Vented attics in PNW with dry and mold issues
I recently came across the attached study. Quote:
“Improvements in attic construction practices and materials in
the past decade have typically lead to decreased interior air leakage in to attics due to improved
ceiling airtightness, with the consequent decrease of the attic moisture load, as illustrated in
Figure 5. Concurrently, attics were also constructed with more insulation (i.e., R-40+) to reduce
the building’s energy consumption. This has led to less heat loss into ventilated attics during
cold weather and, as a side effect, has reduced the drying capacity of the roof sheathing. This
has the effect of potentially exacerbating any inherent but otherwise undetected moisture
problems.”
Summary seems to be the vented attics in the PNW may be more prone to drying issues, mould (and maybe rot).
Another study reviewing recently completed homes at the time of writing show similar issues.
https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/119_Roppeo.pdf
Building in the PNW, it would see an unvented conditioned attic with exterior insulation may be preferable over the long term. Opinions and experiences?
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Replies
Alex,
In the PNW, well insulated and air-sealed vented attics that work elsewhere, can develop visible mold. But I've never seen the problems go beyond discolouration of the sheathing, in the absence of some other exacerbating factors.
To me the message is to stay away from potentially risky assemblies, and roof shapes that you may be able to get away with more forgiving climates, The success of the overwhelming majority of vented attics in the region seems to suggest that abandoning them altogether is probably not necessary.
Malcolm
In your second paragraph, how is success being quantified?
Not exhibiting any damage that affects the viability of the structure. The attics in the houses I've built over the last 30 years occasionally have seen discolouration on the sheathing, but unless this leads to deterioration of the plywood, it's only a cosmetic problem. You can see the difference in the illustrations in the link Jon provided. The areas needing remediation all had other sources of moisture causing the damage.
I've only ever seen or read about the moisture affecting roof sheathing. Buildings with solid lumber decking or strapping don't seem to experience similar problems. However with our seismic requirements, those options are off the table.
If it bothers you, you could paint the attic interior with Kilz paint.
https://rdh.com/wp-content/uploads/2015/03/The-Problem-with-Ventilated-Attics-RCI-March-2015-Nov-5-2014-For-Printing.pdf
More radical solutions would be smart ventilation or making the attic humidity conditioned space but not heated space.
Wouldn't sealing the surface of the sheathing with paint significantly reduce it's ability to dry into the interior of the attic?
The study found it did much more good than harm. But possibly in some conditions, the Boracol 20-2BD (probably quite permeable) would be better.
The study did conclude that in the short term both Kilz and Boracol 20-2BD work great, but they also indicated that more research is needed for long term effects of these solutions.
I am an electrician in the PNW and have been in many attics. I have never seen mold in a vented assembly, even when the venting could have been better executed. I have seen some discoloration.
That said, I live in a drier than average part of the PNW.
Alex,
Thanks for sharing this research.
In my experience speaking with builders and other experts about mold and staining issues like this, it seems to be about tolerance. Some, like Malcolm's comments above, have a higher tolerance assuming that the issues aren't within the conditioned space of the home and are not causing structural damage. Others, won't rest until they solve the issue.
I was very interested to see that one of the potential solutions was to insulate outside of the roof sheathing and to condition the attic. Yet another argument for insulating outside of our building assemblies.
It's not easy to determine in climates that see lots of moisture whether your region and your site mean a net gain in drying potential for vented roofs/attics.
A leading builder in the Northwest, Ted Clifton, seems to go with unvented and cathedraled roofs:
https://www.energy.gov/sites/prod/files/2013/11/f5/hiawinner_cliftonview_100213.pdf
https://www1.eere.energy.gov/buildings/residential/pdfs/doe_ch_case_studies/2016hiawinner_tclegend19th_013017.pdf
This Building America Best Practice Series for Marine Climates does not really call out vented or unvented roofs/attics:
https://www.energy.gov/sites/prod/files/2013/11/f5/marine_40_guide.pdf
Vented attics in humid climates--whether it is CZ 4C or 6A can mean that at times, you will experience spotty mold conditions. I have seen lots of mold in attics in CZ 5A and 6A after sustained periods of high dewpoints, lots of rain, and little sun that resulted in mold in the attic, but neither the conditions nor the mold sustained.
Vented roofs--either a vent space above or below the roof sheathing--is a different proposition in my book. In this case, the roof is just a sloped wall and in any climate, introducing a free-draining and ventilating space between the cladding and the structural sheathing is a good thing.
But I am doing wingnut research on this as we speak; I don't think we understand the impact of roof pitch, depth of the vent space, or types of ridge vents and their impact on how much air actually moves through the soffit-to-ridge spaces.
Peter