Insulation & vapor barrier question
 

I have a Cape Cod style home located in Michigan. I gutted one of the
upstairs bedrooms, removed the collar ties and reinforced the rafters
to allow for a cathedral ceiling.

I have put in new insulation, but have a question about vapor barriers
and some additional foam insulation. Here's my "recipe" that I'd like
some feedback on. Starting from the interior out, the following
materials will be used on all exterior walls AND ceiling:

Tongue & groove knotty pine 3/4" Foamular insulating foamboard 6
mil plastic R-19 kraft-faced insulation Durovent (to provide
airflow from soffit to peak).

My questions a

1) If I put the plastic over the kraft-faced insulation, am I creating
a vapor "trap"?

2) If the above answer is "Yes", will the foam board result in the same
vapor lock situation?

My purpose for wanting to use the plastic is to better seal the walls
from drafts, especially since I'll be using t&g wood for the interior.
The foam board is just to add a little more seal and more insulation.
My concern is that I could create a situation where I'm trapping
moisture and thus encouraging mold growth and decay.

Can anyone with experience/knowledge about the subject provide some
feedback?

wrote in message
oups.com...
I have put in new insulation, but have a question about vapor barriers
and some additional foam insulation. Here's my "recipe" that I'd like
some feedback on. Starting from the interior out, the following
materials will be used on all exterior walls AND ceiling:

Tongue & groove knotty pine 3/4" Foamular insulating foamboard 6
mil plastic R-19 kraft-faced insulation Durovent (to provide
airflow from soffit to peak).

1" FOAMULAR® is only R-5. That plus the R-19 (you should use unfaced) is
only R-24. I thought you need to be R-30 nowadays.

My questions a

1) If I put the plastic over the kraft-faced insulation, am I creating
a vapor "trap"?

Yes.

2) If the above answer is "Yes", will the foam board result in the same
vapor lock situation?

Yes.

My purpose for wanting to use the plastic is to better seal the walls
from drafts, especially since I'll be using t&g wood for the interior.
The foam board is just to add a little more seal and more insulation.
My concern is that I could create a situation where I'm trapping
moisture and thus encouraging mold growth and decay.

Can anyone with experience/knowledge about the subject provide some
feedback?

FOAMULAR® is already is a vapor barrier. You can simply seal the seams with
a non-permeable tape.

I have a Cape Cod style home located in Michigan. I gutted one of the
upstairs bedrooms, removed the collar ties and reinforced the rafters
to allow for a cathedral ceiling.

This sounds a bit odd. Reinforcing the rafters doesn't seem to be a
substitute for collar ties. What did you do here?

-al sung
Rapid Realm Technology, Inc.
Hopkinton, MA

On 20 Dec 2004 21:25:51 -0800, someone wrote:


My purpose for wanting to use the plastic is to better seal the walls
from drafts

Use a "Tyvek" or similar house wrap that is vapor permeable.

-v.




Reply to NG only - this e.mail address goes to a kill file.

Al,

I actually just shortened the collar ties and moved them up about 6',
then reinforced each rafter from the kneewall to the peak. The collar
ties are still in place, so I should have indicated that after I moved
them, I reinstalled them up higher.

Regarding the R-30, since I only have 2x6 roof rafters, I think that
R-24 is probably about as high as I'm going to be able to get. Do you
have any ideas how I could achieve more than the R-24 (afforably, of
course)? :)

Thanks,

Jason

wrote in message
oups.com...
Al,

I actually just shortened the collar ties and moved them up about 6',
then reinforced each rafter from the kneewall to the peak. The collar
ties are still in place, so I should have indicated that after I moved
them, I reinstalled them up higher.

Regarding the R-30, since I only have 2x6 roof rafters, I think that
R-24 is probably about as high as I'm going to be able to get. Do you
have any ideas how I could achieve more than the R-24 (afforably, of
course)? :)

Polyisocyanurate has a higher R-value per inch than does FOAMULAR®.
2" Poly = R-13, 1 1/2" Poly = R-9.8
Here's a link to the John Mansville page on this stuff:
http://www.jm.com/insulation/buildin...022_ap_foil-fa
ced.pdf

Last time I looked at Home Depot, these 4x8 sheets were in the $20-25 range.

-al sung
Rapid Realm Technology, Inc.
Hopkinton, MA