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Old_school (Old_school)
Senior Member
Username: Old_school

Post Number: 885
Registered: 01-2009
Posted on Tuesday, September 04, 2012 - 09:46 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Rich, one reason the roof has lasted as long as it has is the fact that the rooms below it were not insulated or sealed. It was not designed for insulation!

To retro fit it like you are talking about requires a thought process that will allow the house to breathe. Good so far based on your description. I would keep at least one inch of space below the rafters, two would be better, but that would only give you 1 3/4 inches of rigid iso insulation. You will want to make sure and vent the soffit area to have an intake, and then continue that space up into the "attic" above the ceiling. You will also want to drill holes into the rafters at the valleys to give the air a way to get to the dead areas formed by the valley rafters hitting the valley board. I think I would do it with cardboard baffles nailed to the sides of the rafters that leave a space for the air to flow. I would use a spray on urethane insulation over that and beneath the sheet rock. That would be the vapor barrier and also there would be no gaps. I would also tear out the old plaster on the 52 inch walls and fully insulate that. Make sure and leave an opening at the the soffit and you will have to install a fully vented soffit. How will you vent at the ridge? You could almost use a section of Air Vent II ridgevent if you are going to install a copper ridge anyway. IF you do that, vent from end to end in both directions.

As far as the plaster on the lath, No idea. If you have any questions, you can always call my cell phone. It is on the info section.
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Richguibilo (Richguibilo)
New member
Username: Richguibilo

Post Number: 1
Registered: 09-2012
Posted on Tuesday, September 04, 2012 - 07:04 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Let me start by saying how much I enjoyed The Slate Roof Bible. I read it as preparation for addressing the fate of my 135 year old slate roof southeast PA. I learned so much and gained the confidence to make an informed decision. I have contracted to restore the roof with new copper valleys, copper box gutters, copper ridge vents and to replace slates as needed.

In conjunction with this project, I will be insulating the rooms under this roof so they may be used year round. I have removed the plaster from the inside walls that follow the roof to facilitate access. Happily I found no surprises. The wood of the rafters and lathe are like new and there is no evidence of insect or animal presence. The walls are typical for an attic in that they are vertical to a height of 52", then follow the slope of the roof for about 42" to the flat part of the ceiling below the peek. My question is how to insulate behind the wall that is fastened directly to the roof rafters. Some information about my roof:

* The overall profile is a "T" shape. The top of the T is the longer portion (slope = 15:12) intersected at the center by the shorter section (slope = 14:12) creating 2 valleys .
* The actual dimensions of the rafters are 2.75" x 3.75" and they are spaced 24" OC.
* The slates are nailed to slater's lathe (actual 1" x 2.625") spaced 8" OC.
* The slates appear to be Chapman based on the descriptions in your book.

Polyiso rigid panels between the rafters appear to provide the greatest R value per inch. Would this be acceptable for the sloped part of the wall? If so, what is the thickest panel I could use given the 3.75" depth of the rafters? With the rest of the insulation being faced fiberglass batts, should the vapor barrier cover the entire surface behind the new drywall even though the polyiso has a foil skin? If this is not the preferred insulation method, what should I be looking at? I have an insulation contractor scheduled to give me an estimate next week and I like being informed before hand.

I have one more question just out of curiosity. Looking up at the underside of the roof from inside the attic, I see what appears to be mortar or very gritty plaster thinly applied to the upper (1") side of the slater's lathe where it meets the slates and also where the slates butt to each other. The inside surface and lower edge of the lathe is clean. Can you tell me what the purpose of this is?

Thank you for being truly dedicated to this subject.

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