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Community and Q&A

Second chance on a Cape Cod

audiobrad | Posted in GBA Pro Help on

I listened to Martin’s recent FHB podcast #133 with great interest–especially “ways to retrofit insulation”. I own a Cape Cod in Minneapolis which we renovated 25 years ago including insulating the floors and knee-walls, furring down the rafters from 2×4 to 2×6 (couldn’t afford to give up more headroom) and insulating all with fiberglass batt insulation. The rafter areas have vent chutes–with a ridge-vent at the peak, the knee-wall areas are ventilated as well as possible to keep them cold. Despite our efforts, ice dams are still a problem and the melt along rafter lines is visible. We sealed bypasses and vented as much as possible and have made due with it as is with no serious problems. (Fortunately, the steep roof and several courses of ice & water shield have kept the roof leak free).

We recently happened onto a fixer-upper on a bigger, better lot just 3 blocks away that will allow us to (re)build our retirement home just off the Mississippi River with some amenities and features we’ve always wanted, but couldn’t shoehorn into our current home. Oddly enough, it, too, is a Cape Cod. We will be adding a large 1000 sf addition to the back, but we also want to re-side the house–which has the original wood lap siding. The house was insulated years ago with blown in cellulose as evidenced by the plugs in the exterior walls and the approximately 16″ in the attic. This house is a little taller than our current home allowing for an actual attic space almost 5′ high at the peak. From here, we’re able to inspect the rafter bays, chutes, and attic ventilation. This house also has insulated knee-walls and soffit vents. Though no major moisture damage was found in the inspection, there is evidence of frost in some of these unconditioned areas as well as ice dams on the roof. We haven’t even moved in yet and I know I want to re-think the whole insulation plan on this house. I should add that while energy savings are important, the health of the house is the highest priority due to my wife’s lupus and other auto-immune issues.

My plan is to leave the cellulose in the attic as is, but gut the plaster along the rafters and knee-walls and fill the rafter bays with spray foam from the soffits to the attic area (at which point the cellulose would take over). I believe this was basically what was advocated in the podcast. It is also what I’ve read in numerous other articles and blogs regarding the general frustration with insulating Cape Cods.

As for the exterior walls, re-siding the house gives us the option to address wall insulation from the outside before adding LP Smart Side or Hardie Board. Adding rigid foam is often recommended, and we’re fortunate to have this option. But once again, the Cape Cod bites us in the butt as it has virtually no overhang on the gable ends (except for what appears to be about a 3″ crown molding at the roof line). The soffits offer a little more, but probably only about 5″ or so. In the podcast, Martin spoke of a “ratio” when choosing the thickness of rigid insulation relative to the 2×4 framing filled with cellulose. I understand that it’s important that the inside face of the rigid insulation be warm so as not to cause condensation. But I’ve also read that cellulose is very good at mitigating the movement of moist air. The problem is, I don’t know if this cellulose is “dense packed” or free of voids. My guess is that even if done well, there will be gaps due to unseen framing blocks, headers, rim joists, etc. Additionally, I’ve read articles that anything that breaks the thermal bridging is highly advantageous to the overall performance of the wall. So the question is this: would adding, say, an inch of rigid insulation be beneficial, or would it be too thin and risk condensation? Of course the reason for keeping it thin is so we don’t overshoot the trim on the gables or reduce the soffit depth to nearly nothing. Or, maybe I should leave the walls alone. Some even advocate leaving exterior walls uninsulated in old homes and focussing on air tightness. With the re-siding, it’s my last good chance to address this from the outside (including dumping out the cellulose if so advised). I’m assuming that vapor barriers are not needed in the scenarios described, but correct me if I’m wrong. 

Any advice is welcome. I want to do this Cape Cod right (but my bank account has limits). Thank you!


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  1. GBA Editor
    Martin Holladay | | #1

    Lots of issues here.

    First: When insulating a Cape Cod house, it's always better to extend the sloped roof insulation all the way down to the eaves, so that the triangular attic behind the kneewalls becomes conditioned space. To learn more about this issue, see “Two Ways to Insulate Attic Kneewalls.”

    Second: an experienced roofer can extend the roof overhangs on your rakes (that is, your gable-end overhangs). I would advise a combination of:

    (a) Adding trim boards along the cornice of your rake -- these can be fastened with long screws, and should be as thick as you dare. They can be ornamental; get creative.

    (b) Installing wide metal drip-edge along the rake of the roof. It's possible to order extra-wide drip-edge, or to get a sheet metal shop to make wide drip edge out of heavy-gauge metal.

    Third: For more information on the ratio of rigid foam insulation to fluffy insulation on walls, see these two articles:

    "Calculating the Minimum Thickness of Rigid Foam Sheathing"

    "Rethinking the Rules on Minimum Foam Thickness"

  2. audiobrad | | #2

    Thanks for the reply, Martin. I probably wasn't clear, but my intention is to insulate the rafters from the attic down to the eaves with spray foam. This would make the area behind the kneewalls conditioned space. The attic--which has a thick layer of cellulose--would be unconditioned and ventilated.

    As for adding rigid insulation to the exterior of 2x4 walls in Minneapolis (Climate Zone 6), the article ”Calculating the Minimum Thickness of Rigid Foam Sheathing" indicates I should spec R 7.5 minimum—which equals about 2” of EPS or Comfortboard, or 1.5” of XPS . As I cross-referenced several GBA articles, I also learned I should incorporate a rainscreen—probably using 3/4” furring strips to attach new LP SmartSide or Hardie Board. That combination will push my walls out 2.25”~ 2.75” depending on which product I choose.

    Extending the roof overhangs on the gable ends seems unavoidable, and I’ll be reducing soffit overhangs by about half. My primary concern with the latter is aesthetics—I like to have some dimensional depth to work with for the soffits and dormer trim. Basically, I’m fighting for every fraction of an inch I can get. In fact, I will likely be overstepping our property line setback slightly, although I’m hoping this won’t be an issue. I took a brief detour into the notion of cheating the insulation thickness a little while reading "Rethinking the Rules on Minimum Foam Thickness”, but by the end came full circle and reminded myself why I was doing all this in the first place. I need a healthy house—it isn’t worth the risk.

    Which brings me to my final question of choosing which rigid insulation is best for our situation. I’ve read many pros and cons, and noted the different moisture permeability characteristics of each, but I’m not sure how much weight I should give to the perm ratings in our case. I would welcome a nudge if there is a clear winner. In summary, the wall construction will be:

    ▪ Plaster interior walls
    ▪ 2x4 framing with cellulose fill (not sure how dense)
    ▪ 3/4” board sheathing
    ▪ WRB
    ▪ Rigid insulation board R 7.5
    ▪ 1x furring (rainscreen)
    ▪ LP SmartSide or Hardie Board

    Thanks for your comments and for this site. I’m glad to be a new GBA Prime member!

  3. GBA Editor
    Martin Holladay | | #3

    In your case, where every fraction of an inch matters, I would advise you to install 1.5-inch-thick polyiso. Different brands of polyiso are rated differently, but I've seen 1.5 inch polyiso rated at R-9.3 or R-9.6.

    While the performance of polyiso needs to be de-rated during very cold weather, a product rated at R-9.3 still has a comfortable margin before its performance drops to R-7.5.

    1. Expert Member
      Dana Dorsett | | #5

      The performance of Dow Thermax does not need derating for temperature the way most 2lb roofing polyiso does, according to the manufacturer. If using reclaimed roofing polyiso it's safe to assume at least R4.5/inch if on the cold side of a roof or wall assembly in MN from a dew-point control point of view- average performance will be better than that. See Figure 1 on this bit o' marketing fluff from Dow:

      The black curve is Thermax, the weird curve is a low grade Brand-X roofing foam that's only labeled R5.2/inch at 75F mean temp through the foam, but notice that even when the mean temp through the foam layer is 0F (say -30F on the exterior, +30F on the warm side of the layer) it's still running R4.3/inch.

      At 1.5" XPS is labeled R7.5, but over the lifecycle of a building it's performance will drop to about R6.75 or lower. The manufacturers' warranty is only for 90% of labeled R, and it's not clear that it will actually meet that level in 50 years. At full depletion of it's blowing agents it would stabilize at ~R4.2/inch (R6.3 @ 1./5") , equivalent to EPS of similar density. From a design point of view don't count on more than R4.5 per inch.

  4. Expert Member
    Peter Engle | | #4

    You can also save a bit of space on the rainscreen gap. Any gap helps, and anything above 1/8" or so allows water droplets enough room to migrate down the wall. You can use one of the mesh products like Home Slicker. That comes in both 6mm and 10mm thicknesses (about 1/4" and 3/8" respectively). You could also use a more rigid product like MTI's Corrugated Lath Strip. All of these products are designed to provide suitable drainage cavity with the least space.

  5. audiobrad | | #6

    I realized after posting that I may have dismissed polyiso prematurely. Thermax looks interesting—does anyone have any firsthand experience with the product? Dana, your comment that Dow used a “low grade Brand X” foam for their polyiso curve is interesting. I saw the asterisk in the legend, but couldn’t find any reference in the article.

    I’m wondering about the various facing options on Thermax, which leads me to ask if there are any comments regarding the perm ratings on any of the various rigid insulation products—or is it largely irrelevant in this case? I also noted 'crickets' regarding mineral wool. Though it doesn’t help make the wall thinner, just curious if it’s “breathability” or other characteristics are something I should consider.

    Peter, Thanks for the rainscreen comment, but my understanding is that wood siding requires the structural strength of furring strips. I think that would be especially true for Hardie Board due to it’s weight. (“If your walls are sheathed with rigid foam, most types of siding — especially wood sidings — require vertical furring strips.”

    Thanks for the comments everyone!

    1. Expert Member
      Dana Dorsett | | #7

      In Canada there is one manufacturer of somewhat vapor-permeable low-density polyiso wall sheathing. Fiber faced roofing polyiso is denser and less permeable (even without facers), and normally specified as "under 1 perm".

      Thermax only comes with foil facers- either bright aluminum or white, and is effectively impermeable, a true vapor barrier. It's generally only worth the up-charge if it needs to be exposed, since it has fire-rated facers, or in locations where every inch of assembly thickness matters. If one derates other ~1lb density foil faced iso to R5/inch for you climate it wouldn't be too far off the mark. A foil facer adjacent to a 3/4" air gap adds another ~R1 to the performance.

      Where thickness matters less, derating reclaimed roofing foam to R4.5/inch and going fatter works, and is greener than virgin stock insulation of any type. On a deep retrofit project I was involved with going with reclaimed foam saved over 10 grand on the project. The walls had 3-3.5" reclaimed foam and half-inch or 1-inch virgin stock foil faced goods at the exterior facing the rainscreen gap, 4" of foam total. With all foil faced it would have hit the same performance point with an inch less foam, but would have cost quite a bit more.

      Dow doesn't name the manufacturers or exact products in their comparative curves- indeed they may not even know which products were tested, since that's the product of a third party. In the fine print it reads:

      "Published data adapted from BSL
      Thermal metric Project & Other recent
      Research by BSL & RDH – data may not
      be representative of all insulation types"

      More information about the BSL Thermal Metric Project can be found here:

      But the graph indicates the weird-curve polyiso only tested at R5.3/inch at 75F, the mean temperature at which it is tested for labeling purposes, which would only be consistent with a 2lb roofing polyiso (and not necessarily the best performing of what's out there.)

      1. audiobrad | | #8

        Thanks Dana. I'm still not sure what my goal should be regarding permeability with this build. Since I have 1X plank sheathing, I'm not as nervous as I would be with OSB. But do I want the least permeable board insulation I can get (thereby eliminating mineral wool entirely)? Also, I did a brief search on reclaimed and recycled rigid insulation without a lot of hits. Can you recommend a source or the best way to find reliable suppliers in my area?

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