Second chance on a Cape Cod
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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