GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A


eldarchik | Posted in General Questions on

Hey everyone, I’m new here, so please bare with me if I missed something.

After reading this forum for a little while, I’m convinced that doing things right (particularly insulating) is not a question anymore. Me and my fiance just bought our first house (Northern NJ) and need to replace the roof. We got a couple of quotes from the local guys, but most did not react too well when I asked about insulating the roof. GAF does not recommend it, or it is not necessary (one guy said R25 value in the attic is plenty for our area).

Question, does anyone know or worked with a roofer in the Northern New Jersey area that is willing to “do it right”? Or should I just demand this from any roofer and pay extra (but then I have to advise them, and I’m not anywhere near an expert)?

Also, regarding cost, how much extra would it cost to insulate on the outside of the roof?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Andrew_C | | #1

    More details would be useful.
    Is the current attic vented or unvented? Is the roof relatively simple, or are there a lot of ridges and valleys and dormers? A picture is often useful.
    What type of roof are you considering? Asphalt?
    How is the attic or roof insulated currently?
    NJ: does that mean that all of the HVAC system including ducts are inside the house and not in the attic?

    Both vented and unvented attics can be done well.

  2. STEPHEN SHEEHY | | #2

    It might be worthwhile to speak to an architect and see if you can get a detailed drawing of how to install exterior foam on a roof.
    I think some builders are reluctant to do something they've never done before, but once they see a drawing, they'll be OK with it.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Andrew -- you haven't provided us with anywhere near enough details to allow us to help you. We don't even know the area of your roof, or how many feet off the ground it is -- so there is no way that we can estimate "how much extra it would cost to insulate on the outside of the roof."

    The first questions you need to answer are these:

    1. Why do you want to install insulation on the exterior of your roof?

    2. Does your roof or ceiling have any existing insulation? If so, where is it?

  4. eldarchik | | #4

    Guys, thanks for the good start. I will try to answer all questions.


    1. I'm assuming that since we don't have rafter vents (picture attached), our 2nd floor/attic is not vented.
    2. Roof picture attached (google image, I can take more when I get home)
    3. I'm considering an asphalt roof - my roof is approximately 2,200 sf.
    4. Attic is currently not insulated, had very old (30 year batts, which we removed).
    5. No hvac system in attic. No central air (window units), heating is in the basement.


    That is exactly the vibe I'm getting. After reading posts on this forum I see that there is an opportunity to get some good insulation done if I'm redoing the roof. Since I'm in zone 5, it seems I can get the R-value closer to 60 if I can do it from the outside and inside.


    1. I'm in zone 5, since we are re-doing the roof seems like a good chance to tackle insulation there first. Read here on the forums that this can be done.

    2. No and no. Very old roof, and existing insulation that was pulled from the attic was very thin and old bats.

  5. GBA Editor
    Martin Holladay | | #5

    If you don't have any HVAC equipment in your attic, the preferred approach is to install insulation on your attic floor. That's much cheaper than installing rigid foam insulation on the exterior side of your roof sheathing.

  6. Expert Member
    Dana Dorsett | | #6

    With simple roof lines not cut up with a bunch of dormers & valleys, putting 3-4" of reclaimed (re-used) rigid polyisocyanurate above, and R20-R23 batts or open cell foam in the rafters can be cheaper than doing an unvented roof "right" all from the interior. The reclaimed rigid foam itself can be pretty cheap- much cheaper than sprayed on R49 closed cell foam, or 3-4" of closed cell foam. There are several vendors of reclaimed foam operating in your area eg:

    Commercial roofers are more familiar with foam above the roof deck than residential roofers, but it's not rocket science.

    Zip code is important. Northern NJ spans US climate zones 4A and 5A. If you're not sure, the counties in green are 5A, yellow is 4A:

    In zone 4A it only takes 3" of exterior polyiso (or closed cell foam on the underside) for dew point control on up to ~R35 of fiber insulation, to edge over the code-min R49. In zone 5A it takes 4", for up to R30 of fiber insulation. If your rafters are 2x6 the most you're going to get between the rafters would be R23 rock wool batts, but 4" of polyiso above the roof deck brings it up to the right performance range. If you have 2x8 rafters you can get by with 3" of polyiso above the roof deck and more fiber. With used polyiso, for an exterior application like this assume it's only R5 per inch, even though it's labeled (and actual) performance will be a bit higher.

    Who cares what GAF recommends? Shingle color makes a larger difference in shingle temperature than whether the roof deck is vented vs. insulated., which is the most significant variable on shingle life.

  7. Expert Member
    Dana Dorsett | | #7

    Too many interruptions while composing- didn't see responses 4 & 5 until after I hit "Post".

    To hit R49 on the attic floor requires at least 14" between the roof deck and the top plates of the exterior walls at the eaves to have the requisite 1" of vent area clearance between the insulation & roof deck, and 15" would be better. Do you have that much room to work with?

    It looks like you have 2x8 rafters(?), in which mid-density R25 fiberglass would fit just fine, and with 4" of polyiso and an OSB nailer above the structural roof deck it would meet or exceed IRC zone 5 prescriptives for above-deck R, and would meet IRC code minimum thermal performance on a U-factor basis, despite being about R1 shy of R49 at center cavity, due the exterior foam thermally breaking the rafters.

    The roof lines look simple enough to make a foam-over pretty easy, without lots of compound angles to cut, just a couple of valleys where the perpendicular gable intersects the main roof.

    There is a potential thermal bypass where the roof overlaps the porch roof, unless the roof decking continues even below the porch roof.

    In a foam-over give the chimney 2" of clearance from the foam, and be sure to properly flash it where it penetrates the new nailer deck.

  8. eldarchik | | #8


    I'm in the passaic county (07508 zip), so I'm definitely in the 5A zone. The picture of the rafter vents, is not a picture of my house, that is what I do not have at the moment, I was answering Andrew's question whether or not I have a vented roof. I do not. I think I need a vented roof as some roof decking (which is tongue and groove) is discolored (dark grey/black). I think the rafter's cavity is 6", but I have to double check later tonight.

  9. eldarchik | | #9


    What if we decide not to have an attic,the second floor has 1 room (master bedroom) and it previously had an attic, but thinking either raising the ceiling a bit, or not having the attic at all (see example picture) what would be the best approach for insulation be for that?

  10. Expert Member
    Dana Dorsett | | #10

    If that's not your house, what do your roof lines look like? One big single-plane shed roof is ideal, a simple single gable is almost as good when it comes the installed cost of rigid foam.

    A 2x6 cavity is good enough for R23 rock wool or R21 fiberglass or R20-R22 open cell foam. With 4" of reclaimed polyiso above the roof deck (R20-R24) there would be plenty of dew point margin at the roof deck for zone 5A, and it still manages to meet code on a U-factor basis even if it's "only" in the mid-40s for R-value at center cavity.

    If 4" of foam is too awkward or costly, 3" of foam is still sufficient dew point for any fluff or open cell foam in a 2x6 cavity. The magic ratio you're looking to meet or exceed is 40% of the total-R outside the roof deck. So with R15 (3" at R5/inch) outside the roof deck you're good to about R23 in the cavities before wintertime moisture accumulation risks start to become an issue. It won't quite meet code-min, but it'll be pretty good, and has fairly low moisture risk.

    When retrofitting / upgrading a home in zone 5A, the Mass Save Deep Energy Retrofit Builder Guide is a good reference, since the climate zone specific stuff would be about the same for you as in Massachusetts (all of which is in zone 5A.) Download it and save it, to refer to as-needed:

  11. GBA Editor
    Martin Holladay | | #11

    You have listed a few scenarios that would require a sloped insulated roof assembly (cathedral ceiling). If you intend to finish the top floor of your house -- to convert the attic to living space -- that is certainly one possible reason you might want an insulated sloped roof assembly.

    Here is a link to an article that explains all of the options: How to Build an Insulated Cathedral Ceiling.

  12. eldarchik | | #12


    See attached pictures to get a better understanding of the roof lines. Came late and couldn't take pics. Definitely will do it tomorrow.

    Also, my rafters are only 5.5 inches. If they were at least 8, I see how I can pack in more insulation from the inside, but they are only 5.5"


    Yes we would like to make the top floor to living space and have a "cathedral ceiling" style setup.

  13. eldarchik | | #13

    After looking up what dormers and valleys are on the roof, I definitely have them, so based on your article Martin, a cathedral ceiling with proper insulation would probably be hard to achieve. Your article advises on doing a unvented roof. I won't be making any guesses today, but tomorrow I will take detailed pictures of the outside and inside and post them here.


  14. GBA Editor
    Martin Holladay | | #14

    You wrote, "a cathedral ceiling with proper insulation would probably be hard to achieve."

    It's not really hard to achieve -- just expensive. For an unvented assembly, your two choices are:

    1. Install rigid foam above the roof sheathing, followed by another layer of roof sheathing and new roofing. This option usually includes some fluffy insulation between the rafters.

    2. Install closed-cell spray foam on the underside of the roof sheathing. This option usually includes an additional layer of some other type of insulation on the interior side of the closed-cell spray foam (and in direct contact with the closed-cell spray foam).

  15. Expert Member
    Dana Dorsett | | #15

    The roof lines appear to be mostly large rectangles, with only one intersecting gabled ridge line perpendicular to the primary ridge orientation. While the shed dormers will still need new kick out flashing, the fact that the sections of roof are almost all simple rectangles makes exterior rigid foam relatively easy. The bird's eye view of the roof is a bit fuzzy to see, but it looks like 10 planes, but only 2 valleys, one skylight to re-install?

    The opposite of that would be a hipped roof broken up by gabled dormers, where nearly every sheet of foam & nailer deck has compound angles to cut, with less than 25% of the roof area capable of being covered with full un-cut 4 x 8 sheets. (This pretty much describes my house. :-( ).

  16. eldarchik | | #16

    Here are some pictures of both sides of the roof and inside the 2nd floor where we discovered areas where moisture build up might of had happened.

  17. Expert Member
    Dana Dorsett | | #17

    Some of that may have been winter time moisture accumulation from inadequate venting, but some of it reads more like a leakage history.

    What happens on the interior where the roof transitions over the porch? Is it blocked, or is it open to the attic space?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |