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

Cape Cod home insulation, again. Good grief!

Jeff3678 | Posted in Energy Efficiency and Durability on

I am a homeowner trying to determine the best way to improve air sealing, crawl space/attic insulation and roof venting in our circa 1966 Cape in the Philadelphia area (climate zone 4A). We have been in this house for just over one year. I wrote to GBA last fall about sealing and insulating the ceiling of our tucked-under garage. With your advice, that project worked out great – making a huge difference in the comfort of the rooms over the garage.

I have read articles in both FHB and the GBA by Joseph Listiburek, Mike Guertin and Martin Holladay about the difficulties and options for sealing and insulating Cape Cod homes, but remain unsure how to proceed in our specific situation. Since our pre-purchase home inspector advised that we would likely need a new roof within a few years, that presents an additional opportunity to consider our options. I have had two roofing contractors look at our roof and have discussed venting and insulation options with them. To my dismay, neither of them was as knowledgeable about these subjects as I hoped.

The overall house shape is three rectangular blocks in line, with the center block projecting forward about three feet to the front of the house. In the rear, all the blocks line up in a single plane. The three main roof areas are all about 10/12 pitch. There is a single raised-shed dormer across almost the entire back of the house, ending about four feet from each gable end. From the outside, there are modest sized triangular vents at each gable end plus a ridge vent only at the central block. There are no soffit vents present. At the front of the house the central block has literally no soffit present, while all the other areas have about seven inches of total soffit width, with 4.5” to 5” of flat central area.

The visible roof is asphalt shingles.  From the inside, one can see the original cedar shingle roof on 1’x 3” purlins.  No sheathing is present, but when a downstairs bathroom vent fan was installed into the sloped roof below the rear dormer, there was an unexpected layer of ¾” fiberboard found between the asphalt and the cedar.

It is apparent that the upstairs area of the house was finished at some time after the original construction. The two large second floor bedrooms have a 5’6” knee wall towards the front of the house with good access into those triangular crawl spaces. The rafters are 2”x 6”with paper-faced R-19 fiberglass bats more-or-less in place. I suspect that this was the original roof insulation before the upstairs was finished. This insulation extends into the sloped ceiling area above the knee wall – it may go all the way to the ridge. The floor joists are 2”x 10”s with a shallow layer of paper-faced fiberglass insulation inconsistently topped by a few inches of blown-in cellulose. In no way are the joist spaces filled with insulation. There are no ventilation chutes and no joist space blocks at the knee wall bottoms. The knee wall is framed with 2”x 4”s filled with un-faced fiberglass bats. One end-gable wall has paper-faced fiberglass bats between the 2”x 4” studs. The other end-gable has no insulation.

The rear of the second floor has one tiny access hatch into a cramped triangular crawl space, perhaps 30” in height. All the insulation here is in poor shape. I think that there are some remaining 6” paper faced fiberglass bats between the rafters plus some fiberglass and blown-in cellulose in the joist bays. I know that there are first floor kitchen and shower soffits without ceilings under these joists, making all of this very messy. There appear to be un-faced fiberglass bats in the 2×4 knee-wall. The large triangular crawl spaces beyond the rear dormer ends have no access, and there is likewise no access to the small attic space above the second floor central ceiling.

The roofers both suggest stripping off all the old roofing, then adding ½” plywood sheathing on top of the current purlins, followed by the usual underlayment and asphalt shingles. Both also suggested adding shingle covered intake vents near the lower roof edges plus adding ridge vents to the areas that are currently lacking. When asked, one said that he could add ventilation chutes to the lower roof areas, likely needing to cut out some purlins to get them into place. When I said that we might need ventilation chutes all the way to the ridge for them to be effective, the roofer suggested that we could compress the fiberglass bats in the sloped-ceiling areas to accommodate the chutes. I am concerned about compromising this already minimal R-19 insulation in those areas by compressing it.

I also consulted three insulation contractors last year after we had a home energy audit (including a blower door test) by our local power supplier. All three suggested adding appropriately insulated and sealed access hatches to all areas needed, plus doing the usual air sealing and blocking, removing the rafter insulation, adding foam boards over the insulated 2×4 knee walls, then topping up the floor joists with blown-in fiberglass or cellulose to achieve insulation values between R-38 and R-49. Some suggested adding ventilation baffles, but without soffit or complete ridge venting, that seemed pointless. None of them mentioned the rafter insulation over the sloped ceiling areas, and I was not aware of it at that time.

Best insulation practices have obviously changed several times since this house was constructed. We are hoping to be here for many years and would like to do our part to bring it up to date. Needless to say, the upstairs areas are hot in summer and cold in winter. We have experienced only one winter in this house (a mild one), but did not note any ice damming. We are unaware of any current roof leaks or similar issues. In the absence of convincing Mike Guertin to move to our vicinity, we are looking forward to your input. Thank you.

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. MattJF | | #1

    This is a fairly complicated picture to wrap my head around. If I were you, I would start cutting access to any inaccessible areas to access conditions and get a complete picture. A picture of the exterior would help.

    You will have to choose a vented or unvented roof. An unvented roof with exterior rigid insulation is a pretty good option if you are redoing the roofing.

    The sloped ceilings that follow the roofline are going to be the challenging part, there likely isn't room for sufficient insulation and venting.

    Do you have any ductwork in the attic/kneewalls?

    I would draw a profile of each cross section of the house and draw a couple continuous lines on each profile. Sounds like there might be 4-6 profiles? For a vented roof, draw a line for air barrier, thick line for insulation, and medium line for ventilation. For unvented, draw air barrier and insulation lines.

  2. Expert Member
    Dana Dorsett | | #2

    The 1x3s for mounting shingles is called "skip sheathing", though they are similar in function to purlins.

    It's pretty common to lay down full sheet nailer decks over skip sheathing when changing to asphalt shingles.

    With kneewalls blocking vent paths and any hips or valleys it's not really possible to properly vent the roof with insulation at the roof deck. Installing 3" of polyiso above the skip sheathing, held in place with a half-inch or 5/8" nailer deck through-screwed to the skip sheathing or rafters will work. That would allow you to install 5.5-6.5" of blown fiber or open cell foam between the rafters, fully filling the skip sheathing gaps, with a lot of dew point margin at the skip sheathing layer. A 3" layer + nailer above the roof deck would require a 1x4 facia board at the roof edge to cover the foam, or it could also a be a 4" commercial roof drip edge.

    Reclaimed roofing foam is dirt cheap, and available from multiple vendors in the Philadelphia area.

  3. Andrew_C | | #3

    This applies to many who seek advice: a picture is worth a thousand words. Maybe 500 words if you're a Hemingway, but probably at least a 1000 for most people. Simple sketches can also be illustrative, and can help other readers come up to speed on your particular situation much faster. In today's age where attention spans are short, pictures make it more likely that someone will take the time to consider your questions and requests.

    Good luck with your air sealing and insulation. If you end up living there for any length of time, you'll thank yourself for making those improvements.

  4. GBA Editor
    Martin Holladay | | #4

    Cape Cod homes are difficult to insulate well. A golden opportunity comes around every 20 years, however. This window of opportunity occurs during re-roofing.

    You wrote, "Since our pre-purchase home inspector advised that we would likely need a new roof within a few years, that presents an additional opportunity to consider our options."

    What you want to do is install an adequately thick layer of rigid foam on the exterior side of the roof sheathing. The least expensive way to do this is to buy recycled (reclaimed or used) rigid foam. You'll need to seal up all of the soffit vents (if any) and ridge vent (if any) if you go this route.

    Here are links to two articles with more information:

    "How to Install Rigid Foam On Top of Roof Sheathing"

    "Solving an Ice Dam Problem With Exterior Rigid Foam"

    The trick, of course, is to find a contractor who understands the necessary details. Good luck.

  5. Expert Member
    Dana Dorsett | | #5

    >"I also consulted three insulation contractors last year after we had a home energy audit (including a blower door test) by our local power supplier."

    The blower door test numbers (preferably in cfm/50 but ACH/50 would still be good) would be worth sharing here. C'mon now, don't be coy...

    Since you also now have a heating history on the place, you are in the position to run a fuel-use based heat load calculation to track your progress and get a handle on what the next-most-cost-effective measures would be.

    For a house built in 1966 there's probably some ~R11 batts on the floor the attic spaces just as there was in the knee wall attic floors, but there could be something else, or nothing at all. The R19s under the skip sheathing probably came later, and aren't really worth saving, since they don't fill in the gaps between the skip sheathing, and don't have a true air barrier on the interior side.

    To deal with insulating the dormer and attic spaces you'll have to cut one or more access holes in them even to see what's going on. It's unlikely that you'd be able to get R38 cellulose all the way out to the tops of the kneewalls or top plates of the exterior walls in a vented roof situation, given the 2x6 rafters. With a 1" vent channel and 2x6 rafters it won't even hit R17 (less than half of R38) over the top plates of the walls.

    Unvented you'd be able to get R23 rock wool between the rafters, a bit more if using dense packed fiberglass (which would fill in between the skip sheathing. The 3/4" fiberboard is good for about R2, and was pretty common "roof insulation" in the mid-1960s. Typical roofing polyiso is rated R5.7/inch, but derating it to R5/inch for age & temperature is prudent. So at 3" you'd be looking at R15 polyiso, R2 fiberboard, and R23 for rafter fill, which is R40 total.

    With the R17 thermal break over the rafters that surely beats R38 thinning do R16 or so on the attic floors, and comes close to IRC 2018 code minimum on a U-factor basis. If getting it to code min is a goal, go with 4" foam. The end walls of the gables and dormer would need to be insulated to get optimal thermal performance out of insulation at the roof deck.

    A foam-0ver would require some new kick-out flashing & siding work on the shed dormer, but that's not usually a big deal.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |