Can I use aluminium foil on top of my flat concrete house directly facing the sky?
Dear sir,
Greetings.
Please suggest me in using the aluminium foil as radiant barrier. Regarding the word RADIANT BARRIER, all companies are saying to set the ‘aluminium foil wrapped material’ inside the house to the ceiling with some air gap. But here my question is, why can’t we use such any radiant barrier material (especially bonded with better quality aluminium foil) on top side of the flat surfaced house directly facing the sun (with some air gap between material and top surface of the house)? Are there any issues especially in the point of good performance (i.e. avoiding the heat flowing into the house). Please kindly clarify me on this point.
Regards
Visswanth
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Visswanath,
If you want a durable radiant barrier, here is what you need:
1. A shiny material like aluminum foil that has low emittance.
2. An air gap adjacent to the radiant barrier.
3. Protective elements to make sure that the radiant barrier doesn't get dusty or deteriorate due to UV exposure.
I'm not sure from your description whether or not you want to expose the radiant barrier directly to the sky. This approach is possible, of course -- the usual method is to use an aluminized (shiny) paint on metal roofing. (Note, however, that no one has yet invented a paint with emittance that is low enough to qualify as a radiant barrier. But the idea is similar to your concept.)
Any radiant barrier exposed to the weather will deteriorate rather quickly, though. If you include an air space under your roofing, or somewhere in your ceiling assembly, that air space can include a radiant barrier -- and it will be protected from the weather.
The R-value of an air space adjacent to a radiant barrier is about R-2 or R-3. That's something, but it's not a lot. If you want to reduce heat transmission through your roof assembly, insulation -- something with a higher R-value than just R-3 -- is usually less expensive (per R) than a radiant barrier plus an air space.
For more information on these issues, see Radiant Barriers: A Solution in Search of a Problem.
Aluminimum has very low emissivity across the entire spectrum. A shiny low-E surface pointed at the sky is less effective at rejecting solar gain than materials that are emmissive in the deep infra-red spectrum, but reflective across the solar spectrum. The ability to radiate 300-320 Kelvin heat back to a ~270 Kelvin radiant temperature sky while reflecting the ~5800 Kelvin solar spectrum minimizes the temperature of the roof surface. Even though bright aluminum reflects more, it can't radiate heat away, and has to convection-cool, which isn't nearly as effective (especially on low slope or flat roofs.)
A durable pigment that does this fairly well are based on titanium dioxide, which is visually a very bright white. To remain effective over time requires keeping it relatively clean. If the roof collects grit or grows algae it's emissivity is still pretty high, but it's solar reflectivity goes down. It's still pretty good though. Titanium paints, powder coated metal , or mop-on coatings on roofs are fairly popular "cool roof" materials due to their high solar reflective index. There are aluminum mop-on coatings too, which with comparable effectiveness to titanium dioxide, which are more emissive in the 300K range than bright metallic finishes.
A searchable database of third-party tested commercially available (in the US) cool-roof materials is maintained here:
http://coolroofs.org/products/results
If your roof is insulated there is not much need to reflect the sunlight away, because the insulation keeps the heat away from the occupants.
Most houses in India have little to no insulation, where high thermal mass construction and natural ventilation reins supreme. (I'm just guessing location. Visswanath Reddi desi naam hain.) But now that central air conditioning has become affordable for the middle classes, houses are being built tighter. I haven't searched extensively, but there are now websites devoted to newer-tighter better-conditioned housing there, eg:
http://www.gharexpert.com/
But this is a typical "better" flat roof design:
http://www.gharexpert.com/Image_Strip/Single_full_image_for_all_new.aspx?h=1050&w=1680&type=search&CatId=undefined&ImageId=2449&move_to=&ind=undefined&find=undefined&uid=&FID=&GID=#
(An air gap for insulation is not exactly R38 rock wool, eh? :-) )
A mop-on cool roof coating would improve performance quite a bit, but they're counting on the thermal mass of the reinforced concrete slab to moderate ceiling temperatures.
A common roofing insulation material in drier areas is phuska mud, which is a mixture of clay soils and wood shavings / straw/ bovine-dung typically infused or topped with bitumen in the upper layers for better moisture resistance, which has both thermal mass and some insulative properties. There is usually some sort of durable finish like flooring tiles or thin-brick on top to make it a walkable-useable space. Per building code:
https://law.resource.org/pub/in/bis/S03/is.7290.1979/is.7290.1979_011_02.jpg
http://gratitudeheritage.in/images/DSC07811.JPG
Using a light colored top surface on that sort of roof would provide thermal benefits over terra-cotta or red-brick.
Dana, are you saying that a white painted metal roof is more effective at rejecting heat than, say, a shiny bare metal galvalume or aluminum roof? That's interesting.
Depends on the coating, but yes. Unpainted Galvalume has an SRI in the mid-50s, whereas many white or off-white metal roof finishes have 3-year SRIs in the 60s and 70s, some in the 80s.
http://www.ctmrs.com/forms/sri.pdf
https://www.pac-clad.com/color-availability-chart/
Yup, what Dana is saying is well established and based on sound science.