Observations from living in a low thermal mass designed house 2016-04-20
BaanMaeConstruction
0 Comment

We spent several nights last week with Praew’s mother living in BaanMae during some of the hottest days (and nights) of the year. We don’t have air conditioning in the house and we aren’t really planning on getting any. The hope was that the unique design of the house would allow it to be pleasant enough without A/C.

I thought I would post some observations.

But first, a review of some of the design features related to temperature reduction:

  1. Low thermal mass walls. Rather than concrete blocks or double wood wall with insulation between, we have single wood plank walls. This is so that the walls don’t retain heat after the sun goes down.
  2. A “wind tunnel” like design in the attic. This allows any heat built up under the roof from solar radiation when the sun is beating down on the roof from high in the sky to blow out of the house naturally rather than radiate or convect into the living space.
  3. The house is roughly 14m x 4m with the longer axis aligned east-west. This offers less surface area to face the sun earlier and later in the day. The east and west facing walls don’t have any windows, thus reducing the amount of solar radiation seeping into the living space when the sun is lower in the sky. 
  4. The longer north and south walls have many windows which when opened allow the prevailing winds from the south to blow through the house with ease giving the house a very breezy feel, even in the heat of noon.
So that was how the house was designed. But how well did it work?
Quite well, on the whole.
  1. In our previous house, the inside is normally several degrees warmer than the outside at bed time. We have a small custom built air conditioner in the bedroom, but it is very rarely set to a temperature lower than the outside temperature. In BaanMae, the house was pleasantly cool at night. So cool, in fact, that I had to close the windows and turn down the fan at times because I was getting chilly in the wee hours of the night.
  2. The “wind tunnel” attic worked almost too well. Occasionally, the clear polycarbonate ceiling would rattle slightly when the wind blew. And very seldomly, the wind flowing through the attic had enough force to lift up some of the 2.4m x 1.2m polycarbonate sheets and move them. But from a thermal standpoint, it felt cool enough. I didn’t have a thermometer with me, but I will try to measure it again when I get the chance to try to quantify the temperature gradient across the attic again.
  3. When we came back to the house from eating lunch and opened the doors, it could be quite warm inside. But after opening all the windows and doors, the house cooled down to about the same temperature as outside pretty much by the time we finished opening all the doors and windows.
  4. While having smaller walls facing the rising and setting sun (without windows except for a thin heavily tinted one at the top for illumination) is probably better than the alternative, this concept interacted with #1 above resulting in less of a positive effect than I had hoped. The low thermal mass walls get baked by the sun and radiate quite a bit of heat.

Here is a picture of the sun’s rays beating against the west wall at sunset:

I measured the temperature of the wall at between 40C and 45C at various times:
For comparison, the internal walls between the rooms were 5-10 degrees cooler. Still warm, for sure, but quite a bit less warm. For reference, I normally set my air conditioner temperature to 30C and find it to be quite pleasant as long as I have a fan blowing, so in a real sense a 5-10 degree reduction is 50%-100% of the “subjective heat” compared with 40C. When I tried taking a nap in this room around sunset, it was a strange feeling. There was a very nice cool breeze blowing through, but I could feel the heat radiating off the wall at the same time. It was like having a cool fan blowing on you at the same time that a heat lamp is pointed your way.
An additional (and somewhat related) issue we experienced was when relaxing out on the front balcony at noontime. There was a nice enough breeze and the temperature in the shade was warm but not too bad if you are sitting and relaxing with a fan blowing, but the radiated heat from the balcony roof was pretty intense.

So we came up with three modifications to the house that we are now considering.
First, passive cooling design techniques can help a lot with the hot wall without adding any thermal mass to the house. For the east and west walls, our idea would be to add an “outer wall” to block the sun. For aesthetic reasons, we may someday try to build a trellis with vines covering it. But for now, a simple roll-up/roll-down shade mechanism should do. 
I looked around and found this example of the kind of shade I’m imagining at a restaurant in town:

It rolls up and down with a crank mechanism, and has a sling wire on the left and right sides as guide wires:

We would use a more opaque (light tan) shade material since we don’t need illumination and just want to block all of the sun’s rays). By placing it at least 30cm away from the wall, the light colored shade would reflect some of the sun’s light and absorb the rest, while the air would still flow between the shade and the wall convecting the heat away before hitting the wall. This should result in only a slight amount of the sun’s radiation actually reaching the outside of the wall, let alone conducting heat into the house.

The second to-do item is to get some kind of insulation on the balcony roof. We are looking into a foam insulation that is sprayed on. We hired a company to do this at the office about 7 years ago, and it has worked fairly well since then.

The third idea is to add a sprinkler system to the roof. Older tin roofed houses along the canal sometimes have this. Since we are planning to run our grey water sewage as well as rainfall that hits the roof through a slow sand filter and then store it in a cistern (to use for agricultural purposes), it should be easy enough to pump the cistern water up to sprinklers on the top of the roof to use evaporation to cool the roof down during the hottest part of the day. Since we are already capturing the rainwater, this simply cycles the captured rainwater back up to the roof to be recaptured again (going through the filters each time). Not only should this cool off the roof, but it can be used to clean off the solar panels, also.

As with all engineering projects, this house will involve an iterative process of experimenting and making improvements. But I can honestly say that I am happy with the results so far. And I really enjoyed the time we spent there, so I think we will be alternating back-and-forth between that house and this one moving forwards.