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jerod
Posted on Thursday, February 27, 2003 - 09:33 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Hello,
I have just finished a timber frame in Idaho that will have a built up cathedral ceiling with slate roof. There is a lot of debate about the need to ventilate cathedral ceilings if insulation is high (I will have R-60 mineral wool) and air infiltration is stopped. I gather from reading the postings that ridge venting makes roof navigation difficult. My feeling is that soffit to ridge venting will give tremendous insurance against moisture build up/rafter deterioration. Or is the "breathing" of a slate roof sufficient to get rid of moisture? It seems like a ridge vent would be in keeping with the idea of a permanent roof. I would be interested in hearing anyones feeling on the issue. Thanks
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Joe Jenkins
Posted on Saturday, March 01, 2003 - 01:27 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Soffit to ridge venting would be better than no ventilation at all.
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jerod
Posted on Monday, March 03, 2003 - 08:11 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Thanks for the response. I am wondering: How were insulated cathedral ceilings traditionally vented if not through the ridge. Were cathedral ceilings only used on outbuildings or unheated spaces, or did houses just have so much ventilation in other places that condensation was not a problem? My thought was an unvented catedral ceiling with mineral wool would allow vapor to pass to the sheathing. In a vented roof situation it seems vapor would still condense in the air space (on sheathing/ rafters) in winter. So the effect of venting would be the drying caused by air flow. If the roof was unvented vapor would still condense on the sheathing or bottom of slates, but would have the escape route between the sheathing boards and out the openings in slate. With wind blowing across the roof and the sun heating the slates the winter condensation might have a way out. Maybe that wouldn't be enough. Any thoughts?
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Joe Jenkins
Posted on Tuesday, March 04, 2003 - 05:00 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

You should always install a vapor barrier (sheet of plastic, the less seams the better, or the bigger the sheets the better) *interior* to your insulation. So if you have mineral wool between your rafters, then staple up a sheet of heavy mill plastic or other suitable vapor barrier (the best are heat reflective too) to the inside edge of the rafters, making sure you wrap it around the corners and edges of the ceiling if possible in order to seal those likely places from air infiltration. Then place your finish interior surface (drywall, tongue-in-groove lumber, etc.) directly over the plastic. If using lumber that has not been kiln-dried, you have to nail strips of wood on the rafter edges over the plastic to leave an air space between the wood and plastic (you can't put green wood direclty against plastic or it will cup).

If you do this, then no warm moisture-laden air should penetrate into the rafter space and cause condensation. Futhermore, if you allow cold air to flow under the sheathing in a cold air space (ventilation space), then the underside of the sheathing will remain cold and there won't be any warm air there to condense on anything.
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jerod
Posted on Tuesday, March 04, 2003 - 06:58 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Thanks. I have been a builder for a while and have installed vapor barriers in roofs as standard practice. However, when I got into designing my own house I looked for more natural, time tested building methods and decided on straw/clay walls and timberframe with slate roof.
I understand the effect of a vapor barrier, but shy away because it seems a shame to use all these natural materials and then be dependant on a big sheet of plastic for my roof to function. I have read that you can deal with vapor in two ways. 1- vapor barrier 2 - build a roof that will allow vapor to pass without condensing. With my straw clay wall the theory is that if the exterior is at least as vapor permeable as the interior , the wall has "drying potential" - especially in our dry mountain climate vapor should migrate toward the exterior with no "traps". No doubt it is better not to have vapor in the roof cavity at all, but I wonder what water trapped behind wood tounge&groove boards does? Mold? Air exfiltration can allegedly bring up to 100 times more vapor into the roof cavity than vapor pressure, so shouldn't tightly packed insulation stop most of the problem without a barrier? And without plywood(essentially a vapor barrier) sheathing the vapor has a way out. The clay walls and unfinished wood timberframe should also absorb moisture that would otherwise end up in the roof cavity.

Maybe the vapor barrier is necessary. I am just looking for a non plastic alternative. I don't, however, want to end up with a rotten roof system.

Ridge venting a slate roof seems to cause maintenance problems(can't walk or hook ladder) , but do you think it is my best option in this case?
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Walter Musson
Posted on Wednesday, March 05, 2003 - 08:46 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Jerod,
Yes I think that if you wish to avoid the plastic sheet barrier then by all means install the vented ridge along with eaves vents.It is not that inconvenient to use a peak ladder with a vented ridge,especially since you will have a newly installed roof that shouldn't require much maintenance for some time.
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Joe Jenkins
Posted on Wednesday, March 05, 2003 - 11:22 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Vapor barriers were never used on the older buildings with slate roofs, nor were vented ridges. The entire roof system seemed to breathe on its own. After a century of successful performance, it's hard to argue about the design. However, a lot of the older buildings weren't very well insulated either.

You won't get condensation behind tongue in groove ceiling boards because it will be just as warm behind the boards as interior to them. That's why the vapor barrier is installed interior to the insulation, so it's the same temperature on both sides.

I also originally built without a vapor barrier for the same reasons - because I didn't want plastic in my natural house.. However, I soon learned that the small additional cost of such unnatural things as a vapor barrier and caulk (around windows and doors) could save a lot of heating fuel. So I opted for fiberglass insulation and an invisible interior vapor barrier and it works well. We heat with only wood.

The section of the house where no vapor barrier was used (other than that on the fiberglass insulation itself) has a slate roof and a finished attic space (bedroom) and no ridge vent or any vent. We have never had condensation problems.
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Joe Jenkins
Posted on Wednesday, March 05, 2003 - 11:36 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

BY THE WAY, WE ARE MOVING THIS ENTIRE WEB SITE TO A NEW SERVER AND THE SHIFT WILL PROBABLY TAKE PLACE TODAY, WEDNESDAY, MARCH 5TH. THAT MEANS THAT ALL MESSAGES POSTED IN THE PAST WEEK WILL DISAPPEAR. AND THERE MAY BE AN INTERRUPTION IN THIS MESSAGE BOARD SERVICE FOR A DAY OR SO.
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jerod
Posted on Friday, March 07, 2003 - 12:08 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I am interested in the part of your house that has no venting with slate. Is there a bathroom/other source of moisture?

You use the vapor barrier for energy efficiency more than moisture I take it. Will tightly packed insulation not stop air exfiltration, or will air flow right through insulation without some type of film (vapor barrier, paint, etc...) Also, you noted that older homes do not have vapor barriers but also are not well insulated. Wouldn't a poorly insulated house have even more moisture laden air entering the roof system with the potential to cause problems?

I like the simplicity of the unvented roof and would like to do it on my house if it makes sense. It seems like it could work. The major concerns seem to be ice dams and moisture.
As for ice dams I am thinking the 10:12 pitch and R 60 roof should be good prevention. (Will snow slide easily of a 10:12 slate roof?)
The moisture issue seems more tricky. I wonder if the vapor barriers on the walls of many houses cause vapor to be concentrated in the rising warm air (since the walls can't absorb any of it)? With clay walls and unfinished wood it seems some of the moisture could be taken up in the structure and not forced to leave through the roof. Also, our bathing essentially takes place in an outdoor sauna so that moisture source is eliminated. I wonder if the wood and other natural materials in older homes regulated interior moisture levels to some extent. The vinyl, thick finishes on everything, and plastics in modern buildings don't absorb moisture so it ends up in the roof. Tightly packed insulation, I was thinking, would prevent much of the moist air that did end up at the celing from entering the roof. If it did get in, the mineral wool should allow the vapor to pass without a trap. And since there is a way out between the sheathing boards and slates the water has a better way out than in and should dry. With the wind that would blow across the roof and sun heating the black slates having drying effects it seems like it might be able to dry. Especially since our area is usually drier outside than in, more so in winter. The summer shouldn't be a problem because vapor would likely stay vapor all the way through the roof and not condense.
Does this sound reasonable, or am I talking myself into it. I will ventilate the roof if it makes sense, but I am not sure it is the best. I worry that the air space puts moisture laden air into a cavity with air that is cold and therefore less able to handle moisture. In damp times it might actually bring moisture into the roof. Also I wonder if the air flow would make the roof less energy efficient because it would also pull warm air from the insulation?
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Larry Roberts
Posted on Saturday, March 15, 2003 - 11:47 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Our slate roof house was built in 1925 and we have lived there going on six years. We have properly maintained the roof and expect another 100 (of course we won't be around). We have a very large walk up attic with the floor insulated but not the rafters. Our roof base is tongue & groove. It gets nice and cold in our attic, no moisture, never a problem. The attic is plenty cold in the winter. I am sure that our attic floor could use more insulation. Our roof pitch is 10/12 and yes, the Indiana snow & ice slides easily off the roof (we get an average snow fall of 50 to 60 inches per year). You didn't mention your average snow fall. If it is like our northern Indiana climate, approx. 50 to 60 inches, then I would recommend at least installing good quality snow guards on the back side of the home. We won't install on the front because of the looks. However, if you forego installing roof guards on the front. Don't make the mistake that I made when it comes to land scaping. We had some new bushes & miniture pyrimidal yews (five feet max). Guess where the landscaper planted them? After the first major snow fall, the snow & ice sliding off the room damaged some of the plantings. Now, we need to locate the plantings out in front of the avalanch area.
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Willy Gorrissen
Posted on Tuesday, April 06, 2004 - 01:30 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

My house in Idaho has cathedral ceilings throughout. In the summer the inside of the house stays much warmer at night than the outside. Therefore I'm interested in increasing the evening/night time air flow through the house to draw in the cool night air.

How can I do this with cathedral ceilings?

Thank you,

Willy
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vsovbma@google.com
Posted on Wednesday, June 21, 2006 - 06:08 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

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