Improving the building envelope for existing bump-outs
We are in the process of addressing repairing some structural framing issues (dry rot caused by bulk water intrusion due to missing flashing) in our 110-year old home, a two-story balloon-framed foursquare in Manhattan, KS (climate zone 4).
Since we have to remove the siding to repair the framing, we’re taking the opportunity to tighten the envelope and add insulation. The new wall structure will be (from interior out): original lathe-and plaster, 2×4 framing on 18” centers with cellulose-filled cavities, existing board sheathing, taped zip-system panels, two staggered layers of taped 1” polyiso, a ¾” rain screen gap created with 1×4 furring strips secured to the framing, and finally LP SmartSide lap siding (using the original siding’s reveals of 4” and 2.5”).
We’ve already converted the primary roof to an unvented assembly by spraying 6” of closed-cell foam under the roof deck. However, there are a couple of bump-outs that I am uncertain of how best to handle the details on. One is relatively small – a 10’ wide section cantilevered a couple of feet out of our kitchen. It is a major source of drafts in the winter. Would wrapping the roof and base of this bump out in the same layers as the walls of the house be sufficient, or is there a better strategy?
The other area is a 6’ x 20’ one-story room on the north side of the house built on top of a crawlspace. I’m planning on encapsulating and insulating the crawlspace (as its dirt floor is a major source of moisture in the adjoining basement), but I’m not sure how to best handle its roof. The roof is hipped, and a relatively gentle slope (about 5/12). Currently it has about 3.5” inches of blown cellulose on the ceiling, plus R-19 fiberglass batts laid on top of that. It shares a water wall with the house that contains the main sewer stack and pipes serving the upstairs bathroom. This space has also been used to route HVAC flex-duct to several upstairs rooms (though I am in the process of replacing those inadequate ducts with new runs in the conditioned space). Before the attic was spray-foamed, the water pipes would occasionally freeze in the winter. I am also dropping the ceiling in this room by eighteen inches to conceal the new HVAC supply ducts and an ERV, both of which are routed on the south side of the space. The ERV ducts will angle over the HVAC supply to reach the joist spaces of the upstairs.
What would be the best strategy for insulating and air-sealing this space? I can’t add any height to the existing roof deck – it is too close to one of the second story windows.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part