Air-Sealing and Insulating in a Hot-Humid Climate
We’re in the design phase of a remodel/ addition for a 1903 house in Houston (hot and humid climate zone 2). The siding and roof need to be replaced, so there’s an opportunity to improve air sealing and insulation.
The goal is to make it a Pretty Good House, and I’m looking for advice.
The house: (also see attached drawings for wall and roof details)
– Pier and Beam foundation, 1-1/2 story
– Wall construction, from inside out: drywall -> 2×4 framing -> [no sheathing of any kind] -> tar paper -> wood siding
– Attic was finished at some point, resulting in rooms with 3′ knee walls around the perimeter, a small portion of horizontal drywall under the ridges, and in between there’s drywall attached to the underside of the rafters (see ‘existing roof section’, attached)
– Roof construction, from inside out: drywall -> 2×4 rafters -> sheathing -> tar paper -> asphalt shingles
– Roof is vented at gables and soffits, but no ridge vents.
– Blown-in insulation between roof deck and drywall (where drywall is nailed to the rafters), behind knee walls, and under ridges.
– The area behind the knee wall is vented at the soffits, and the area under the ridge is vented through the gable walls, but airflow is blocked between these two areas by blown-in insulation behind drywall
-The roof lines are very complicated, with dormers, pitch transitions, and valleys — basically breaking all fo Martin’s rules for roof design.
My rough plan:
– Walls: remove old siding, add batt insulation in cavities as needed, then: taped ZIP sheathing -> rigid foam -> furring strips -> cement board siding. Also replace windows.
– Roof: convert to unvented attic, both for air sealing and to bring ductwork inside envelope. Cut new ZIP wall sheathing around rafter tails and tie in to roof sheathing for continuous air seal at sheathing. Continue rigid foam from walls to roof. New roof over foam — standing seam if I can afford it, but probably asphalt shingles.
– Floor: currently uninsulated. Add FG batts between joists, then taped rigid foam for air sealing attached to bottom of joists (tying in with walls), then OSB to protect foam and keep out rodents.
I’d love to hear any criticisms of this plan, either to improve performance or reduce costs.
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Replies
Brian,
On the walls, I might consider ZIP-R for convenience or skipping the exterior foam if budget is an issue. It might make more sense to use these funds for spray foam in the attic area. I agree with you that foaming the attic may be the only way to improve how it performs. Otherwise, I like your plan.
Thanks for the reply, Steve.
If the ZIP/ rigid foam envelope encloses the non-vented attic, and is continuous from walls to roof, is there any need for spray foam in the attic?
Brian,
Let's see if an expert weighs in.
Using taped ZIP to create air and water barriers is a good thing. But the attic won't be "conditioned" without a sufficient layer of insulation on top of the roof sheathing, on top and below the roof sheathing, or immediately below the roof sheathing.
You could use air sealing with exterior rigid rock wool to create a conditioned space. But rock wool is pricey and challenging to install.
In your climate, it's also possible that you could install air permeable insulation with a vapor diffusion port. But the room located in the attic complicates things. When people post about trying to insulate 1.5 story capes, for example, they are generally advised to apply spray foam to the roof line. It's much easier to get into the nooks and crannies with foam than other methods, especially with dormers and complicated roof geometries.
At least that is my recollection.
I got to ask what are you really going to save form the original house?
It does not sound like much to my ear. New windows, doors, roof, siding, sheathing and insulation are in your question. My guess is the kitchen and bath need to be redone the electrical a plumbing are overdrew for an upgrades, by the time you reside and resheath rewire and replumb the plaster will look like Swiss cheese.
I think if you add up the costs for this plan you could build a better new house for less money in less time on this or any other lot.
I do like your exterior roof insulation plan.
Walta