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I am thinking about adding a little over a foot of insulation to my attics.

I understand that there shouldn't be gaps between fiberglass batts (I read that this can reduce efficiency by 25%, whatever that means).

However, my attics have some areas that look very tough to do:

1) It's very difficult to get to the far edges in my attics. Can I get away with NOT going all the way to the edges? (I understand that I shouldn't block my soffits / baffles.)

2) There are all sorts of conduits and junction boxes sticking up out of the floor. Can I just lay the insulation over it?

3) There is a 3' x 3' area that is densely packed with pipes. Can I skip this area or do I need to squeeze insulation around them?

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up vote 3 down vote accepted

Insulation is helpful even if it's not perfect. Ideally, you should cover the attic floor completely, without reducing airflow between any vents (soffit, ridge, gable, roof, whatever). Whether a specific area is important to insulate depends on your house, but as a rule, you should aim to insulate any walls/ceilings between living space and unconditioned space. You should also use some sort of vapor barrier at the edges, to prevent air and moisture from moving into your insulation.

Based on your question, I'm assuming you're looking at using rolled insulation such as fiberglass. You might also consider blown-in cellulose insulation, which is installed using a blower machine that can force loose insulation into cavities. If you buy cellulose insulation at a home center, they will often rent you the machine for free. Two reasonably handy people can do this installation -- you need one to feed the machine and one to operate the hose.

For insulating around wall/ceiling penetrations, you can also use a canned spray foam product (such as "Great Stuff" or "DapTex"). These will allow you to insulate tricky areas, and the foam will fill whatever voids you have -- perfect for pipes, around wiring, and around electrical boxes. Make sure you do not use these products in electrical boxes or around chimneys or other hot areas, as they are flammable.

I believe it's OK to place insulation on top of junction boxes. If you have recessed lighting in your ceiling, a special housing is required -- placing insulation over a regular recessed light housing can cause overheating and is a fire hazard.

One thing I'm not sure of is whether it's code-compliant to hide a junction box under insulation. Code disallows putting a junction in a hidden location, like inside a wall without an access plate. I don't think this would apply to insulation laid in an attic, but you may want to contact your town's inspection services department and ask.

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My understanding is that you cannot bury the junction boxes if they are not accessible from the other side. –  BMitch Nov 8 '11 at 19:30
    
I contacted my city's electrical inspector, and he said that it was fine to lay insulation over the boxes. He recommended that we have some way of marking the locations of the junction boxes. –  anon Nov 17 '11 at 15:30
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