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Community and Q&A

Attic Insulation / Ventilation for 100-Year-Old House

Cameron Patterson | Posted in Energy Efficiency and Durability on

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.

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Replies

    1. Cameron Patterson | | #2

      No I had not come across that, very helpful! In the case of full air permeable insulation, let’s say 4" min wool in rafter bays and supplemental under rafters to R38, would there be a need for air sealing any leakage above the attic floor? As in, at the roof-wall interface there are occasional gaps, and where the gable vent will be closed up wont necessarily be airtight without some significant foam fill at corners and panel seams. My thought was that spray foam seals all that up inherently, but does this fully air permeable insulation offer forgiveness for that outside air getting in essentially up against the roof sheathing?

      See below picture for the light coming through above the top plate and to the right is the backside of the cedar shingle stud assembly that would be closed off with sheathing to cut off the north porch and sole gable vent.

      1. David Argilla | | #3

        So I am just an interested home owner, so not qualified to comment on what you need to do. I'll give your post a bump, hopefully some one qualified will chime in.
        I can say if it was my house, I would keep the roof structure insulated from colder temps from AC (i.e. roof framing and sheathing stay hot, so outdoor humidity doesn't condense in your roof), then the research in that article suggests that any moisture problem should dry out through the vapor portal. I personally would want to seal any openings that I could reach though.
        Up here in the mossy PNW, the roof structure of my 1906 house with original wood shingles was not airtight at all, so I am sure the framing got wet every winter. But the original skip sheathing and wood shingles let the structure dry out , and framing is still in great shape. Obviously much more vapor open, but same process of allowing moisture to dry as roof heats up.

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