Foiled by the foil

Now I'm not supposed to show you this, but my brother Jack is building this house, he's very proud of it, but he's made a couple of mistakes and this is one of them. It's a common enough mistake in Thailand, you might say that the Thai building industry is commited to making it, because you see it everywhere. Aware of all the problems we've been discussing here in regard to tin, tile, and concrete roofs, the futility of dropped ceilings as insulators, and so on, they have turned to the use of insulation, which is all to the good. Unortunately they've all been conned into one of the great snake-oil building techniques of the 20the century, aluminum foil insulation.

The type of roof my brother Jack had in mind was cement tile underlain with teak planking ceiling, with a little space in between. Anyone who has observed Thai construction practices knows that the usual heat protection with this arrangement is the addition of a sheet of reflective foil insulation between the two materials (the space is about two inches.) The purveyors of the foil will tell you, while catching their breathe in the course of praising their product, that you only have to leave a little space between the foil and the next layer in the sandwich, then you will be rewarded with a reduction in heat of more than 50%, etc etc

Jack succumbed aboriginally to the magical promises of aluminum, and decided to use foil insulation.

This was a mistake.

We see above the guys putting on the roof panels, over the foil which (shiny side down) is in turn over the teak panels. And you can see from the picture that there is not going to be much space, maybe very little space at all, between the bottom of the foil and the teak ceiling. And very thin or irregular spaces are not very effective at interrupting the flow of heat from the roof tile, which as we've seen may reach 60 degrees on a sunny day.

So Jack thought to himself (after I toldf him about the importnace of the space) , should I start shouting at those guys not to walk on that foil when they are putting on those roof panels? What do you think? Jack thought not.

Let's see what theory tells us about the effectiveness of foil like this, whether applied correctly or not.

The first little sketch below shows the R value for the roof "sandwich" without any foil at all. We assume the roof temp is 50 C, the room temp is 36 C in each case. The foil-less sandwich delivers an R value of 2.7 altogether, and the estimated temp of the underside of the teak roof is 40 C--much too warm. Moreover, the heat transfer into the room is 28 watts per square meter, or in excess of 500 watts for a room. Not as bad as a simple tin roof, but not what we'd like.

Now suppose wwe add foil. First, we do it badly, the way Jack did it, so the foil is mostly resting its shiny side on the top of the teak. No surprise here, the foil is not very effective, as shown on the third of the little sketches.

Now suppose we do it right, we leave a space of an inch or more beneath the foil. That's the middle sketch. This time the R value is 3.85, a littlle improvment over the no-foil case(R=2.8), bringing down the underside of the roof to 38.5 C. The heat flow drops from 28 watts to about 20 watts -- hardly the "more than 50 percent" claimed by the aluminum salesmen.

What you get from the theory is that the correct spacing would improve things a little, but not much.

One other conclusion here: whether we use foil or not, the fact that we've got a roof "sandwich" has reduced our heat flow from more than 100 watts per square meter (the hot tin or tile roof) by about 75 percent. This eases our ventilation problem a bit, we don't need a howling wind blowing through the room to blow away the heat seepage like we did with the tin roof. As to how much ventilation, we'll develop some guidelines in a future posting.

Conclusion: foil may have it's uses (more on that later) but the way it's used in Thailand does not do very much to improve either ceiling temperatures or heat flow through a roof.

But let's ask this: what constitutes "improvement" anyway?

The simple answer is: "a cooler room below". But what is cool? The answer is "not just air temperature", but also a cooler underside to the roof, because the heat you feel in a room is about fifty percent air temperature and fifty percent radiant temperature, ie the temperature of surrounding surfaces. And as long as you leave the windows open so the place doesn't heat up like an oven, the air temperature will not rise much. But if the ceiling is say 37 or 40 degrees, you are going to wish you were back in misty England or foggy San Francisco, that ceiling will deliver maybe 35 unwanted watts per square meter to your suffering fahlang head.

So how did Jack's roof turn out?

Pretty much as predicted by theory. With outside temps about 32, the interior surfaces got up to a somewhat uncomfortable high 30s, but as long as the windows were open and the breeze was good, the place was bearable, even in the afternoon, or at least not much worse than a shady spot outside where you would have to live with a slightly sweat-inducing 32 C.




Warm but bearable all around. The temperatures shown around the room are temperatures of the wood interior surfaces, which radiate heat
to all objects of lower temperature within the room. Such as Jacks's 34 degree bald head.This accounts for about half of the perceived heat, the rest being related to air temperature, which with good ventilation would reach 32-33 C, as you can see from the floor and shady wall temps.

But here is the big question: what would have been better than foil?