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I'm installing 10" thick R-30 fiberglass insulation in my basement ceiling in addition to 5/8" drywall in RC channels for max soundproofing on a budget. My question is, how should I handle installation in the roughly 90 sqft of area where I have insulated PEX and wiring running through the joists for my hydronic cast iron radiator system.

I'm concerned that placing the 10" insulation beneath the piping especially will put pressure on the pipes or just won't even fit. I'm also concerned that having the air just open above the piping will aid in sound travel between the floor. I'm going to assume that cutting 10" insulation into two 5" strips is near impossible.

What's the best approach here?

A.) Cram the 10" underneath

B.) Cram the 10" underneath and above

C.) Buy some thinner (~5") insulation for these sections, running one strip above one and beneath, tape all the batts together

D.) Burn my house down and collect the insurance money.

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  • We did 10" of insulation fitted between studs etc much like this. We used a wood fibre based insulation with the same thermal performance characteristics. The best bit was it does not irritate the skin so we only needed dust masks and there are knives like bread knives on steroids (about 14" long) that cut it fine.
    – Solar Mike
    Commented Nov 2 at 8:54

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I'm going to assume that cutting 10" insulation into two 5" strips is near impossible.

Nah yeah nah, mate.

Cutting even 10" insulation isn't that difficult with a basic utility knife, and you don't need to maintain any particular orientation, so you can even just peel it into layers and lay them in however is convenient.

You have two primary goals:

  • Full - fill the cavity completely, at least near the cold side of things
  • Loose - don't pack it tight or you reduce effectiveness and can cause other problems

In this case I'd simply split the batt and run about 2/3 of it over and 1/3 of it under the plane formed by the pipes and wiring. This keeps the pressure above the pipes and cables where it won't cause a problem with your ceiling. I wouldn't bother trying to fill any small voids between these layers, but you could fit strips after positioning the upper layer if you wanted to be fussy about it.

If you did want to cut a 10" batt to width, just slice, open, slice, open. It might take four or five passes, but it's easy enough.

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  • Gloves/mask/eye protection is good idea. Most batts should be easy to peel by hand. Close is usually good enough.
    – crip659
    Commented Nov 1 at 21:53
  • Very solid answer. Cut it with a fresh utility knife or peel it apart.
    – KMJ
    Commented Nov 2 at 19:19
  • @JeremyP.Beasley Glad to hear it, but you still seem to have forgotten how to resolve a question post. None of your last 15 or so questions have accepted answers. In fact, you're shooting 4 for 26 overall. Take the tour for a reminder.
    – isherwood
    Commented Nov 19 at 19:39

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