Solar-pumping of slate roof
Hello to the specialists !
My south-facing lounge & kitchen area has a 20-foot cathedral ceiling and PU foamed external insulation. The south-facing roof-slope is around 1000 square feet.
My question concerns the do’s and don’t of penetrating this ceiling in order to access the roof cavity and extract air from the 2-inch gap between the back of the slates and the insulation layer.
Normally as a basic DIY-er I would never even touch the roof, but in this case I feel myself getting tempted by the cost-benefit analysis of a large-scale transpired solar collector !
My mezzanine provides easy access and to boot has a serendipitously-situated roof light that makes things a lot easier: by removing the siding I can see inside the roof structure, including the area of interest (cavity) in the neighboring bay (height 4 ft x width 3 ft).
I propose to remove the existing strip-pine T&G, penetrate the underlying roof deck (= 22 mm particle board), then cut a pilot hole through the 70 mm of PU foam, and end up in the cavity. If all looks OK, I’d then enlarge the offtake port to full size.
As I see it, I have the choice of making a single opening the net size of the duct, or otherwise several openings, in which case I’d build a sealed offtake plenum over the full bay, with the aim of dropping the air pressure and getting an even flow.
The (horizontal) duct would run across my truss tie-beam, to my technical shaft that descends another 20 feet into my (concrete) crawlspace. A fan would blow air down the crawlspace (12 meters long) then out into the garden.
The crawl is already fitted with a whole-house fan that works fine (on fair days): encouraged by three winters of use, I now seek to augment this “solar hypocaust”, using massive roof air over the summer and tentatively get towards annualized geosolar storage…
In a few years we will be down-sizing and moving on; it would be nice to have an up-and-running green energy solution on the house, especially if it included an air-source heat pump capable of providing for our DHW needs (currently a quarter of our electric bill).
Thanks for reading and for any feedback !
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