GBA Logo horizontal- lakesideca.info Facebook- lakesideca.info LinkedIn- lakesideca.info Email- lakesideca.info Pinterest- lakesideca.info Twitter- lakesideca.info Instagram- lakesideca.info YouTube Icon- lakesideca.info Navigation Search Icon- lakesideca.info Main Search Icon- lakesideca.info Video Play Icon- lakesideca.info Audio Play Icon- lakesideca.info Headphones Icon- lakesideca.info Plus Icon- lakesideca.info Minus Icon- lakesideca.info Check Icon- lakesideca.info Print Icon- lakesideca.info Picture icon- lakesideca.info Single Arrow Icon- lakesideca.info Double Arrow Icon- lakesideca.info Hamburger Icon- lakesideca.info TV Icon- lakesideca.info Close Icon- lakesideca.info Sorted- lakesideca.info Hamburger/Search Icon- lakesideca.info
Musings of an Energy Nerd

Insulating a Cape Cod House

Capes are notoriously difficult to air seal and insulate

This photo of a Cape was taken in Levittown, New York, in 1947. When these homes were first offered for sale, they cost between $7,000 and $9,000 each.
Image Credit: Image #1: University of Illinois at Chicago library

If you own an older Cape Cod home, you have my sympathy. If you’re the type of homeowner who regularly tackles DIY projects, you’ve probably spent weeks chasing air leaks with a foam gun, lying on your back in a cramped attic. And there’s a good chance that, in spite of your efforts, your house still suffers from ice dams.

I’m sorry for your troubles. You deserve better.

If you are thinking of building a new Cape, it’s not too late to get the details right — as long as you’re still at the planning stage.

A Cape is a story-and-a-half design. Most Capes have second-floor bedrooms featuring 4-foot kneewalls and two sections of sloped ceiling. The sloped ceiling sections usually rise to meet a narrow horizontal ceiling in the center of the house. Most Capes have triangular attics behind the second-floor kneewalls and a tiny third-floor attic that is too cramped to stand up in.

Cape-style homes are associated with a famous peninsula in Massachusetts as well as a well-known 17,000-home residential development built in the late 1940s in Levittown, New York. Most of the original Levittown Capes were sold with an unfinished second floor; it was up to the homeowners to figure out how to install insulation up there. You can imagine the results.

What’s wrong with a Cape?

The basic problem with a Cape is that most examples have a poorly defined thermal boundary.

Should the triangular attics behind the kneewalls be considered indoor space or outdoor space? From a building science perspective, the answer is clear — these attics should be inside the home’s thermal boundary. However, most builders don’t have the foggiest idea where the thermal boundary in a Cape belongs. That’s…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

13 Comments

  1. Brian Wolfgang | | #1

    Cape Cod with SIP roof
    I don't have a ton of experience with this style of home, but I was wondering if the use of SIP's at the roof in new construction would have any merit. You would still need to provide structural support, but it would solve some of the difficult air sealing challenges. Any thoughts?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Brian Wolfgang
    Brian,
    I addressed your question in my article, which notes, "There are many ways to build an insulated sloped roof assembly. This type of roof can be vented or unvented. It can be insulated with fiberglass, cellulose, mineral wool, spray foam, rigid foam, SIPs, or nailbase."

    Depending on the span, a SIP roof many not need any structural support. Roof SIPs are engineered and manufactured to handle a variety of spans. Some SIPs include integral structural members. If you are uncertain about the structural needs for a planned SIP roof, talk to your SIP manufacturer.

    Of course, SIP roofs still have air sealing challenges. In a cold climate, it's a good idea (a) to seal SIP seams with interior tape as well as spray foam, and (b) to include ventilation channels between the exterior OSB face and the roofing. For more information on sealing SIP seams, see .

  3. User avater
    Jim Baerg | | #3

    Been there, done that.
    As one who has pushed through the difficulties of insulating the attic of my own story-and-a-half home, I very much appreciate the clarity of this article. I was fortunate having a house that was 26 feet wide. That gave me pretty good head room in the kneewall and upper attics. Many of the post war Cape Cod houses are narrower and are much tougher to work in.
    In my case, I thought that insulating the knee wall attic floor (R-60) and knee wall itself (2" of taped ISO) was preferable because it allowed for higher levels of insulation than just filling the rafter cavity. The most difficult area to insulate was the sloped rafters between the two attics. I was able to slip cardboard baffles into those cavities, leave a 1" air space up against the roof sheathing, and still get 5" of cellulose blown in when the upper attic was insulated. Later I checked those sloped roofs with an IR camera and found some of the cavities with insulation voids. We drilled 2" holes through the plaster and dense packed. I'm pretty satisfied with the result and think that I've got a continuous, fairly air tight, insulation package.
    On a more technical note, I notice that the drawings in the article show new construction, rather than what typically exists in older houses. Still very useful, but I would caution readers to think through each building junction as it really exists when they plan their insulation and air sealing project. I've always found that drawing up detailed sections of existing conditions is very helpful before work begins.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Jim Baerg
    Jim,
    Thanks for sharing your experience.

    I agree completely with your point that older homes are rarely framed the way that new homes are framed. Almost everyone who owns an older home learns this lesson quickly. I've seen log joists, rough-sawn joists, and planed joists. I've seen 16-inch-on-center framing, 24-inch-on-center framing, and lots of other types of framing -- like 26 inches on center, or 30 inches on center, or absolutely random spacing.

    I've seen balloon framing and platform framing. I've seen too much blocking and not enough blocking. I've seen double top plates, single top plates, and no top plates. I've seen oversized window headers and no window headers.

    So if you've got an older wood-framed home, you can't assume that the pretty pictures in Fine Homebuilding or GBA represent your house. They probably don't.

  5. Ed McGuire | | #5

    proper roof ventilation
    I would like some feedback, please.

    I live in a story and a half cape. The main roof to the side of the dormer was looking lumpy, so I set up the roof jacks and opened things up. The 1/2" sheathing had delaminated and fiberglass insulation was against the underside of the sheathing. My solution was to rip and install 1 x 2's to the sides of the rafters flush with the top of the rafter. I had some 1" polyiso scraps waiting for place to spend eternity, so I loose fit them under the 1x2's and foamed the gaps. Installing the polyiso this way did compress the fiberglass about 2", but I created an 1 1/2" airspace below new sheathing. Is there a better way?

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Ed McGuire
    Ed,
    While the preferred solution -- adding rigid foam to the exterior side of the roof sheathing -- would have provided a higher R-value for your roof assembly, your approach is a good lower-cost Plan B.

    Of course, your approach only makes sense if you have soffit vents at the bottom and a ridge vent on top.

  7. User avater
    Dana Dorsett | | #7

    Potential trouble ahead.
    How much fiberglass is between the 1" polyiso and the interior ceiling?

    The roof deck will be fine now that it's vented, but unless the ratio of the polyiso R to total R is sufficient for dew point control in your climate zone, you may have a moldy moisture trap on your hands.

    In zones 1-3 you would be fine with 1" polyiso.

    In zone 4C it will be OK as long as there isn't more than R20 of fiber (2x6 rafters with compressed batts are fine, 2x8s maybe not, depends on the final thickness of the fiber.)

    In zones 4A & 4B it'll be fine as long as there isn't more than R16 in the fiber layer, which is fine if it's R19s in 2x6 raftes, could be a problem with high density batts or deeper rafters.

    For zone 5 & colder you have to derate the 1" polyiso to ~R5 for wintertime performance. For dew point control that R5 needs to be at least 40% of the total R in zone 5, 50% zone 6, and 60% in zone 7.

    So, in zone 5 it's good for at most R7.5 of fiber, which could be a problem even with compressed R11s in 2x4 framing, since an R11 compressed to 2.5" is about R9- you compressed it a bit more, so it's probably OK with 1.5-2" of fiber, but not more. With any 2x6 or deeper rafters it's definitely a problem.

    And in zones 6 & higher it's a problem.

    If the interior has an air tight polyethylene vapor barrier it might do OK even without sufficient foam-R for dew point control, as long as the rafter bays were pretty dry when you sealed it all in. If there are air leaks to the interior side or no vapor retarders other than latex, the risk in zone 6 or higher (or zone 5 with 2x6 or deeper rafters) is pretty real.

    If there's any salvation at the margins here, it appears to be a south facing pitch means that the average winter temps at the roof deck are a bit higher than on the north side, which means you can cheat the minimums a bit. If it's not even close, there's a good chance you'll have wet insulation at the end of the winter in any rafter bays that leak air or have more than 1-perm of vapor retardency on the interior.

  8. User avater GBA Editor
    Martin Holladay | | #8

    I disagree with Dana Dorsett
    Ed,
    I wouldn't panic yet. I disagree with Dana's analysis.

    Dana is right that if you are building an unvented roof assembly, you need to pay attention to the ratio between the rigid foam layer and the layer of fluffy insulation (in this case, fiberglass).

    However, yours is a vented assembly (assuming that you have soffit vents and a ridge vent), so there will be at least some airflow through the ventilation space. This airflow will keep your roof sheathing safe.

    Concerning the question of whether site-built ventilation baffles need to be made from a vapor-permeable material, I answered that question in my article called Site-Built Ventilation Baffles for Roofs.

    The paragraphs below are quotes from my article.

    * * * *

    One of the reasons that builders install ventilation channels is to help damp roof sheathing dry out. Researchers now realize that ventilation channels can help a little bit at this task, but not as much as some people think. (In fact, during the summer, ventilation channels can actually add moisture to roof sheathing rather than remove it; for an example of this phenomenon, see Image #4, below.)

    The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusion. Because of this fact, installing ventilation baffles that are airtight makes this type of roof assembly more, not less, robust.

    What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?

    The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.

    That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso— to make ventilation baffles. The main reasons:
    Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
    The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
    Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting.

  9. User avater
    Dana Dorsett | | #9

    The problem isn't at the roof sheathing.
    As I stated: "The roof deck will be fine now that it's vented..."

    The problem is that the first condensing surface (the interior side of the foam) is colder than the wintertime interior air's dew point. As long as it's air tight to the interior and low-perm, it's not a problem. But with air leaks it's a potential problem. With high vapor permeance at the interior it's a potential problem, but mitigated by solar gains on the south facing pitches.

    Counting on sorptive transfer of moisture around the vapor barrier via the rafters into the vent channel might be OK if it's just a thin sheet, with less than 200 mils of rafter to traverse, but with 1" polyiso there is 1" of wood that it has to get through. An inch of wood will run 1 perm or less unless it has a high moisture content (high enough to support mold), a class-II vapor retarder, which isn't providing a high drying rate for the cavity.

    This isn't time to panic, but it's not best practice by any means.

  10. Charlie Sullivan | | #10

    A rare disagreement
    If Martin and Dana disagree, it must be an interesting and challenging question! So I can't resist adding my $0.02.

    It seems to me that the opportunities for moisture to escape when an impermeable baffle is used are much greater with an unconditioned attic than with a cathedral ceiling. With the unconditioned attic, the insulation in question fills a triangular area, and moisture can escape by traveling laterally through the insulation to the vented attic space, even if it can't escape through the baffle, and even if the drying through the rafters is minimal. I wonder how much of the field experience Martin's article refers to is with impermeable baffles installed in unconditioned attics rather than cathedral ceilings.

    Several other factors that may help explain why there aren't more reports of problems are 1) Many baffles are installed with without the edges air sealed, 2) Many are installed without any real protection against wind washing coming from the eaves, so there is outside air flowing across the bottom surface as well as the top, and 3) Unlike OSB, the vapor impermeable materials themselves are pretty tolerant of moisture, so if there is some condensation on that surface, it might never got noticed and might never cause a real problem.

    In any case, this case seems like reasonably low risk--with some foam, the condensation potential will be reduced relative to a simple uninsulated impermeable plastic baffle, and everyone agrees that the roof deck will be fine.

    More generally, it might be that the supposedly best commercial baffle product, DCI Smart Baffle, is also the most dangerous. It has 2" plastic flaps on each side, to provide spacing from the roof deck. Thus, the vapor path through the wood goes up from the 1/5 inch that Dana think might work to two inches, but without any of the benefit of the R-value of foam. Perhaps "Smart Baffle" refers to the fact that smart people are baffled by the 3D hygrothermal analysis that would tell us when, whether, and how it actually works well. I'm hoping that it works OK in an unconditioned vented attic, because I am in the process of installing a similar baffle configuration in my attic.

  11. William Sherman | | #11

    2x6 rafters, a roof leak, PV, and a raccoon
    The roof of my 40's Cape in central Rhode Island has sprung a leak -- under PV panels, of course. The asphalt shingles seem to be at the end of their life too. One end of the 2nd floor is a bedroom. The leak has already badly effected the original plastered and calcimine-coated rock lathe. The other end is unfinished attic space which had 1" of fiberglass insulation face-stapled to the rafters (seriously!) which was used for storage. Yes, it had 1" of fiberglass until a raccoon turned the attic into a B&B/rave last winter. The shredded insulation is all over the place. I can now see light coming in through the ridge vent when I look up at the attic ceiling. That attic wasn't conditioned at all, but the minimal insulation did help a little in moderating temperatures somewhat. And the air leakage from the living space below ... well, you know! ;-)

    I'm at a crossroads. Now is the best time for me to take action. Either I find somebody who can design and install R-49 rigid foam above the rafters, and some sort of roofing on top of that which will "solve all of my problems". Or I find a decent roofer who can just fix the leak and replace the existing shingles, then I can do the best air sealing and insulating that I can given the confines/restrictions of 2x6 20" o.c. rafter construction with no soffit vents. There's a gable vent for each "behind the kneewall" area, and a ridge vent. It seems impossible to do anything about the 5 1/2" space between the sloped part of the bedroom ceiling and the underside of the roof deck (currently T&G board sheathing). Does code allow for "sorry but this can't be brought up to R-49 without complete demolition" in cases like this?

    How would I go about finding a builder/roofer who "does" rigid foam insulation on top of rafters? I'm sure they exist. Surely there's a better way than calling up every builder/roofer within 50 miles?

    Thanks,
    Bill
    ... --- ...

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to William Sherman
    William,
    Q. "Does code allow for 'sorry but this can't be brought up to R-49 without complete demolition' in cases like this?"

    A. Yes. In most municipalities, homeowners who engage in a renovation project are not required to meet all modern building codes. That said, it's always best to check with your local code inspector to learn about local ordinances. One other point: Just because it is legal to ignore insulation requirements doesn't mean it's a good idea.

    Q. "How would I go about finding a builder/roofer who does rigid foam insulation on top of rafters? I'm sure they exist. Surely there's a better way than calling up every builder/roofer within 50 miles?"

    A. You might want to call up a local energy rater -- one who is certified by RESNET or BPI -- and ask the rater if they know any local contractors who understand building science issues and green building methods. Lists of certified raters can be found by visiting the RESNET and BPI websites.

    You might want to follow the approach used by Pauline Guntlow, the author of a relevant GBA blog called "Solving an Ice Dam Problem With Exterior Rigid Foam." Guntlow wrote, "I called a number of general contractors to hear about their experiences using continuous rigid foam insulation above the roof deck on a sloped roof. (Commercial roofers have successfully used this approach for years on low-slope roofs.) Of the people I called, only two — a G.C. from Long Island and a building inspector from Massachusetts — had used this method."

    Good luck.

  13. Rick Evans | | #13

    Raccoon Rave
    Bill, I know your situation is unfortunate but I enjoyed the description of your roof issue. Too Funny!

    Good luck on finding a solution

Log in or become a member to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |