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Off topic "thermal" question - Page 2

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Posted by Chuck Jensen on May 8, 2004, 9:45 pm


Blue board (XPS) is quit a bit more vapor and water resistant than
beardboard (EPS).  HD carries an EPS product that incorporates a plastic
sheet on one side and a foil sheet on the other.  It's supposedly vapor
proof when properly applied.  I would expect to tape seems, caulk boxes, and
put a bead of silicon on each plate and stud.  Also, when wearing the
framing hat, I'm not a stranger to a caulking gun.
Integrity of the interior vapor barrier is crucial when foam is placed on
the exterior.  Again that's why I question that practice - people cut,
drill, and punch holes inside their house with little regard to the vapor

Exactly, in fact 1x2 stringers are sometimes applied to shim the siding away
from the housewrap so the wall cavity can dry out easier.

I just found and interesting PDF article about air/vapor barriers on Fine
Homebuilding's website so I've got further reading to do..

Yes, good questions.  Thanks,

Posted by LarenCorie on May 10, 2004, 11:52 am

 Low/no perm insulations, like isocyanurate, have in the past,
suggested actual vents through their product, which tends to
negate part of the insulation value.  Contractor/installer
resistance made that approach an overwhelming failure.

A good rule of thumb to follow, is to keep at the very least 2/3
of the insulation on the cold side of the vapor barrier. One foam
product that works fairly well (on the outside) is the many brands
of expanded polystyrene, with a facing of silver cardboard, that
has slits cut into it for even better breathing of the moisture.

The best approach is to seek an intelligent balance between factors.
Lower perm VBs, if lower perm exterior sheathing and finishes are
used, and higher standards in colder climates. Here is what ASHRAE


-Laren Corie-

Posted by Ben Simons on May 9, 2004, 6:32 am
 Am Fri, 07 May 2004 03:03:14 GMT hat Chuck Jensen

Yes, there are reasons. You loose living space if you put the insulation
on the inside. If that is not a problem for you, then ok.

Imagine also two rooms beneath seperated with a door. It is easy to see,
that from the door frame (the 'I's) to the outside wall (the '-'s) you
have to wrap insulation around the 'I's also. If not, the 'I's will act as
a cool fin in your room. If you just insulate on the outside, you have to
insulate just along the '-'.
MUCH, MUCH easier.



The vapour barrier must be really closed. There are special self adhesive
butyl-strips available for the joints to the wall an between vapour foils.
Don't just tack the foils at the wall.

If you insist to insulate on the inside, install an "wire-layer". this
way, you don't have to put the wires and boxes into the insulation.

Best regards

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