Attic Insulation / Ventilation for 100-Year-Old House
Hi all, looking for that sweet collective GBA wisdom to definitively decide what my moves should be for my 100 yr old craftsman insulation / attic ventilation system. I’ve poured over these articles off and on since last summer, hoping to come to a clear direction and be prepared before the heat this year, but now within the weeks of signing estimates I’m back to uncertainty, primarily b/c the end work involved is not what I was expecting. Would love your insights.
Here are my existing conditions:
– Climate Zone 2A (South Central Texas)
-longways N-S (afternoons are v hot)
-uninsulated, ventilated crawlspace
-uninsulated, cedar shingle walls (covered with 2+ coats of paint)
-Nearly unvented attic (only the leaks at the roof-wall connection and a measly N gable vent that is on the outside face of the north porch) with minimal compacted loose fill over some portion of the attic floor of TnG wood ceiling and gyp at conditioned interior
-Leaky, kinky ducts are in the attic (of course)
-Newish roof (within 5 year) dark shingles (of course) on OSB sheathing on original purlins and 2×4 rafters
-2’ overhangs with no soffits
-Most of the roof is a gable, but the S is a hip, and there is a low slope extension over the sun room
-mechanical system is 2.5 tons which could be more or less ‘right-sized’ if it weren’t for the immense heat loads of this attic that is 160+ Degrees from 11am to 9pm b/c of the stagnant air, AHU has a few more years on it, and compressor is 3 years old.
Some quick notes on the below: It seems worth it in any case to remove the old, old nasty insulation, air seal at ceiling, and reseal the ducts. Removing the roof is a non-starter, as there are so many years remaining in its life, so insulation above sheathing is a no go.
After researching, it has occurred to me that sealing off the north porch is all that it would take for the attic to be practically unvented. Follow this with 2” closed cell spray foam and supplemental open cell spray foam or mineral wool to reach R38, follow up with an attic dehumidifier, and voila – an unvented attic, right? Peter Pfeiffer seemed to like this approach for his homes in nearby Austin. This seems too easy though, and I start imagining potential problems – Would closing off the exterior top plate would stop the walls from drying out? Surely there would be an unforeseen consequence to sealing off any portion of a 100 year old house in Texas that has known nothing but airflow its entire existence? Then there’s of course the environmental impacts, the risks of installation, the irreversibility of it all…. perhaps the ccSPF w/ finish of cellulose is a better low foam option?
So then maybe a ventilated roof? And it seems in this case a radiant barrier at the bottom of rafters would be in order as an easy way to keep some heat out. But it seems in order to do this right, I’d need to probably also close the gable vent and north porch, and then add ventilation at the eave closure or introduce an Edge Vent, and then lastly add in a ridge vent. This doesn’t necessarily provide great ventilation at the S hip portion though, so maybe it just doesn’t work? Plus, in total it’s very nearly more work and cost than just sealing it up, for what is on all accounts, the worst way to handle ductwork in this climate. So that seems like a lot of effort to reinvent the current attic into what would be a cooler, but in some sense, worse attic. But it forgives a lot of sins, right?
Lastly, an unvented assembly that doesn’t implement SPF seems like it would be perhaps the most work, as it would require the ventilation of this second course, but then lots of baffling, sealing, and furring out under the air space to get a sufficient depth for mineral wool or fiberglass.
It’s a lot, but it seems like perhaps unvented with spray foam is actually favorable, all things considered…?
Thanks for any insights or contrary opinions.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part