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Insulating a CMU frost wall - OT for alt.solar.thermal

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Posted by v8z on September 20, 2009, 8:14 pm
Recently built a framed and truss shop building in Ohio.  Will not be fully
heated full-time, but want to maintain just above freezing, so it will be

There are concrete footers below frost line, and 8"CMU frost wall to above
grade.  The site slopes both directions, resulting in the uphill sides
having a frost wall that is approx. 3-1/2' above the floor slab on the
inside of the shop.  The CMU is waterproofed and parged above grade.

The roof has had baffles and stuffed batt installed at the eaves prior to
the drywall going up, to allow blown-in in the ceiling, and the walls will
be done with dense pack cellulose ( blown in behind reinforced poly stapled
to the 2x6 studs then drywall).

What I'm trying to decided is what's the best approach for insulating the
frost wall on the inside.  Can't fill the CMU's since there are too many
that were grouted for strength to prevent the hill from pushing the wall in.
And the building is already backfilled and graded so insulating the outside
is not an option.

There is a small step where the wall sole plate / treated sill plate sit on
the CMU.  The options I've thought of so far are:

1. Build a "stud wall" out of treated 2x2, anchoring a sole to the slab,
putting 2x2 "studs" 48"O.C., and a  2x4 top plate ripped to fit against the
shop wall's sill plate.  I'd cut 1-1/2" DOW stryrofoam to fit in between the
"studs", and top with fire-taped drywall.  This would let there be a
mechanical attachment for the drywall.

2. Glue 1-1/2 or 2" DOW foam right to the CMU then adhere drywall to it,
taping the seams with fiberglass mesh tape.  Might use the ripped 2x at the
top, nailed to the sole to reinforce the step that would result. Not sure if
adhesives are up to the task?

3. "stud wall" as in 1, but using poly vapor barrier over batts.  This is
probably least attractive due to possible moisture infiltration into the
batt from block side.

4. ?????????

Anyone have thoughts, observations, experience, or a suggestion for #4?
Thanks in advance!

Posted by PeterD on September 20, 2009, 9:52 pm

If your goal is just to keep it above freezing, you may not have to do
much if anything more other than the insualtion you have already in
the walls and ceilings.

I have a 30x60 ft steel building, with insulation (fiberglass). Two
insulated 10x16 ft doors, some (reasonably good) windows.

I run a small 1500 watt electric heater full time, and that keeps the
temp above 32 except for the very coldest times--and even if the temp
drops below freezing on a really cold day, nothing inside freezes.

(when I do work, I use a 200,000 BTU gas heater, which brings the temp
inside up to 55 degrees in about 15 to 20 minutes--I do use a ceiling
fan to move the air around, as I found that witout the fan, when it
was 55 at the floor level, it was close to 80 at the ceiling!)

Posted by Ecnerwal on September 20, 2009, 10:04 pm

Well, that was huge mistake. It's trivial to do it right when the hole
is open. And the way to do it right, now, is to open the hole back up
and insulate the outside down to the footing (and out from the footing a
foot or two). In the lifespan of the building it's probably worth doing
it right, now - MHO.

You could modify your option 2 and use concrete screws (blue things, no
anchor required, if they work right) for some mechanical attachment
through the drywall and foam, though the better quality adhesives are
pretty good - it's just hard to say in the long term.

Cats, coffee, chocolate...vices to live by

Posted by willshak on September 21, 2009, 7:31 pm
 on 9/20/2009 4:14 PM (ET) v8z wrote the following:

4. Skylight(s) to let in the sun during the day?. They don't have to be
high end, just the plastic bubble skylights(s). They may also supply
some light.
Windows on the sunny side to let in more sun, and light?
Both the above won't help at night, but you may not have to run a heater
as much, or not at all, during the sunny days.

In Hamptonburgh, NY
In the original Orange County. Est. 1683
To email, remove the double zeroes after @

Posted by willshak on September 21, 2009, 7:32 pm
 on 9/21/2009 3:31 PM (ET) willshak wrote the following:

One more thing. A black roof.


In Hamptonburgh, NY
In the original Orange County. Est. 1683
To email, remove the double zeroes after @

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