[Greenbuilding] Foam installed between framing during framing

[gromette at frontier.net]

About three summers ago my husband and I found about 1200 sq ft
of 2.5 inch poly-iso board at the Denver Habitat store that had
been removed from a local movie theater that had been damaged by
a tornado. We decided to purchase it, haul it 120 miles to our
building site--we're building a rammed-earth-in-tires-house--and
install it on the interior below a plywood and metal roof and
between engineered I-joist/rafters. To maximize the efficiency
of the material and design, we decided to cut each piece to exact
width, notching the long side of each piece to fit around each
I joist.  We knew this would be hugely labor intensive, and we
were right. But since labor (namely: mine) is cheap we decided
to do it.

We found out that, even though the rafters are nominally x inches
apart, there is no way they actually are. If one was off 1/8 inch
another one somewhere was, also.  We ended up measuring each space,
writing the measurement on the rafter so I could see it from the
ground, and cutting and notching each individually.  The poly-iso
wasn't too difficult to cut, because the fibers are so tiny we
could get a fairly precise edge using a circular trim saw.
AWFULLY messy, though! I had to be careful about all that fiberous
dust: I covered up completely with a good mask, goggles, hat, neck
scarf, etc.

Long story short, after we got the poly-iso all installed (it took
the two of us WEEKS) we decided to go ahead and have
a contractor seal it with an inch of closed-cell foam to ensure a
perfect seal, and then finished up with R-30 fiberglass batts and
a vapor barrier.

If we were to do it again, I would cut each piece to width, but
probably not try to notch it around the "I" portion of the rafter,
and instead use the closed sell spray foam to make up that little
bit of depth as well as seal it.

We haven't finished the house yet (it's our PRP--Perpetual Retirement
Project and it's summer-only construction) but the building is
staying amazingly warm even at 8,000 feet and well-below zero winters.
Not to mention it's cool in the summer.

I've cut enough EPS (styrofoam kinda stuff) to suggest a tight fit
with it and its bead properties are more difficult to control an
accurate cut. Dunno. I'm sure the pros out there would be way better
than I am at it, so it may work. I'd still plan on spray foam to
seal it completely, though.

Good luck and I hope this helps.

--Cathy W.

On Thu, 3 Jan 2013 11:04:14 Eli Talking wrote:
>
> I am wondering if it would be reasonable to install accurately  
> precut eps foam between joist, rafters, or top chord of truss during  
> the framing to achieve a cheap relatively tight installation when  
> compared to fiberglass with the air seal being achieve outside the  
> framing with continuous 6? eps foam managed for air tightness....