[Greenbuilding] energy retrofit, asphalt shingle roof, integrated solar thermal air collector

Haudy Kazemi kaze0010 at umn.edu
Fri Nov 18 22:47:36 PST 2011


Hello,

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?

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.

When I looked at this site: 
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
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).

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.)

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.

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?

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.

Thoughts and suggestions are welcome!

Thanks,

-hk


Project details:
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.

Existing roof structure:
-inside ceiling finish
-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.
-1x6 roof deck

Planned additional layers:
-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)
-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.
-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.
-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.
-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.
-regular roofing felt + asphalt fiberglass shingles on top of the OSB




More information about the Greenbuilding mailing list