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Abby Kende
Posted on Saturday, November 10, 2001 - 07:17 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

I want to finish the attic under our slate roof. Are there any problems with ventilation if I put insulation under the slate?
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Joe
Posted on Saturday, November 10, 2001 - 08:22 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

It's always preferable to leave an airspace (an inch or so) between the insulation and the roof sheathing (boards). If there is a space above the ceiling in the attic, it should be ventilated if you insulate the ceiling. If a space is not there and you're insulating between the rafters up to the peak of the roof (unlikely), then you have to rely on either roof top ventilators, or just let the roof ventilate itself. I built a home 20 years ago with no ceiling in the top floor (I insulated between the rafters to the peak) and no added ventilation, and I have had no problems. But I do have a slate roof with a board roof deck (not plywood), and my soffit and fascia is not airtight, so the roof breathes.
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Maarten DHaese
Posted on Wednesday, November 13, 2002 - 01:38 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Joe:

In another message you said: "It depends on whether you want to use your attic space as a living space or not. If not, you can insulate your attic floor, otherwise you have to insulate the rafters. If the floor is insulated, the attic must be ventilated (out gable end windows or louvered shutters). If the rafters are insulated, there should be an air space (1/2" or 1") between the insulation and the roof sheathing, and maybe some soffit vents and gable end vents or roof vents to allow for some air flow or breathing. Also, if the rafters are insulated, there should be a vapor barrier (sheet of plastic) interior to the insulation (under the wall board or paneling)."

Your message here does not mention the vapor barrier. Do you need this barrier to prevent condensation from warm moisture-laden air that goes up to the roof, or is it to prevent moisture from outside coming in?

Also, if you need a vapor barrier, can you use a breathable membrane (i.e. lets vapor from the inside of the house through, but not the other way around)?

I have a house in London with a natural slate roof that is about 80 years old. The basic architecture is slate nailed on battens nailed on rafters. There is no felt paper under the slates and it doesn't look like there's ever been any. From your book, I understand that this is not a problem. I now want to insulate between the rafters (there is currently no insulation on the loft floor or in the roof), but I am getting mixed messages from contractors and general literature. My rafters are 4" deep. Basically, I want to add 2" of insulation material, leaving a gap of 2" between the insulation and the slates. Then I want to fix plasterboard to the rafters to hide the insulation. Your comment seems to suggest that I need to put a vapor barrier between the insulation and the plasterboard. Is that correct?

Also, various contractors have told me that they want to put felt paper underneath the slates (underneath the battens). One contractor even suggested to 'wrap' the rafters with felt paper as follows (felt paper is ==; rafter is ¦¦; batten is --):

-------- -> batten
= ¦¦¦¦ =
= ¦¦¦¦ =
= ¦¦¦¦ =
======

The idea I think is to prevent a leak in the slate roof from directly affecting the insulation material. I am concerned though that this method may bring its own problems re: ventilation. Am I right to be concerned?

Even if it doesn't bring additional problems, is it useful to do this?

Many thanks, and thanks for writing the book.
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Melanie Bates
Posted on Monday, March 24, 2003 - 09:37 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Joe

My question is about the same as Maarten Dhaese.Though i could not find the response that you gave to his question.
My house was built in 1938 it has a slate roof with a lime cement pointing to the back of the slates, the usual rafter and batten size are all there with no ventillation to the eaves or ridge and no felt. I am now converting the loft into a habbitable room and wish to insulate between the rafters and then plasterboard the whole lot on the top of the rafters. I will be creating a small lowered flat ceiling area at the ridge to help with the air flow.
What type of insulation do you suggest?
And would it be sufficient to use foil backed plasterboard? or should i be including some kind of extra vapour barrier?
And lastly what do you suggest i should do to add ventilation to the roof itself, there are two gable ends that i could add air bricks to if needed.
any help with this would be much appreciated.

Melanie B
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Anonymous
Posted on Wednesday, February 11, 2004 - 01:31 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

you guys should check out www.buildingscience.com and also buy the book "insulate and weatherize" by bruce harley at www.taunton.com for $20.00.

no Im not bruce. but it helps to read more about vapor barriers - which prevent moist warm air from moving through materials into enclosed spaces where it can condense.

regarding ventilation, this is on the exterior side of the insulation, and it helps wick out moisture that may have gotten in there.

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