Hot Tile Roof Iii

[Swelters]

Some of you worry that you've done it wrong.

But you didn't do it as wrong as I did, the first time I built in Thailand.

We see above the guys putting on the roof panels on a traditional Thai house I built in Bangkok, over the foil which (shiny side down) is in turn over the teak panels that my wife wanted on the ceiling. She didn't want to look at aluminum. 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, you really want an inch or two of space.

Should I have start shouting at those guys not to walk on that foil when they are putting on those roof panels? I thought not.

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

The top 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-- too warm. 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 which would deliver maybe 2000 watts to the room, but not what we'd like.

Now suppose we add foil. First, we do it badly, the way I did it, so the foil is mostly resting its shiny side on the top of the teak. 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 little 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 -- some improvement but hardly the "more than 50 percent" claimed by the aluminum salesmen. It will cut our airconditioning cost by up to 25 percent. (Research at Univ Florida shows foil cuts power cost by about 15 percent.)

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. It's the same layering effect we use in cold weather clothing. In the tropical heat, this eases our ventilation problem, we don't need a howling wind blowing through the room to blow away the heat seepage like we do with the tin roof.

Conclusion: foil is pretty good at cutting radiant heat, as when used alone, in the country house I showed earlier. In a layered roof, it's moderately helpful, adds perhaps an R value of 1.

As to how much ventilation, I'll develop some guidelines in a future posting.

[Artisi]

That sure is wrong - but at least you have learnt something from your mistake - more than we can say for some of the others who know it all.