Jason, I think we will put foam board over the batts in the lower attic. Do you know what type of baffles you have to vent from the lower attic to the upper attic? A 10' pre-fab baffle would be perfect, but those don't seem to exist. –  Michael Schmitz Feb 28 '13 at 18:06
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The foam board is an insulation board, and is rated as such. An example is this from Home Depot: www.homedepot.com/p/t/100320335?catalogId=10053 –  Jason Mar 13 '13 at 15:33
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Sorry, my previous comment got cut short. When I say rated as such, what I mean is that it has the same fire rating as any other insulation, and is rated for crawl space use. You'd also want to use the right fasteners, like this: lowes.com/pd_285633-67865-TMF2.5PA_0__?productId=3199781 Those will secure the board without ripping right through. You can then seal the seams with foil tape. The nice thing about the foam board is that it is easy to cut. In my attic, they used foil backed board. Here's a photo: !i.imgur.com/LBMsoA1.jpg –  Jason Mar 13 '13 at 15:51
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