Insulating a Cape Cod House

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Insulating a Cape Cod House

Capes are notoriously difficult to air seal and insulate

Posted on Dec 11 2013 by Martin Holladay

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 why builders and homeowners are always scratching their heads when they contemplate where to put the insulation in a Cape.

[Photo credit: Black Dog Builders, Salem, N.H.]

If you squeeze behind the kneewall in a Cape, you never know what you’ll see. You might see a space with no insulation at all — or perhaps just some thin insulation between the kneewall studs.

[Photo credit: Curtis Dean]

You might see a space with insulation on the floor, but not along the roof slope.

You might see a space with insulation everywhere — on the floor, between the studs of the kneewall, and following the roof slope — with all of the insulation in terrible shape.

[Photo credit: Patrick Dunnigan]

You will almost never see any attempts at air sealing work.

[Photo credit: A.M. Kuchling]

Performance problems

Most Capes are lightly framed. It’s not unusual to see 2x6 (or even 2x4) rafters. Light framing only complicates insulation decisions, since there is rarely enough room between the rafters to install code-minimum levels of insulation.

Because it’s hard to insulate the area behind a kneewall, almost all Capes leak a lot of heat at the eaves — a recipe for ice dams. It’s no surprise that owners of Capes in snowy climates usually own a roof rake, a ladder, and a collection of hatchets.

You have two basic choices

Some builders assume that the triangular attics behind kneewalls shouldn’t be brought into a home’s conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. . “Why heat those areas if you don’t need to?” they ask. But if you try to make your thermal envelope follow the kneewalls, you end up with a complicated shape — one that is extremely hard to air seal.

If you want the triangular areas to be outdoors rather than indoors, you will need to install blocking between the floor joists under the kneewall bottom plate, to prevent the movement of air through the joist bays. (Without the blocking, conditioned indoor air can leak into the unconditioned attic, or outdoor air can leak into the home through the joist bays.) Each piece of blocking needs to be sealed at the perimeter with caulk, spray foam, or tape.

You will also need blocking between the rafters above the kneewall top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate.. This blocking needs to extend up to the ventilation baffles between the rafters. (Someone remembered to install ventilation baffles, right?) Of course, each piece of blocking also needs to be carefully air sealed at the perimeter.

Do most builders remember to install all that blocking? No.

Is the blocking easy to install? No.

Should you try to do it that way? No.

The right way to do it

The triangular attic behind a kneewall needs to be within the home’s conditioned space. That means that the insulation needs to follow the roof line.

Unfortunately, it’s really hard to retrofit insulation along the roof line of an older Cape. To do this work, you need to have access three areas: (1) the cramped attics behind the kneewalls, (2) the rafter bays behind the sloped ceilings that are probably finished with plaster or drywall, and (3) the tiny third-floor attic. Working in all of these areas is difficult. Moreover, installing enough insulation to meet minimum code requirements will lower the ceiling so much that most homeowners complain.

That doesn’t mean that owners of Capes haven’t tried to insulate their roofs from the interior. Attempts to accomplish this feat are made every week of the year. The usual result is thin insulation, performance compromises, and frustration.

Is there any good solution to a poorly performing Cape? There is. The necessary roof insulation has to be installed above the roof sheathing. To learn about this type of fix, see How to Install Rigid Foam On Top of Roof Sheathing.

But… but… but…

Although installing rigid foam insulation above the roof sheathing is the best approach, most Cape owners don’t want to do this work, either because (a) the work is expensive, or (b) they recently installed new roofing without considering whether or not they should have installed rigid foam at the same time. No one wants to demo new roofing.

There are other possible objections to installing foam above the sheathing. For example, some homeowners worry that their builders won’t come up with soffit and rake details that look attractive. Moreover, the work gets quite expensive if the house has skylights, dormers, or complications to the roof line. In short, there are valid reasons for rejecting the “foam above the sheathing” solution.

In cases like those, homeowners may just have to accept that life is imperfect.

Building a new Cape

If you are designing or building a new Cape, you shouldn’t have to worry about the air-sealing and insulation problems discussed in this article — as long as you plan from Day One to include an insulated sloped roof assembly that brings all of your attics inside the home’s thermal barrier.

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. Any of these insulation approaches can work, as long as:

  • The designer has settled on an insulation approach early in the design process;
  • The insulation details are well thought out and are consistent with best practice recommendations;
  • The R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the insulation meets or exceeds minimum code requirements for ceilings; and
  • Workers pay attention to airtightness at all stages of the work.

For more information on insulated sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.

This Cape is so cute! Let's buy it!

If you are considering the purchase of an old Cape Cod home, ask yourself the question, “Can this Cape be fixed?”

Sometimes the answer is, “Yes, but it’s going to cost $30,000 to fix it.” As long as you’re signing your purchase agreement with open eyes, aware of the scope of work you face, that cute Cape may still be worth it. Or not.

Martin Holladay’s previous blog: “Flashing Brick Veneer.”

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Image Credits:

  1. Image #1: University of Illinois at Chicago library
  2. Image #2: Southface Energy Institute

Dec 11, 2013 10:49 AM ET

Cape Cod with SIP roof
by Brian Wolfgang

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?

Dec 11, 2013 11:10 AM ET

Response to Brian Wolfgang
by Martin Holladay

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 .

Dec 16, 2013 12:21 PM ET

Been there, done that.
by Jim Baerg

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.

Dec 16, 2013 12:32 PM ET

Edited Dec 16, 2013 12:37 PM ET.

Response to Jim Baerg
by Martin Holladay

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.

Jun 16, 2016 3:09 PM ET

proper roof ventilation
by Ed McGuire

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?


Jun 16, 2016 4:23 PM ET

Response to Ed McGuire
by Martin Holladay

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.

Jun 16, 2016 4:53 PM ET

Potential trouble ahead.
by Dana Dorsett

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.

Jun 16, 2016 5:32 PM ET

I disagree with Dana Dorsett
by Martin Holladay

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.

Jun 16, 2016 6:01 PM ET

The problem isn't at the roof sheathing.
by Dana Dorsett

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.

Jun 18, 2016 10:19 AM ET

A rare disagreement
by Charlie Sullivan

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.

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