I’m a journeyman builder from New York who is lucky enough to experience the wide range of weather all seasons can bring. So far I’ve been up to the task, but my new shed dormer on our small cape has not. It was cathedral framed and roofed last winter, and finished off this summer as time allowed. This winter we have discovered a large amount of moisture soaking the top of the R-38 fiberglass insulation, and collecting on the underside of the roof sheathing and rafter vents (one small 3×3 section remains to be rocked). Though the roof is low in pitch (3/12) I have ruled out a roof leak at this point and can only assume this is moisture generated from within the house. I have noticed that with a good snow the ridge vent may become obstructed, which I would assume reduce the air flow through the vents, and wonder if this could be part of the problem. I hope to find someone who has run into this before, along with any ideas as to what is going on, and a source that details the correct method of insulating a cathedral ceiling in the north east (I had thought I did it correctly, obviously not). Thanks for your time.
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You have an easy path for interior moisture to reach a cold roof surface. The insulation helps to keep that surface cold. If the sheetrock was finished and painted, the amount of moisture that is able to reach that cold surface would be reduced to around 2-5% of what it experiences now. Attic ventilation can help in this situation but it is like using a water hose to put out a house fire. There is just too much moisture available as long as the sheetrock is unfinished. If you had used sprayfoam insulation, you would probably not have a problem even with the sheetrock unfinished due to the reduced airflow and the fact that the dewpoint would occur inside the insulation. There is a lot to learn about the science of moisture and heat flow in building environments. Building science is a fascinating subject. Check it out.
http://www.buildingscience.com/resources/resources.htm
'Nemo me impune lacesset'No one will provoke me with impunity
Edited 1/25/2006 2:02 pm ET by razzman
Thanks for the interest, and the website seems to be a great source of information. Thank You.
Thank you for the reply, I had assumed lack of sheetrock to be part of the problem, however I had no idea it was so critical. It makes sense that without a sheetrock buffer, the path of moisture to the cold underside of sheathing is that much easier. I also like this explanation because it is easy to fix, and not endemic of the entire roof. As you have pointed out building science is a very interesting, and extremely important part of any successfully built house, especially when the climate is not so forgiving. I know I certainly have a lot learn in this subject. It may help to view the house as an entire system, so that things like moisture movement don't to take a back seat to other critical elements. In this case I let it take a back seat to everything else that needed to be done. Thank you again for your reply.
How is the ceiling finished? Where in NY are you? Are there penetrations, such as can lights, in the ceiling?
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
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The ceiling is sheetrocked with the exception of a 3x3 area in a closet/unfinished bathroom at the top of the stair. In this part of the ceiling there is also a bath ceiling fan. (I should have probably put in the wall.) Thanks for the interest.
I'm dealing with the same problem in a rental house in MA. I too believe the roof is sound - it doesn't leak in the summer. I concluded that warm moist air is getting into the attic, which is poorly ventilated (I'll fix the ventilation in the spring). For now, I'm focused on the attic door / hatchway as the source of the moist air - it is not very tight and not insulated. You might want to look at that, in addition to closing that one 3x3 section of missing rock. A lot of air can be escaping into the attic.
By the way, I think my tenants go into the attic a lot (every time I go over there is new stuff stored up there), and I suspect that when they do, they open the hatch and leave it open for quite a while. Do you have a lot of traffic in and out of the attic?
The bedrooms are basically in the attic, with the insulation in the rafter bays. There are soffit and ridge vents, with popper vents connecting, however it looks like the problem may be with the lack of sheetrock. It seems without that extra moisture buffer, the venting system just cannot handle the moisture load. Thanks for the interest.
Do you have soffit vents too?
do you have VB installed?
Is there an air channel under the sheathing or is the R-38 tight up against the sheathing?
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We have soffit and ridge vents, with pink rafter vents between, and 22" kraft faced R-38 stapled over the framing. There is enough space on each side of the vent that some fiberglass may touch the sheating. Aside from the kraft, there is no other vapor barrier. Thanks for the interest.
SO far, so good, except that I don't think much of kraft as a VB in this scenario. Is the cieling finished yet?
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Check for leaks in any bath or dryer vents that run through the space. I just looked at a friends attic with similar problem. At first I suspected the drop down stairs in the hall ceiling but after I got into the attic I discovered that his second floor bath vent runs along the attic floor about 10 feet and then exits at a soffit so much of the moist air was coming right back in (there are three teenagers sharing that bathroom and they apparently take long showers).
Secondly, the vent (alum. flex) was completely choked with condensation, sagging way down between a couple of joists, and therefore wasn't actually venting outside at all. Thirdly, the flex pipe was not sealed to the fan unit, the duct was attached to a plastic box that just snaps into the sheet metal housing. The fit of the plastic to the sheet metal was sloppy, barely enough friction to hold the piece in place let alone actually seal anything. The sum of these conditions was that he was essentially venting a heavily used bathroom right into his attic.
I drained the tube and rearranged it so any condensation would drain outside. I may recommend replacing with rigid tubing. Then I sealed all the connections with aluminum tape. Will eventually re-route the vent to a side wall. Until then, I cracked opened the gable windows to help get out the existing moisture, and told the guy to turn the fan on before the showers, and then to let it run ~10-15 minutes after the shower. Told him I'd look into fan units with timers but haven't done that yet.
It's been a couple of weeks and the attic is almost dried out.
good point - but I think you meant to reply to the OP, Jallen clark
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My applogies for not posting sooner. I refinished some floors and we had to move out for a week. The only thing we have venting in the area is a plumbing vent pipe, and I supose a leak there would cause excess moisture to collect in the insulation. At present I covered the doorway in plastic and put a dehumidifier in there. Within three days the sheathing was almost completely dry. There is a 3'x3' section of exposed insulation from which I can gain access to the sheathing. After it is rocked and finished we will see if we have any more trouble. (Water collecting on the window sill in that area and water stains on the sheetrock.) Joe Listeburk of B.S.T. recomends a full 2" air gap and at least an insulation value of R-40 beneath. Pertaining to your post, we bought time delayed bath fan switches from a company called EFI. They are made as a line buttons with which you can select a series of time intervals from 5 to 30 minutes. They fit in a standard switch box and use a GFCI cover plate. Thanks for the reply.
OOps. I messed up the post. The last was for RedfordHenry as well. As for vapor barriers we only have the kraft facing on the insulation, stapled over the rafters. I have been told, however, that latex paint also serves as a vapor barrier. I would not be suprised if many people in colder climates have a similar problem but just don't know it. Moisture collects on the sheating in the winter and it dries out in the summer. I remember when a contractor I worked for recieved a call from a homeowner who's house we had just finished. The homeowner chose blown in cellulose (wet blown) as the insulation and a sub was brought in to install. Upon entering the attic during late winter, (truss roof with attic storage) the homeowner found the top few inches of the cellulose to be extremely damp. It was a concern to him then, but the weather soon lifted and with the warmer weather the cellulose dried and it was a situation of out of sight, out of mind. What was suprising to me is that I had used the same exact detail (by instruction) on many houses before my own and have to conclude that the same problem existists with these structures. I imagine it equates to cutting the housewrap in an X pattern over the R.O. and slapping the window in without any butyl tape. Thanks again.