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

Joe Killian kaa-ajk at sonic.net
Sun Nov 20 22:24:47 PST 2011


  Haudy,
   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.
   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.
   Joe

On 11/18/2011 10:47 PM, Haudy Kazemi wrote:
> 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
>
>
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