A reader's Roofs and roofing question: Tropical ventilation?

Tim W.   from   Toronto, Canada   had this Tropical ventilation question.

Hi Bill,
Your site is a thing of beauty.

My wife and I are building a small retirement community in the Philippines (lat. 14 degrees, similar climate to yours) and we're carefully studying the great Troppo Architects, Murcutt, etc. for sound ideas about how to build, esp. with corrugated.

In the Philippines the preferred material of the wealthy is concrete hollow block, so we have been encountering some incredulity. We'll live with it.

A question for you: do you ventilate the interior of the roof (i.e., between the corrugated and the ceiling) with your whirlybirds, the living space, or both? And are there rules of thumb for how much air you should be able to move, based on floor area,etc.)
Best wishes,

Bill's answer

Hi Tim, We are split something like 50-50, solid reinforced concrete block walls and lightweight steel frame construction.
My feeling is that the block walls are great if they are shaded from the direct sunlight so that they can't soak up the heat.

What you get over there might be the same as we are seeing here, the designers of houses using blockwork tend to build box type houses that are eventually going to be fully air conditioned. This in spite of the fact that they fully comply with BCA requirements for ventilation breezeways etc.

Steel wall frames clad with metal sheeting are great also, they may pick up some heat quickly but they also lose it quickly when the sun goes down, unlike the masonry that holds the heat it may have picked up for some hours.

On the whole the designers of steel frame houses are a lot more responsive to true tropical design. There are many fine examples of really innovative design that can only be done using steel.

It is rare that the two camps come together and combine the strengths of each method unfortunately.

  • The whirlybirds are a cheap and common way of reducing the build up of heat in a roof space.
  • Typically we have them as high up the roof as we can get them, at the ridge, just tucked under one side of the ridge flashing.
  • The ceiling inside the house is not vented. (Usually, there are always exceptions)
  • The eaves are vented, so that outside (cooler) air from under the overhanging eaves is sucked into the roof space, as the rotating whirlybird sucks the hot air out at the ridge.
  • This creates a circulation of air inside the roof space, which is obviously cools the space down, but in the wet season it also inhibits the growth of mould etc caused by the high humidity.
  • If the eaves are lined, say with cement sheets, then we fix a few plastic clip-in type vents around the perimeter to let in the air. In the old days before these were available, the guys that fixed the eaves would drill holes in the cement sheets to let the air in. Say 8 or 10, one inch holes for each sheet.
  • There are no hard and fast rules that I know of to guide with the number of vents or ventilators. I do know that static ridge vents are rarely seen on residential construction, and rotary types are more or less universal up here. Off course they only work when there is a breeze, the more the breeze the more the ventilation.
  • If the eaves are open, then there is no problem with ventilation, but we have a requirement in the BCA to seal the eaves for vermin proofing. That is we fix a mesh to stop vermin getting into the roof space.
  • Don't treat this too lightly! Seal the eaves. A family of rats or possums running around on top of you ceiling can keep you awake for hours.
  • I've seen electric cables and also PVC water pipes eaten through by rats.
  • Also I've had to replace sections of plasterboard ceiling that were so badly stained by Possum piss that the stains could not be painted out.
  • The reason we don't vent the ceilings directly inside the house is that mostly the modern house buyer wants nice flat ceilings, easy to clean, no cobwebs, spiders, geckos etc.
  • But mainly they want to have air conditioners installed for those very hot nights.
  • I have seen a few houses with absolutely no ceilings at all, (and no air-conditioners). You can see the rotary vents turning away from underneath. They are cool, but they have the problems previously mentioned.
  • One of my mates had a Troppo house like that and he had a sheet suspended over his kitchen area to stop errant geckos from dropping into his cooking pots!
  • One system that was used a few years ago which I like the sound of, but which I have not seen in action, was what they called a whole house fan.
  • A large, 2ft dia. slow revving industrial type fan was fixed in the ceiling of say a corridor in the bedroom area.
  • At night the bedroom windows (usually louvres) are left slightly open and also the doors.
  • This large fan would suck air out of three or four rooms and into the roof space where it gets out through the rotary vents.
  • So a steady current of cool air was entering the area through the windows and then out through the roof.
Cheers Bill

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