We come now to a problem that is in my experience the most misunderstood thermal building issue in the tropics, the source of more than half of the discomfort in non-air conditioned buildings, that being the lack of understanding of cooling ventilation. By cooling ventilation I mean ventilation that aims to reduce the temperature of the air of building materials within the building or within any space (such a an attic) that is subject to heating from an overlying roof.
When I was a kid growing up in Boston, Massachusetts during the "dog days of August" when the heat and humidity would sometimes reach the levels of Bangkok in March, my mother would leave the shades dawn and windows closed in the morning, then in the late afternoon when the inside of the house would get too warm, she would open everything and then in the evening turn on a big attic fan, that would roar away all night supposedly pulling cool outside air through the house.* The afternoon-evening ventilation was meant to cool the house -- which was probably 30-35 degrees, with outside air that by late afternoon might be cooler than the interior temperature.
Now times have changed and in the US people are trying to build airtight houses with central air conditioning which is considered a sort of basic right similar to the right to own a big gas guzzling SUV, with much the same wasteful effect, in my humble opinion. As a result the defintion of ventilation has undergone a drastic change, it now being something like "providing just enough replacement air to prevent the house from stinking of cabbage or cigarette smoke or poisonous off-gassing of all the crappy modern furniture and devices people put in their houses these days." This means only a tiny amount of air, not enough to cool anything. Anything more than this minimum to prevent you from getting nauseous is considered wasting energy and bringing in outside pollution (presumably from the SUVs).
Yes, the modern world and it's lobbyists have really created some big unsolvable problems that they then propose to tackle with even more ridiculous ideas like wind farms. Fortunately Thailand has not yet copied these bad ideas and is still pursuing old-fashioned bad ideas by burning cubic miles of coal (with associated planetary detriment) in order to cool leaky concrete structures.
But let's say you want to go back to the real old days, like my brother Jack and his partner Dah, and live in country-style house with low thermal mass (e.g. wood) and a lotta ventilation, enough to keep the house from getting too hot. Then you 've got the same problem, providing enough ventilation to exhaust any heat buildup either in the house or in the "attic" espcially in the mid-late afternoon when you want to realx with a big bottle of Singh beer and a ciager. The real problem where almost everyone goes wrong is getting sufficient ventilation, which is a lot of ventilation, not the amount of ventilation you'd find recommended in any fahlang websites or literature, since this is all aimed at the new paradigm of sealed houses.
We touched on this topic earlier when we noted the oven effect that you usually find with a dropped ceiling, as shown on the sketch and previously discussed:
Now what a lot of people do is put in some kind of a dropped ceiling, which of course intercepts the radiant heat from the underside of the roof. But even though you can turn off the radiant heat, but the convective transfer will soon enough heat up the air in the "attic", maybe all the way up to the high 40s. Then the gypsum ceiling will heat up, then....you are right back where you started, a hot gypsum ceiling instead of a roof. Sure, there is a little time factor here, each square meter of hot roof has to heat up a couple of cubic meters of air and a little bit of gypsum, but that may be as little as a few minutes, since the hot roof with an R value of less than 1 will deliver about 100 watts per square meter of heat.
But how much in the way of openings for ventilation is enough?
To find the answer in a proper engineering way you've got to know the R value for the roof, from which you can determine how much heat is going to flow through the roof into the space below. Given the rate of air heating, you then need to reckon the air replacement rate necessary to prevent the temperature from rising more than a nominal amount, say 1 degree C. Then you've got to figure the sizes of openings (we'll save fans and turbines until next time) necessary to deliver this amount of air to the space under normal conditions -- let's say a breeze of 1 meter per second, typical for hot weather in Bangkok.
Now I have done this for you generically (and will post the derivation here some time for your criticism) but am going to summarize my results below to save you the pain (while meanwhile adding to your suffering when you realize that your vent area probably doesn't meet the standard).
Standard for wall openings to prevent interior heating significantly above outside air temperature**
1. No roof insulation, eg metal or thin tile roofs only: Not recommended because underside of roof will get very hot and radiate onto your head, and wall openings will have to be 20 percent or more of the wall area. Better just lie under a grass rood with no walls.
2. "The usual" roof insulation -- a layer or two of air, maybe a sheet of aluminum, plus a wood or gypsum dropped ceiling. R value on the order of 5: vent must be 10 percent of wall area. For example for a 4 m high by 5 m long bedroom wall, you will need 2 square meters of open windows on opposing walls (ie full cross ventilation) to maintain temp at 1 degree above outside air temperature.
3. Good roof insulation, eg at least 3 inches of fiberglass or equivalent foam material, R value of 15: Same as 2 above, except area can be reduced to 2 percent of wall area, eg a grill with about 0.5 square meter opening area.
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* In retrospect I remember the fan as being about 15 inches diameter, which would move about one-half cubic meter a second -- not enough to cool several rooms very quickly. But the R value of the roof was probably pretty good, there was about 6 inches of fluffy stuff -probably asbestos, now that I think of it, over the ceiling.
** Don't ever forget, you're not going to "cool by ventilation" anything to below outside air temperature, which may be pretty unbearable on April afternoons. (Then it's time to go under the house...you do have an underhouse, don't you?) True, as we shall see, you can cool surfaces like nightime roof surfaces, to below air temperature, more on this disappointing topic later.
We talked last time about the need for major ventilation. If we hope to cool spaces under roofs that a lightly insulated with "passive" openings, say windows or grill vents we will need very large openings to keep the temperature rise less than about one degree C. when the roof gets hot. Here you can see an example, a high roof vent that provides about a square meter of open ventitlation at the peak of both ends of a traditional Thai house.The house is oriented N-S, so it presumably does well at catching the southerly breezes characteristic to the hot season.
