Basic building science question: Where does normal condensation occur in a properly insulated roof?
I have decided, after receiving all the advice on GBA about cathedralizing my attic ceiling, to just abandon the project and restore the flat ceiling. The house is in Worcester, MA, climate zone 5.
But I have a newbie, perhaps dumb, building-science question, not pertaining to my particular situation, but about in general, about condensation in a cold-dominated climate, such as zone 5 or 6: In a properly constructed vented cathedral ceiling where there are soffit vents, ridge vents, and a continuous air gap in each rafter bay between fluffy insulation and roof sheathing from soffit to ridge, won’t the cold air entering from soffit vents still encounter warmed up air inside the insulation and condense there?
And if outside air is warmer than conditioned ceiling, won’t there be some condensation
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Joel K,
The question you are asking is not as simple at it might seem. GBA advisors and contributors have been addressing the management of airborne moisture in walls and ceilings for a number of years. There are pro's and con's to the various methods of construction and detailing that are heavily dependent on what your local climate is. The southeastern parts of the US will require very different management approaches than the northeast or western desert regions.
I have not followed the discussion notes about cathedral-izing your ceiling so I can't really know what has put you off the project. I would first question the engineering aspect of removing the ceiling joists that make up your current flat ceiling. They may well be the elements keeping your walls from spreading apart under the roof loads being transferred downward. If you have already done the proper engineering work to assure creating a cathedral ceiling is safe, then you can move on to management of heat and moisture.
Insulating a cathedral ceiling (actually any ceiling) is fraught with peril only because details matter so much. They are doable however. I would urge you to go to the Building Sciences site and start reading as much as you can handle. There are vast, useful, and very understandable discussions of how and why you want to control air flow in your home and the negative outcomes from lack of control.
There is so much to the question you pose than saying yes or no. It is a bit like asking a symphony conductor why there aren't the same number and type of musicians each concert. Depends on what and where you are playing.
As a short thought experiment, imagine you live in northern Maine in February. You have filled a large trash bag with air scooped from the living room. (not blown into by yourself) Take it outside with you, and then take a deep breath of air and exhale. A white cloud appears in front of you. Open the bag of air from the living room a little bit and squeeze the bag. I doubt you will see a cloud form. So you can see that warm air can SOMETIMES "create" condensation in the presence of colder air (or surfaces). (sorry about the caps, no italic choose)
There is nothing inherently better about flat ceiling insulation techniques. They are just typically a bit less costly than cathedral ceilings for a variety of reasons. Flat ceilings can be screwed up just as badly as any other choice, be it wall, floor, roof or basement. If you have the option and time, understanding moisture management will better guide any decisions you make. If time and money constraints are forcing your hand, then a flat ceiling is generally easier to detail and make more resilient. There are also more flat ceiling DIY resources geared to guiding people wanting to get a job done well, not become a building expert.
Thanks Roger. I edited my question to say climate zone 5 in Worcester, MA, and make it clear that the question I posed was supposed to be a general building science question, not soliciting advice about my particular remodel. We have already decided to keep our flat ceiling because we no longer feel the cathedral ceiling is worth the time and money. Raising the ceiling was just an idea that seemed appealing after we opened up the low bathroom ceiling (because bathroom was dated 1940s and needed an update).
Joel,
Q. "In a properly constructed vented cathedral ceiling where there are soffit vents, ridge vents, and a continuous air gap in each rafter bay between fluffy insulation and roof sheathing from soffit to ridge, won’t the cold air entering from soffit vents still encounter warmed up air inside the insulation and condense there?"
A. The short answer to your question is that if the roof assembly is, indeed, "properly constructed," the ventilation baffles will be installed in an airtight manner. (Ideally, the ventilation baffles form an air barrier above the fluffy insulation.) So the cold air traveling through the ventilation channel never encounters any warm air.
If the fluffy insulation is enclosed on six sides by an air barrier, there will be a temperature gradient from the top of the insulation (which will be near the temperature of the air in the ventilation gap) to the bottom of the insulation (which will be near the temperature of the drywall ceiling). No condensation should occur; but if moisture does accumulate against the underside of the ventilation baffle, the moisture will either diffuse through the baffle (if the baffle is vapor-permeable) or the moisture will raise the moisture content of the rafters, which have the ability to evaporate that moisture (because portions of each rafter are in contact with the ventilation air).