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<font size="+1">Haudy,<br>
Some years ago my brother did a very low tech similar setup to
gather winter heat off his roof. This was admittedly New Mexico,
not Minnesota, but was even simpler than you're describing and my
brother reported very good results.<br>
He stapled a plastic sheet on the bottom of his rafters in the
attic (on a southish facing sloped asphalt-shingle roof) to create
a contained space below the sheathing. He added a fan and a
couple ducts to pull air form this space into the living space.
He reported being very pleased with collecting quite a bit of
heat.<br>
Joe<br>
</font><br>
On 11/18/2011 10:47 PM, Haudy Kazemi wrote:
<blockquote cite="mid:4EC75108.1000108@umn.edu" type="cite">Hello,
<br>
<br>
I'm planning an energy retrofit on an old house in Minnesota and
was wondering if anyone has used an asphalt roof as a solar
thermal air collector, especially for the shoulder seasons and
times where the roof is not covered in snow?
<br>
<br>
The idea is to use a 1.5" air gap (created by 2x4 furring strips
between the shingles+OSB and 6" of polyiso) as a solar air
collector to heat air for a porch and/or a heat pump water heater
like the Nyletherm Geyser. The front porch is on the south side
of the house; it would be necessary to use a fan to force the warm
air out of the collector, probably done by blowing porch air into
the collector rather than trying to pull air out. (Keeping a
positive pressure in the collector so air inside the home doesn't
get pulled into the collector in case of any missed gaps.) I
expect the home itself to be at a slight negative pressure due to
bathroom and kitchen exhaust fans. The slight negative home
pressure will also decrease the possibility of semihumid home air
from getting into the unvented roof assembly.
<br>
<br>
When I looked at this site:
<a class="moz-txt-link-freetext" href="http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm">http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm</a>
<br>
I saw the 'Greenward Ridge Vent Water Heating System' which looked
interesting, if I wanted to use a ridge vent system. I'm going
for an unvented design (if I scrap the integrated solar thermal
collector idea, the same 1.5" air gap could become part of a
hybrid vented/unvented roof design).
<br>
<br>
I also saw the 'Roof Integrated Solar Absorbers' (RISA) section
but those designs used PEX with water or antifreeze circulating in
them and apparently a metal roof. I think installing a regular
shingle roof over a liquid-based integrated collector is just
asking for a leak from a nail in a PEX line. In comparison, a
nail in an air space is not really a problem. (It might be
technically possible to install an asphalt roof shingle over a PEX
liquid based RISA, if the nails are carefully positioned, but it
seems like that is far from foolproof if anyone else but oneself
is doing the actual reroofing in the future.)
<br>
<br>
Is there a way to install PEX as part of a integrated roof liquid
based solar thermal collector that allows for foolproof shingle
installation? I'd love to hear about it.
<br>
<br>
Should the rigid polyiso insulation need to be isolated from the
solar air ducts for air quality reasons? What is a suitable
separator? Housewrap? #15 Roofing felt?
<br>
<br>
Routing small diameter piping through multiple floors is ever so
much easier than transferring the same amount of energy using air
ducts, and there are no air quality issues. Air collectors can
cost less to build, however.
<br>
<br>
Thoughts and suggestions are welcome!
<br>
<br>
Thanks,
<br>
<br>
-hk
<br>
<br>
<br>
Project details:
<br>
The basic project plan is to convert a vented roof into an
unvented roof by adding sufficient layers of external rigid
insulation and airsealing ceiling penetrations. (Project plan is
based in part on materials the available through Building Science,
CCHRC's REMOTE Manual, and other sources). The local building
planning office is familiar with this technique and references
Building Science in their checklist when considering whether to
approve an unvented design. The basic local guideline is 60% of
the R-value for the unvented roof should come from the rigid
insulation.
<br>
<br>
Existing roof structure:
<br>
-inside ceiling finish
<br>
-2x4 rafters with 4" blown-in cellulose in most areas (sloping);
some areas with flat ceilings on collar ties have a layer of
polyethylene and more than 4" of cellulose+fiberglass.
<br>
-1x6 roof deck
<br>
<br>
Planned additional layers:
<br>
-building paper/WRB/housewrap layer over old roof deck (IIRC,
Building Science shows this in at least some of their unvented
roof designs and in the perfect wall diagrams)
<br>
-4" strips of 1/2" OSB or plywood screwed through the old roof
deck into the rafters. This is intended to firmly fasten the old
decking to the old rafters in preparation for the long roofing
screws in case some of them miss hitting the rafters. It will
also provide a breather space for the WRB layer.
<br>
-add 6" of reclaimed polyiso foam sheet (2 layers of 3") onto the
old 1x6 roof deck, using cans of spray foam to fill any gaps in
the foam.
<br>
-use 2x4s laid flat as furring strips over the polyiso with 10"
long roofing screws to hold them down. The 2x4s will also serve
as washers/load spreaders for the screws on the foam. 2x4s are
less expensive than 1x4s and give a larger air gap.
<br>
-add a new layer of 5/8" OSB roof sheathing over the 2x4s.
Radiant barrier OSB could be used in a warmer climate, or if an
integrated solar thermal collector wasn't under consideration.
<br>
-regular roofing felt + asphalt fiberglass shingles on top of the
OSB
<br>
<br>
<br>
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