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Extending R-value for cathedral ceiling

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Extending R-value for cathedral ceiling Chris 12-11-2006
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Posted by Chris on December 11, 2006, 11:11 am


I have to add fiber glass insulation on my cathedral roof, for a small
cottage 16' x 24', new construction, in Canada.
The roof rafters are 2" x 8". For that size, I should use R20
(6") in order to keep 2" for ventilation on the top of insulation.
I though I could to be able to use add another 2" x 2" lumber in
order to extend the rafters wide to 2 x 10 (instead of 2 x 8) and to
use R28 (8") insulation instead of R20 (6"). Is that a big
difference for R-value? The cottage is used to spend weekends and some
vacation days and not (yet!) for permanent living.
What would you recommend to me?
And another question: In order to keep the 2" for ventilation I
though to use raft-R-mate. Is it wrong to put this raft-R-mate the
entire long of rafters or only at the beginning should be enough?

Thank you,
Chris


Posted by Goedjn on December 11, 2006, 12:32 pm



>I have to add fiber glass insulation on my cathedral roof, for a small
>cottage 16' x 24', new construction, in Canada.
>The roof rafters are 2" x 8". For that size, I should use R20
>(6") in order to keep 2" for ventilation on the top of insulation.
>I though I could to be able to use add another 2" x 2" lumber in
>order to extend the rafters wide to 2 x 10 (instead of 2 x 8) and to
>use R28 (8") insulation instead of R20 (6"). Is that a big
>difference for R-value? The cottage is used to spend weekends and some
>vacation days and not (yet!) for permanent living.
>What would you recommend to me?
>And another question: In order to keep the 2" for ventilation I
>though to use raft-R-mate. Is it wrong to put this raft-R-mate the
>entire long of rafters or only at the beginning should be enough?
>
>Thank you,
>Chris

You can add rigid foam insulation crosswise over the joists,
as long as you cover it with sheetrock afterwards.
That's why sheetrock screws come in a 3" (75mm) version.



Posted by on December 11, 2006, 12:58 pm



>I have to add fiber glass insulation on my cathedral roof, for a small
>cottage 16' x 24', new construction, in Canada.
>The roof rafters are 2" x 8". For that size, I should use R20
>(6") in order to keep 2" for ventilation on the top of insulation.

Why not put up a good plastic film vapor barrier under 8" of insulation?

Nick


Posted by Chris Friesen on December 11, 2006, 1:10 pm


nicksanspam@ece.villanova.edu wrote:

>>The roof rafters are 2" x 8". For that size, I should use R20
>>(6") in order to keep 2" for ventilation on the top of insulation.

> Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Chris

Posted by on December 11, 2006, 1:19 pm



Chris Friesen wrote:
> nicksanspam@ece.villanova.edu wrote:
>
> >>The roof rafters are 2" x 8". For that size, I should use R20
> >>(6") in order to keep 2" for ventilation on the top of insulation.
>
> > Why not put up a good plastic film vapor barrier under 8" of insulation?
>
> You're supposed to leave space for airflow between the insulation and
> the underside of the roof.
>
> Chris

I would extend the rafters down so you can use the thicker insulation.
I'd also use the air channel baffle material the entire length, as that
is the best insurance that you will have a clear space for air movement.


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