Now we noted and you might recall that those big churches in Spain or Mexico are pretty cool inside and imagine that thick masonry walls or roof would do the trick of keeping you cool in your house. So let's exchange the thin hot roof (metal or cement tile) which we discussed last time for a nice typical 4 inch concrete roof slab. What happens?
Not only does the 4 inch concrete not slow the heat penetration down much (R= about 0.5 which is still a long way from the 10 or so we would like*) but the heavy concrete stores much of the afternoon heat about (10 kilowatt hours for a 25 square meter room), then releases it back into your room in the evening until 9 or 10 o'clock. This is not the time when you want a lot of heat!
And if you deal with this by turning on an overhead fan, you just accelerate the rate of heat transfer from the warm underside of the roof to the lower part of the room--and yourself. Sure, you can turn on an air conditioner, but what kind of "green" living is that? First you go and use solar heating to store unwanted heat in your concrete roof, now you use fossil fuel to suck it into the room, pump it back out in the form of vaporized refrigerant gas, then blow it on your neighbors, just like that hi-so Italian restaurant does on Soi 23, blowing hot air from its battleship array of compressors on lowly pedestrians.
Not only does the 4 inch concrete not slow the heat penetration down much (R= about 0.5 which is still a long way from the 10 or so we would like*) but the heavy concrete stores much of the afternoon heat about (10 kilowatt hours for a 25 square meter room), then releases it back into your room in the evening until 9 or 10 o'clock. This is not the time when you want a lot of heat!
And if you deal with this by turning on an overhead fan, you just accelerate the rate of heat transfer from the warm underside of the roof to the lower part of the room--and yourself. Sure, you can turn on an air conditioner, but what kind of "green" living is that? First you go and use solar heating to store unwanted heat in your concrete roof, now you use fossil fuel to suck it into the room, pump it back out in the form of vaporized refrigerant gas, then blow it on your neighbors, just like that hi-so Italian restaurant does on Soi 23, blowing hot air from its battleship array of compressors on lowly pedestrians.
Increase global warming! What a waste!
But wait. Why do thick walls work for those cool churches?
First, the walls are truly massive, maybe several of feet thick, so the heat takes a long time to penetrate (just as the ground deeper than a few feet is not heated much by the day's sun). Second, the dry desert regions have night sky radiant temperatures that are very low, maybe even below freezing, and whatever heat collects in the masonry bleeds back out to the outside sky at night, not into the church. Solar heating is offset by effective nocturnal cooling. But in the humid tropics the night sky temperature drops to only only 15-20 degrees. So there may be a little bit of night cooling (maybe 50 watts per square meter), but not enough to offset the effect of strong daytime heating (which is closer to 500 watts per square meter).
So if you are unforunate enough to be living in the top floor of a house or an apartment I have a two ideas that I think are better than blowing your budget on ai rconditoners that (like most in Bangkok) mainly cool solar-heated concrete.
First, you can create what I call an "artificial tree," which is almost as good as a real shade tree over the roof ( which would be the best of all solutions to the problem). I first noticed this technique at fish markets on the Mekong River in Laos, where the market folk created shade nets made from the plastic netting sold for use to cover plant nurseries. The netting, usually either green or black-- both seem work as well-- is suspended like a Berber tent a meter ot two above the roof, absorbs most of the solar heat and allows it to dissipate away into the air, so that the netting material itself (amazingly) doesn't get very warm. Two layers work better than one, but you don't want to use an unperforated fabric or plastic or canvas cover, because this will immediately heat up to 50 C or more and radiate this too the roof, just as bad as the dreaded tin roof we discussed yesterday. This all requires some effort because suspending a net big enough to cover several rooms requires some support structure. I did it at both his top floor apartment and a top floor office, each 150 square meters area, at a cost of about 6000 baht each for installation. The comfort level was immediately improved, and much cooling money saved (though some air conditioning was still necessary to reduce humidity). In both cases the results were worth the effort.
Second, you can paint the surface of the roof bright white to reflect half or more of the solar radiation back into the sky. This is what governement energy folks in both Australia and Florida have been recommending for years, and several readers brought up this idea in my last entry. I have not tried this but it makes sense and the improvements are well documented. There are special paints for this, they appear to be more effective than ordinary white paint.
But what about a new house that aims to have a bit of architectural style? Nets and white paint?
Neither of these approaches may be acceptable on esthetic grounds. A white roof would look wrong, and be an annoyance to neighbors as well--they don't want our glare and heat. And a traditionalThai house covered by a black net?
But wait. Why do thick walls work for those cool churches?
First, the walls are truly massive, maybe several of feet thick, so the heat takes a long time to penetrate (just as the ground deeper than a few feet is not heated much by the day's sun). Second, the dry desert regions have night sky radiant temperatures that are very low, maybe even below freezing, and whatever heat collects in the masonry bleeds back out to the outside sky at night, not into the church. Solar heating is offset by effective nocturnal cooling. But in the humid tropics the night sky temperature drops to only only 15-20 degrees. So there may be a little bit of night cooling (maybe 50 watts per square meter), but not enough to offset the effect of strong daytime heating (which is closer to 500 watts per square meter).
So if you are unforunate enough to be living in the top floor of a house or an apartment I have a two ideas that I think are better than blowing your budget on ai rconditoners that (like most in Bangkok) mainly cool solar-heated concrete.
First, you can create what I call an "artificial tree," which is almost as good as a real shade tree over the roof ( which would be the best of all solutions to the problem). I first noticed this technique at fish markets on the Mekong River in Laos, where the market folk created shade nets made from the plastic netting sold for use to cover plant nurseries. The netting, usually either green or black-- both seem work as well-- is suspended like a Berber tent a meter ot two above the roof, absorbs most of the solar heat and allows it to dissipate away into the air, so that the netting material itself (amazingly) doesn't get very warm. Two layers work better than one, but you don't want to use an unperforated fabric or plastic or canvas cover, because this will immediately heat up to 50 C or more and radiate this too the roof, just as bad as the dreaded tin roof we discussed yesterday. This all requires some effort because suspending a net big enough to cover several rooms requires some support structure. I did it at both his top floor apartment and a top floor office, each 150 square meters area, at a cost of about 6000 baht each for installation. The comfort level was immediately improved, and much cooling money saved (though some air conditioning was still necessary to reduce humidity). In both cases the results were worth the effort.
Second, you can paint the surface of the roof bright white to reflect half or more of the solar radiation back into the sky. This is what governement energy folks in both Australia and Florida have been recommending for years, and several readers brought up this idea in my last entry. I have not tried this but it makes sense and the improvements are well documented. There are special paints for this, they appear to be more effective than ordinary white paint.
But what about a new house that aims to have a bit of architectural style? Nets and white paint?
Neither of these approaches may be acceptable on esthetic grounds. A white roof would look wrong, and be an annoyance to neighbors as well--they don't want our glare and heat. And a traditionalThai house covered by a black net?
I don't think Jim Thompson's ghost would stand for it.
So another approach is required here: insulation . We'll get into that, and all the ways you go wrong with it too, next time.
So another approach is required here: insulation . We'll get into that, and all the ways you go wrong with it too, next time.
----------------------------
Just for handy reference, here are the R values, English units (Btu/hr/sq ft/deg F) for the common building materials in roofs and walls:
concrete 0.1 (per inch thickess)
wood 1 (per inch thickess)
foam 7 (per inch thickess)
material/air interface 0.6 (still air)
material/air interface 0.2 (15 mph breeze)
to figure heat transmission you can use the following:
Watts/square meter=5.6*(temp difference C) /R
So a 4 inch concrete slab is
R=4*.1+0.2=0.6,
and heat transmission for say a temp drop across the slab of 50 to 30 deg is
Watts/square meter=5.6*(50-30)/0.6=200 watts.
Which is A LOT!
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