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How does this attic insulation plan sound?

Matthew Fleck | Posted in General Questions on

Hi all,

First, this site, and the community on it, has been very helpful to me already.  Thank you!

Here’s what I have:
My wife and I bought a 2-story 1930s brick home in the Bronx, NY a year and a half ago.  It has a large attic that is used for storage, but we want to make it livable.  As far as I can tell, the only attempts to limit energy loss in the house consist of newish vinyl windows and about 3 inches of rock wool batts between the 2×6 rafters in the attic that could be as old as the house…  We have steam heat, and radiators in the basement and 1st, 2nd floors, but not attic.

Here’s my plan (starting from the top of the house and working down):
The house had new asphalt shingles with venting at ridge and soffits installed about 8 years ago (note, however that the current rock wool batts are pressed firmly against the underside of the roof deck, so venting attempts by PO’s roofer were negated).
1.  I want to add venting using 1-inch Foamular spaced 1 inch from the underside of roof deck between the rafters.  I’ll cut the rectangles a little narrow and seal gaps with great stuff.  R5
2.  I’ll pack in 3.5 inch unfaced fiberglass batts below next so that they’ll be flush with the ends of the rafters.  R15
3.  I’ll tack 2″ polysio faced insulation boards to the underside of the rafters, with the faced side facing in towards the attic.  R13
4.  I’ll put up 1/2″ gypsum drywall fastening it through the polysio and into the rafters.
5.  Then there will be the attic space.  There is no source of heat or cooling there at present (but there are new vinyl windows).  I’m planning to install a mini-split system to condition the space.  I recognize that this may come up a little short in the colder winter days, but hopefully opening the door to the attic will let up some warm air from the lower floors too.
6.  Finally, I want to blow 8 inches of cellulose into the attic floor–I’m going to let a professional do this.  They will also fill the stairwell and any attic walls that I don’t get to.  I recognize that most encourage you to pick either the attic roof or floor, but I think it will be worth it.  I don’t anticipate making the attic a bedroom for now, and it won’t be heated by our current radiator system, so it makes sense to keep as much of the heat on the first two floors as possible, and think of the attic as a separate zone.

Here’s what I’ve got for questions:
1.  What do you think of the plan?
2.  I’ve heard that polysio should only be used on exterior.  Is my application (between undersides of rafters and drywall) ok?
3.  I eventually want to do some air sealing work on the lower floors, is there any other air sealing work I can do in the attic/attic floor before I get into this?
4.  I’ve heard that rock wool may have asbestos in it.  A contractor told me my batts likely did not, that asbestos was only in loose rock wool.  I was planning to remove the old rock wool myself with dust mask, eye and skin protection.  Sound ok?

Thank you again.  I’ve learned a lot from the articles and posts on this website already.  It is a great resource!!!

Matt

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Replies

  1. Expert Member
    Zephyr7 | | #1

    It sounds like you’re planning to insulate both the roof AND the floor of the attic. Why? You only need to do one or the other. If you insulate the floor, your attic is unconditioned space. If you insulate the roof, your attic becomes conditioned space. If you are planning on using the attic as living space at some point, insulating the roof is likely your better option. There is no need to do both.

    You’re fine using polyiso indoors, and the drywall will serve as the thermal barrier for fire safety that code requires. You just need to use at least 1/2”. I like to use 5/8” myself which is less prone to sagging and just more solid all around. 5/8” drywall is the commercial standard used in office buildings and the like. 5/8” drywall is heavier though per sheet.

    I’d use polyiso in place of the foamular. Polyiso is a bit more R per inch, the foil facing gives you a bonus radiant barrier in your roof, and it’s a greener product too. Note that polyiso typically has a facer on both sides too. I’d leave 1.5” vent gap too, 1”, while meeting code, isn’t really enough in the view of many. The BSC people like a 2” gap.

    I’d use mineral wool instead of fiberglass but that’s just because I like working with the product better. Mineral wool gets a bit more R per inch, and it’s better in terms of fire safety. It’s more expensive too.

    Bill

  2. GBA Editor
    Brian Pontolilo | | #2

    Hey Matt.

    You should be insulating either the attic floor or the roofline to R-49. So, instead of insulating both with neither achieving the appropriate R-value. Why not pick one and make a plan to get there? The attic floor is an easier and less risky approach in my opinion. Have you read these articles? They should be helpful:

    https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
    https://www.greenbuildingadvisor.com/article/creating-a-conditioned-attic
    https://www.greenbuildingadvisor.com/article/attic-insulation-upgrade-2
    https://www.greenbuildingadvisor.com/article/air-sealing-an-attic

  3. Matthew Fleck | | #3

    Thank you for the replies!

    Zephyr—thanks for the recommendation on vent gap, drywall, and using polyiso to make the venting.

    Is it ok to use faced polyiso in all of these applications?

    I can increase the vent gap to 1.5”, but that leaves less room for insulation material. Can I compress the fiberglass slightly from 3.5” of space to 3” of space to make an extra 1/2 inch of space for venting?

    Thank you Brian for your response as well. Let me share my rationale for insulating both the attic floor and ceiling. It still makes sense to me, but I’ll yield to the experts! I don’t think I can get to R49 in the ceiling—would just lose too much headroom. There is a good chance that we will only use the attic once a week or so. Our regular source of heat comes from steam radiators on the first floor, second floor and basement. There are no radiators in the attic. I would rather not use steam heat to keep the attic at a comfortable temperature all of the time. I would prefer to think of it as a separate zone that will be conditioned by a mini split that we will install in it. By insulating the attic floor as well, I am hoping to keep the heat that we generate daily in the space we spend the most time in. Whenever we plan to use the attic, we could flip on the mini split or open the attic door to let heat up. Of course, a lot of this will come down to budget. But that was my thinking for a now

    At this point, I’m debating between this plan, or just blowing cellulose into the rafters behind some polyiso on the underside of the rafters.

    Anyone else have feedback?

    Matt

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