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Insulation installation help

sandking's picture

Hello all.  My wife and I purchased a new home and unfortunately have had to almost gut the whole thing so our budget is extremely tight.

We live in NY and I'm in the process of reinsulating the whole house.  The house is 2x4 construction for the walls and 2x6 in the attic (upstairs ceiling) and in the cathedral ceilings as well.

We can't afford spray foam so we purchased R15 for the walls and R19 for the attic. In the attic I need to put down some plywood for storage down the middle, so anywhere we don't have a floor I will add some R30 going across the R19 (won't be too much).


Here are our questions:

1. When installing the R15 in the walls, I've been told to use spray foam (Great Stuff) up and down the cavity where the 2x4 meets the sheating so it seals each cavity, as well as behind each electrical box.  Does this make sense? Is it necessary? Will it help?

2. For the cathedral ceiling we were going to put a 1.5" baffle under the roof sheathing, then R15.  We plan on installing recessed lights so i want to create a cavity for the heat to move.  I've received other responses here that say to stuff with R30 and add foam board, but I thought compressing R30 from 9.25" to 5.25" would be lossing Rvalue and usless?

3. A neighbor recently reinsulated his daughter's room with R13 and then some 3/8" foam board (the fan type) before putting up sheetrock.  Do you think this adds anything and worth the trouble?

Thanks in advance.

Look, your other post already (post #206931, reply #1 of 4)

Look, your other post already has 8 replies.  Don't start a new one, that just makes people work twice as hard if the answer to something was given in one post but not seen on the other.

YAY!  I love WYSISYG editing!  And Spellcheck!


Thanks but (post #206931, reply #2 of 4)

my other post was in the construction forum due to questions more on how to furr out the ceiling.  This post is only regarding Insulation so I put it in the insulation forum.  In addtion, questions 1 and 3 weren't asked in the furring out post.  If I did anything wrong I'll delete the post, but I thought it would be more relevant here???

1) No.  The spray foam, as (post #206931, reply #3 of 4)

1) No.  The spray foam, as described, will do very little good -- sealing between stud bays has negligible effect.  Save it for sealing around windows and other spots you can't insulate properly with your regular insulation.

2) Cathedral ceilings are always a compromise.  Compressing fiberglass insulation cuts it's insulating value, but compressing, say, 9.25" insulation down to 5.25" will usually leave you with more insulating value than insulation that is initially 5.25" (though the difference may not be enough to be worth the added trouble/expense).

3) The thin fan-fold foam board has very little insulating value -- it's not intended as insulation (even though siding companies will call it that to sell you siding).  It's really used (by the siding companies) because it's the cheapest stuff that can be applied over existing clapboard siding to flatten it out before applying new vinyl/steel siding.  Doesn't have any other common use.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

1) Maybe. A lot of air leaks (post #206931, reply #4 of 4)

1) Maybe. A lot of air leaks into walls through joints in the sheathing. You can seal as you describe with spray foam or caulk, or you can glue the drywall to the studs when you hang it. IMO reducing air leakage through the walls is well worth it.

2) Skip the can lights and use surface mounted fixtures. Put as much insulation into the cathedrals as you can, and don't punch holes in the ceiling. Put more than R19 in the attic--build up your attic floor areas on additional framing so you can use more insulation under the floor.

3) No, but you could do some good with real rigid foam insulation, like maybe 1" or 2" polyiso installed over the studs.