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Community and Q&A

Rafter insulation – design

jpcjguy | Posted in General Questions on

Hi all,

So am revisiting insulating my attic rafters. I am in Richmond Va and have a 2 year old roof. I debated spray foam (closed) but it is very expensive. So I am looking at doing a 1 inch air gap soffitt to ridge and using Roxul R30.
I will have to fur out my 2x8s for this to work.
My plan is as follows:
R-30 Roxul – allowed by my building inspector – 7.25″ thick
Ripping a 2×4 in half – gives me 1.75″ of depth for furring out the 2×8
To ensure the correct depth, rip some 2x4s to 1″ high and 1/2″ wide and nail them to the rafters along roof line.

as for what the most cost friendly, install friendly and effective “barrier”, I was thinking of using this:

Then the Roxul would go against this and end up the proper depth.
Otherwise I am open to other suggestions to keep “air chase” sealed and in place that is cost effective and not a pain to install.


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  1. jpcjguy | | #1

    I have not ruled out doing flash and batt - 2" closed cell - for about R12 and then 5.5" Roxul - for about R23 - gets me to about R-35. Need to get some estimates and compare the two. Also need to look at my time value. 54 total rafter bays at 17 feet each - that is a lot of material either way!
    Part of me thinks the venting solution - while labor intensive completely removes the possible condensation risk on the inside foam side - but I think that is minimal because VA is in zone 4 and we don't have sub zero temperatures here.

  2. jpcjguy | | #2

    So after more searching I did find this article by Martin:

    Great article and I want to make sure that I am not intending on using the product for any R value - just a near airtight seal between my air gap and the roxul insulation. I figure it is already cut to 16" width with staple tabs - I can then go back and tape seal the edges where they butt up together and along the staple edge along the rafter - or find a thin spray foam to help seal it. (like a great stuff)

  3. Expert Member


    A couple of thoughts;

    - I'm not sure either product is rigid enough to ensure the 1" air gap is maintained. Typically rigid foam, plywood or even house-wrap is used as baffles. Somewhere in one of his blogs Martin lists the usual options.

    There is some debate as to whether high-perm or variable perm materials make more sense as air baffles, especially when they are not just being used near the soffits, but over the full length of the rafters.

  4. jpcjguy | | #4


    Thanks for the reply. I understand what you are saying and agree that it is not rigid. My thought is if I fur out the rafter, it will give me a true 9 inches. If I use the rockwool, it is only 7.25". As long as I don't push it in deep, the foil should not collapse. While using a thin plywood would be ideal, I was looking at the 100 feet of already set up 16" wide with staple tab for $42 as a pretty good deal from a standpoint of not having to rip plywood in to 16" sections and fit it it and mount it.
    Again - the foil is strictly an air barrier.

  5. Expert Member


    You are probably right. The Roxul is a lot more even than fiberglass batts and will provide a stiffening surface for the foil. I'm not that comfortable with impermeable air-baffles, but that's based on nothing concrete. I've never heard of them being a problem.

  6. Expert Member
    Michael Maines | | #6

    I am leery about using impermeable vent baffles, because they are the condensing surface in the assembly. There are many products you could use to make your own. Another poster recently that they plan to use a vapor-open, airtight WRB in this application.

    On the other hand, Martin's article here says that with the small amount of moisture that makes its way to a baffle, the moisture can work its way through the rafter, so impermeable baffles are OK.

  7. jpcjguy | | #7

    Good point Michael - Martin does make a case against impermeable baffle, but a good air barrier at the ceiling would help. I am thinking that is a bigger risk in much colder climates also - not so much an issue in Virginia.

  8. jpcjguy | | #8

    Anyone else have any thoughts on this? Using the foil barrier vs. a thin foam board or plywood?

  9. gotakeawalk | | #9

    Joe, We are in similar situations and are rather close by. I am near in Charlottesville. I decided to make my own baffles and but use a vapor permeable material. I have a few rolls of Mento 1000 roof underlayment on the way that we will be installing in the next few days. Like you we will be using Rockwool batts. Feel free to drop by and take a peak...

  10. user-723121 | | #10

    1/2" fiberboard is inexpensive and has a high perm rating, it makes a good airspace baffle.

  11. jpcjguy | | #11

    Ryan, That is interesting. How are you attaching it to the rafters? Do you have any pics you can share?

  12. jpcjguy | | #12

    So is using a material like Mento 1000 roof underlayment an acceptable air barrier? What other options would be good?

  13. GBA Editor
    Martin Holladay | | #13

    Q. "Is using a material like Mento 1000 roof underlayment an acceptable air barrier? What other options would be good?"

    A. Many of the comments posted here have already answered the question. In general, stiffer materials are preferred, but with conscientious installation (aiming for airtightness) and the use of mineral wool batts, you can probably get away with using a membrane product like Mento 1000. All of the usual options for site-built ventilation baffles, as Michael Maines explained to you a couple of weeks ago in Comment #6, are listed in my article. Again, here is the link: Site-Built Ventilation Baffles for Roofs.

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