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Off topic "thermal" question

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Posted by Chuck Jensen on May 7, 2004, 3:03 am
I have an 1100 sq ft (2 story) garage/rec-room that I will heat with a
radiant floor driven by solar + auxilliary.  It is 2x6 conventional framing
with osb+housewrap+cedar on the outside.  Stud spacing will be R-19
fiberglass.  I would like to put 1/2" vapor barrier foamboard with taped
seams on the inside under the code required thermal barrier sheathing
(drywall, plywood, or t&g).  Question is - is this right to do?  Most
everyone I talk to, and most articles I read, tend to place foamboard on the
outside of the studs or osb.  This does not make sense to me at all.  The
key is to prevent not only air leakage but to also prevent warm moist air
from the interior space infiltrating into the insulation and condensing
there.  The insulation can still breath to the outside through the house
wrap and sheathing.  Here in the southwest, indoor RH is usually higher than
the outside RH, which may be as low as 5% at times.  Any reason I shouldn't
put the foamboard on the inside?
Chuck Jensen

Posted by David Baillie on May 7, 2004, 1:44 pm
I can only answer you as a carpenter in Canada but we often use the foam
board on the inside.  When I say often I mean on houses that care about good
insulation.  You get a lot of advantages with it.  First off you get R 5 per
inch, then you get a good vapor barrier- very important in controlling
moisture transfer which kills R value, finally you get a lessening of the
transmission of heat through the stud material by providing a thermal
break...  Wow i actually got to write about something I know instead of just



Posted by Chuck Jensen on May 7, 2004, 8:47 pm
 David, thanks for the reply, and coming from Canada where energy efficiency
seems to be taken more seriously than around here where I live, I will
follow your method.  I believe that contractors often take the path of least
resistance (yes, lower R-value), and place the foam on the exterior because
it's easier.  Most electrical boxes are not designed for a deep offset from
the stud face so extra box collars have to be installed, standard door and
window depths become a problem, the longer finish nails required for
trimming out, etc...

Posted by David Baillie on May 8, 2004, 3:22 pm
 Glad you liked the reply Chuck but don't kid yourself about energy
efficiency we're quite the energy pigs up here too.  Good luck on the




Posted by daestrom on May 8, 2004, 6:28 pm



Some interesting questions.

Well, certainly installing foam on the outside is easier.  Electrical boxes,
window trim and all would make it harder to install foam board on the
inside, not to mention the labor of more cutting/fitting on the inside.  And
installing on the outside, it is easier to get 100% coverage.  Inside, the
space between first/second floors could be missed and more joints to fit
around rooms means more chances of leaks.

Regarding moisture, that's another interesting thing.  If you have
fiberglass or other 'air-filled' insulation, moisture can really cut down
it's R-value.  Condensation can be a problem (lower R value and mold) and
actual frost can reduce the R value even further.  So the rule of thumb is
to have a good vapor barrier on the 'wet' side of any such 'air-filled'
insulation.  In cold climates like NY, that means the inside face of the

Foam board doesn't suffer from moisture effects though.  So it can be either
inside or outside.  If it's put on the outside, then something *else* must
be used as a vapor barrier on the inside (in cold climates).  If you use
foam board on the inside, it must be very tightly fitted to avoid moisture
leaking through.  So you may need a separate vapor barrier (plastic sheet)
*anyway*.  If you need a plastic vapor barrier *anyway*, why install the
foam board on the inside where it's a lot harder, when installing it outside
provides the same effects and is easier??

With foam on the outside, the air-filled fiberglass insulation is less
likely to see frost form (depending on just how cold it gets outside).  But
with a plastic vapor barrier on the inside, and foam board on the outside,
how to keep the air-filled fiberglass dry?  Of course, don't let it get wet
in the first place.  And perhaps provide a small amount of 'breathing' by
not cementing the joints in the foam board?

And remember, the vapor barrier is *not* the same thing as 'house wrap' like
Tyvex.  House wrap will 'breath' quite well (designed to do just that), it
merely cuts down on infiltration/wind leakage.


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