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rakuz66 07-22-2011 10:00 AM

Adding attic insulation
I'd like to add some insulation to my attic. I just had the house sided and re-roofed, so I now have a vented soffit and ridge vent. I know I will need to put in some rafter shutes, baffles or whatever you want to call them in between the rafters. I presently have about 7 inches of blown in insulation on my attic floor which is under the reccomended amount for S.Eastern Pa. So my question is this , Can I use unfaced batts on top of the blown in insulation running them perpendicular to the joists, or do I need to use more of the blown in insulation? Thanks.

operagost 07-22-2011 12:54 PM

You can use the batts. That's a great way of mitigating the thermal bridging caused by the joists. While you're up there, you might want to check the existing insulation for dirty spots, which would indicate air intrusion. Seal up any gaps.

AGWhitehouse 07-22-2011 01:12 PM

While your crawling around adding the chutes and batts, add a tyvek air barrier on top of everything as well. Air movement within fiberglass batts greatly reduces the effectiveness. So if you put an R-30 batt up there without an air barrier, then you only really be getting an R-20, or so gain, as the top layer of batts will be compromised do to attic air movements. So, for the $150 or so that a tyvek roll costs, you'll reclaim that lost R-10 or so.

operagost 07-22-2011 01:18 PM

Does the Tyvek block moisture? That would be bad.

Gary in WA 07-22-2011 05:02 PM

If blow-in fiberglass, yes that will reduce the R-value, by compressing the existing. If blow-in cellulose, just use more, it will also stop any convective loops in your blow-in f.g.:

Tyvek (58 perms) won't block moisture, (I've repeatedly sited that) compared to Typar (16 perms), which will. Moisture movement optimum is 18-32 perms....


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