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Old 11-23-2010, 11:36 PM   #1
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Insulating Attic Floor - Which?


So I'm going to tackle insulating my attic floor after I'm done with some electrical updating and was curious to know which I should be using?

It had some areas that have insulation in thick cardboard looking paper, and when I look inside it has some type of cellulose. However, I gutted a bedroom and removed the insulation there, so some areas are bare and just lathe and plaster coming through.

I was told at Home Depot that the R-30 unfaced is what I want. Do I need a vapor barrier of some sort if I am putting it between the joists? I also want to put plywood over it in some areas for storage. Wanted to also mention that the attic ceiling does not have insulation and I will not be insulating it since the attic isn't really used.

Always thought unfaced was good if you were going over old stuff, but if you were touching down onto the lathe or 2nd floor ceiling below that it needed a vapor barrier.

I have access to a bunch of R-30 unfaced rolls so I'm hoping I can use that. I'm guessing I need a vapor barrier of some sort so do they sell a paper or plastic that I could use since I already have the unfaced insulation?

Can someone help me understand if unfaced is ok if I plan to remove all the current insulation and start over up there? And if I need a vapor barrier, what could I use with unfaced rolls?


Last edited by HandyFrank; 11-23-2010 at 11:40 PM.
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Old 11-24-2010, 01:12 AM   #2
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Insulating Attic Floor - Which?


First, if at all possible, blow in cellulose and leave the FG for someone else. It is not very good insulation, despite it being ubiquitous. If you have to use batts, try to get cellulose, cotton (both treated w/ borates, not phosphates) or mineral wool. Install it as thick as your area requires, plus 4", IMO. Codes are minimums, remember. It is almost impossible to retrofit a vapor barrier, and unless you are in a very cold spot, you don't need one; check w/ local officials. Do the absolute best you can to AIR seal, anywhere there is a penetration into the attic. If you need boards to walk or store on, I'd raise them up and insulate underneath.

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Old 11-24-2010, 10:38 AM   #3
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Insulating Attic Floor - Which?


Thanks for the reply.

I was trying to avoid blowing the insulation but am not completely against it. I just had these unfaced rolls already so I was going to use them instead.

My worry is that people seem to say that you should get faced insulation and put the faced side downwards towards the house for moisture issues.

When you say seal the attic, can you elaborate a bit? I saw a link to an article with pictures that talked about this but I always thought that you were supposed to leave some venting around the perimeter of the attic so it could breathe and such?
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Old 11-24-2010, 01:27 PM   #4
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Insulating Attic Floor - Which?


I just had these unfaced rolls already so I was going to use them instead. FG is not good, esp is lying open like that. Being handy and "free", the temptation to use them is strong, and I see that. If you use them, blowing cellulose over them, even a few inches, will fill in the unavoidable gaps and "seal" them; they are notorious for air blowing through them.

My worry is that people seem to say that you should get faced insulation... That is IF you need a vapor barrier, which they won't be great at anyway because of edge leaks, IMO. See buildingscience.com, BSC Information Sheet 310, and BSD 106. Your best and first priority is an AIR barrier, which is really hard to retrofit, as is a vapor barrier. Air leakage is how most moisture gets into insulation, not diffusion.

When you say seal the attic, can you elaborate a bit? You ventilate at the perimeter (soffits) and gables, to let any moisture that sneaks out of the house, out of the attic area. You seal as tightly as possible to keep damp air, inside the house, from getting into the attic . Any penetrations into the attic (elect boxes, exhaust fans, wires, etc) should be impeccably air sealed.
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