Posted By Cgallaway on 23 Mar 2010 01:40 PM
It seems to me that one problem with energy efficient roofing is that it seems to be all made for only hot climates. That is, the products all seem to be reflective, to drive heat away from the home. Nice in the summer, but I don't live where it's always sunny and warm. I would hate to drive out all that sunlight in the winter. Has anyone tested using some sort of paint coating like this? http://www.paintwithpearl.com/chromics/blacktempchange/black_temperature_change_paint.htm
I'm curious if this would work for cooler climates, where in the winter, we do want "free" heat in our houses so the HVAC unit doesn't have to work so hard....just like in the summer we don't want to have the HVAC work so hard for cooling purposes. It seems like this product could be applied to paint for, say a metal roof?
Could you imagine this product painted on the sides of a house? Might really help reduce the need for additional heat/cooling.
Some thoughts?
Playing games with solar absorption & infra-red emissivity works for limiting the solar gain of a roof, but does nothing in the "right" direction for heating dominated climates. Peak heating loads happen at night when it's coldest. If it happens to be a very clear night, a lower emissivity of the roofing material results in a slightly lower heat load, but low-E materials also have low solar absorption, so you're limiting the heat gained during the sunny days at a net loss. Also, low-e materials only work when dry. A skim coat of dew/frost/snow is HIGHLY emissive, ruining that already modest heat-saving effect. If you design the roofing material for high solar gain you lower the heat load somewhat under sunny conditions, but those sunny hours are few, and the convection-cooling of the roof in the cold winter atmosphere is huge, and the net heat gain is fairly modest. Here again, a snow load on the roof is highly solar-reflective, and the solar gains further foiled.
Compare that to summer time heat-rejection strategies of high solar reflectance, high-E: Summertime solar intensity & duration is high, so lowering the cooling load by reflecting 80% of the heat then radiating the heat that was gained to the sky limits the temperature of the interior of the roof deck. Under the roof deck the heat transfer to conditioned space can be further reduced with reflective low-e foils (radiant barrier) In order to get anything out of a radiant-reflective type of insulation strategy requires two things: A sufficient air gap between the radiating and radiated-upon elements to thermally isolate them, and a sufficiently high temperature difference that the radiating heat is the primary heat transfer mechanism. When you have an attic floor at 100F under a 140F roof deck slipping in something IR reflective between them cuts the radiated portion of the heat transfer by 80-90%, which can be half the total. But in a wintertime situation with a 10F roof deck above a 15-20F attic floor (above the attic-floor insulation) the difference in temp is small. If you regularly get -30F night-time temps (do you live in Antarctica?) a reflective layer might buy you something, but otherwise, not much. The difference between winter/summer attics is that roofing can easily be 30-50F above the outdoor ambient, whereas in winter under high heat load conditions the roof deck is typically within 10F of the outdoor ambient, and there is not enough temperature difference for radiated heat to be a significant portion of the total heat-transfer. (Conductive and convective heat transfer dominates.)
High solar reflectance-high-emissivity exterior paints will modestly reduce cooling loads in much the same way as cool-roofs do, but the convection cooling on walls is much higher than on even steeply-pitched roofs. Cool-roof materials have the greatest effect on low-angle (under 2:12 pitch) or flat roofs, where the induced convection of the heated surface is low. On walls it may be measurable in a lab, but probably not-so-much on your cooling bill.
As for the funky temperature sensitive paint, they don't specify it's solar absorption or infrared emissivity at any temp- not likely to be much different than any other paint. White in the visible spectrum doesn't necessarily mean it's reflecting the bulk of solar radiation (much of which is in the near-infra-red), and if it's emissivity is low it'll retain heat. Bright aluminum reflects over 90% of solar radiation, but get's VERY hot in bright sun since it doesn't efficiently radiate away the fraction that it didn't reflect. There have been other paints & additives marketed as insulation, but most of it is sheer hopeful thinking wrapped in techno-babble snow. For heating dominated climates if it doesn't have an ASTM C518 rating for R value, it's usually worthless in independent real-world testing. In cooling dominated climates it's a combination of solar reflectance AND infrared emissivity that can make it work (or not.) Both are necessary, neither is sufficient on it's own.
The best energy efficient "roofing" for heating dominated climates is rigid foam board applied above the roof deck. By placing the insulation above the roof deck the thermal short-circuit of the rafters are eliminated, and you get the full rated R of the material. Several vendors (Hunter, Atlas etc.) sell it pre-lamnated to OSB nailer deck. Some have product with ventilated nailer deck as well. This approach is a sure-fire way to eliminate ice-damming issues in most of the lower 48, and can be used in combination with attic-floor insulation if you balance the R-values to keep the average wintertime attic temp below the dew point of 30-35% relative humidity 68-70F conditioned space air. Soffit & ridge venting needs to be blocked when you go this route, or it'll be a thermal bypass of the roof-deck insulation. But a 3-4" panel of iso is worth R20-R25, and will out-perform R22-R28 between the attic floor joists. In combination with a unventilated attic and a bit of attic floor insulation the total performance is excellent.
But don't count on paint or emissivity/solar-gain ratios to buy you anything at all in a heating dominated climate, and treat any claims of heat savings with such materials with great skepticism without independent 3rd party testing by multiple parties.