# insulating a slate roof



## Windows on Wash (Aug 30, 2011)

The roof doesn't need to "breath". Ventilation is all about controlling moisture in the home. 

If it is already being treated as an unintentionally conditioned space, insulate the roof deck. 

Don't spray right to the sheathing if I have a suggestion. Put in a skim layer of rigid foam and spray overtop that so that the foam doesn't seep out onto the slate. 

How about a picture? 

For the record, batt insulation does not work well here. Nothing to control the vapor migration.


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## i07nyc (Nov 18, 2017)

First pic shows finished area through hall into unfinished. Second pic is the rafter bay. Third pic is half the area. The pic came out as green and moldy for some reason, but it is not. I was told as slate is watertight but not airtight, if insulation is put up against the wood, condensate will form on the backside of the slate and rot the wood. An airspace is needed so the roof can breathe.


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## Windows on Wash (Aug 30, 2011)

Whomever told you that it needs to "breath" is incorrect. Unless you have some sort of continuous ventilation pathway, i.e. soffit to ridge, an airspace is useless. 

A vent space is not for trying to dry out a roof. It is for handling interior moisture that collects in the attic.


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## Bud9051 (Nov 11, 2015)

What you have currently doesn't have a moisture problem, per your post. What you are proposing may create one.

Let's start at the beginning. Give us your approximate location so we will know what climate we are dealing with. You didn't state it but I see nyc in your forum name but I've learned not to assume. But if that is where you are located then you will need R-38 in those rafters, per the 2009 energy codes. If your local building department has adopted newer codes you will need to check with them.

Building codes are not there just to meet your needs but to ensure all future owners benefit by knowing the work was done to some minimum standard. R-13 is woefully inadequate.

In addition, an unvented roof assembly still has requirements. I'll just give you a link to get you up to speed.
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Bud


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## i07nyc (Nov 18, 2017)

Building a channel then using rigid foam as a backer for spray sounds like a good idea. It still leaves an air space to let moisture escape. And no place for it to absorb.


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## Windows on Wash (Aug 30, 2011)

That does work quite well but you still haven’t addressed the lack of intake (ie at the eaves) and exhaust (ie at the ridge).

Both of these will require modifications to the slate roof.


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## i07nyc (Nov 18, 2017)

I had a revelation this morning while staring at things. I can pull up some of the floor boards then use foam board to cover the back side of the plaster walls. Leave it as a cold space. Then I won't have to worry about moisture buildup on the roof deck and when I have a leak, I'll know where its coming from. Should I use faced or unfaced batts? Does this sound like a good idea? I included a pic of. Where I'd put the foam board.


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## i07nyc (Nov 18, 2017)

I. Should clarify I mean sticking r30 fiberglass under the floorboards. The boards are 1x4's with a 3/16" gap between each one.


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## Windows on Wash (Aug 30, 2011)

http://www.finehomebuilding.com/pdf/021221068.pdf


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