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How to get R38 in low/slope truss roof?

Waters's picture

How to get R38 in low/slope truss roof? (post #191616)

Waters on Sat, 07/24/2010 - 16:11
in

I have a dormer area in my current project where low slope trusses won't allow for R38 in much of the celing area--near 6' up from the eaves.


I had thought to use a high density batt--I like "Ecobatt" insulation--and then add a layer of rigid foam atop the sheathing, under the sheet membrane roof that will be installed.


There is an open eave, with strip vent atop all the birdblocks.

If there is insulation added above the sheathing, does the ventilation pose a problem  or nullify the R value of the added foam?  Where will vapor condense in that situation?

I'm familiar with a spray foam 'hot roof' with no ventialtion, but I don't know if that's applicable here and I don't know if any of the insulation outfits in town are doing it yet...


Suggestions welcome, thank you.

-A +A

Anyone? (post #191616, reply #1 of 7)

by Waters in reply to Waters [original] on Sun, 07/25/2010 - 12:58

Anyone?

where is here? (post #191616, reply #2 of 7)

by Piffin in reply to Waters [original] on Sun, 07/25/2010 - 16:08

where is here?

 

 


or try this site for the whole gang

http://forums.delphiforums.com/breaktimeclass/start

Eugene OR (post #191616, reply #3 of 7)

by DanH in reply to Piffin [original] on Sun, 07/25/2010 - 17:01

Eugene OR


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

To get R38 with solid foam, (post #191616, reply #4 of 7)

by xxPaulCPxx in reply to Waters [original] on Mon, 07/26/2010 - 14:05

To get R38 with solid foam, you would need 5.84" of foam... that doesn't leave much room for an air channel under the deck.

It would be easier to put 6" of rigid foam panels on top of your existing deck, then fur out a new vent channel on top of that (over the foam), and put a new deck on top of that to attach shingles to.

Then you would cover the exiting vent channels.

YAY!  I love WYSISYG editing!  And Spellcheck!

____________________________________________________

Insulation on top of the (post #191616, reply #5 of 7)

by Clewless1 in reply to Waters [original] on Fri, 07/30/2010 - 19:13

Insulation on top of the truss ... and the vented area from the eave ... does no good.

Consider using e.g. Thermax along the perimter just above the drywall. You can step it in thickness and achieve a result of higher R near the perimeter. We used to use this strategy when space was at a premium. The rest you do in the batts. You maintain the ventilation air flow and get higher R near the edge.

There ain't NO free lunch. Not no how, not no where!

So good it bears repeating (post #191616, reply #6 of 7)

by davidmeiland in reply to Clewless1 [original] on Sat, 07/31/2010 - 18:15

Insulation on top of the truss ... and the vented area from the eave ... does no good

This is important. A vent space in between insulation layers damn near kills the value of the upper layer. You want all of your insulation right on top of the drywall. You do not want air between the drywall and insulation.

Exactly (what I said) ... or (post #191616, reply #7 of 7)

by Clewless1 in reply to davidmeiland [original] on Sat, 07/31/2010 - 19:31

Exactly (what I said) ... or at least meant.

There ain't NO free lunch. Not no how, not no where!