External Condensation on Cape Cod Roof
I know all too well how a Cape Cod roof looks in the winter with the rafters telegraphing through when there is frost or light snow.
I’ve noticed a couple mornings this summer (been in the high 90s) that some of those same areas are showing up as dark. I assume this is due to those rafter bays being cooler (poor insulation) and condensation forming on the black asphalt shingles?
I just don’t know that I’ve seen it happen in the years living here prior to this summer so it’s been bugging me.
These are on the cathedral portions between the half-upstairs kneewall and attic. I plan to remove the drywall and correct some of the bays this fall but wondering if there is a more pressing concern.
Is it likely this condensation is just external?
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Replies
That is an interesting effect caused by night sky radiation, and it is related to the wintertime effect. In all seasons, on clear nights especially, roofing loses heat directly to space via radiation--the form of heat transfer that lets us feel the sun's heat or the warmth of a wood stove.
Night sky radiation cools the roof below the ambient air temperature. It also contributes to dew on grass, foggy car windshields, etc.. Normally those things would match the air temperature, but when cooled below the air temperature, moisture in the air condenses on their surfaces.
On roofs with vaulted ceilings, the framing lumber transfers indoor heat to the roof's surface, keeping it warm enough to prevent condensation. In some cases it may also be the daytime heat stored temporarily in the lumber moving to the cold side. But insulation in the rafter bays slows the indoor heat from reaching the roof, so condensation forms.
With vented attics you usually won't see the framing patterns in the condensation, since there is no insulation slowing heat flow. But on occasion I have seen framing telegraph through on vented attics, which makes me think that thermal mass plays a role.
kramttocs,
https://www.greenbuildingadvisor.com/article/night-sky-radiation
Thanks both of you. This is another one that I knew had been covered but for the life of me I couldn't think of the right search terms. 'Radiation' definitely didn't come to mind.
I am trying to digest this but don't think I am quite there. But now I know what to research.
For my immediate peace of mind, it sounds like this is all external moisture happening and unlikely for there to be moisture under the roof deck?
Yes it is all external, nothing to worry about.
Again, thank you. I'll be creating a new post in the near future (again, I know it will be a rehash since Cape Code/cathedral ceilings always are) and look forward to feedback on the idea I've been kicking around for alleviating this stuff.