Feb 15, 2015: Case Studies – Heat Doesn’t Rise

It is a commonly held misconception that heat rises. In fact heat travels equally well in all directions. While it’s true that hot air and hot water will rise due to convection currents, this is only one of the ways that heat moves. The truth is that heat moves in three main ways; conduction, convection and radiation and, particularly with conduction and radiation, heat will travel just as easily downwards as upwards or sideways.

So why the science lesson? Over the last week or so I’ve been involved in consulting to a couple of clients who have problems (or potential problems) with heat going downwards. These are common issues in houses so I thought it worth sharing some information about them.

Case 1: The Unventilated Black-tile Roof

The north wall of a house in the Southern Highlands of NSW. Note the black tile roof with no eave overhang and no ventilation.

One of my regular readers has recently bought a house near the Greeny Flat in the Southern Highlands of NSW. Although this has been a very cool summer, she is finding it hard to keep the building cool on hot days. She has tried doing what I recommend for the Greeny Flat, which is to close all the windows and blinds during the day and open everything up at night, but it isn’t working for her. She thought it was because there was too much sun coming in through the north and west facing windows so she asked me to come and have a look and make some recommendations on how best to shade these windows.

While this is generally a cool climate by Australian standards, it can get pretty hot in the summer time. There is an unfortunate tendency these days to build houses like this one with brick-veneer walls, dark tile roofs and no eave overhang. In my opinion this should be illegal in Australia. Thermally massive materials are only helpful to the energy performance of a building if they are placed inside the thermal boundary (the insulation and air-sealing layer). Brick veneer puts the thermal mass on the wrong side of the insulation. So does a tile roof. These will not help to keep the building warm in winter and will only serve to keep the place hot well into the night in summer. So when I arrived at this house the first thing I noticed was the black color of the tiles and the lack of any eave overhang on the hipped roof. I don’t know why anyone would build a house with no eaves but I suppose it’s a combination of a current trend and a cheaper way of building. Nevertheless, eaves do several important things: they help to keep rain off the walls, windows, and doors; they help to shade the walls, windows and doors in the summer; and, particularly in a house like this one that has a hipped roof, they provide a way ventilate the attic. With other roof forms like gable roofs it is possible to ventilate the attic by providing louvre vents in the gable end walls. But with hipped roofs there are no gable end walls and the only places you can put ventilation are in the eave overhangs or directly through the roof (using something like a Whirlybird). This house has neither of these.

After hearing what her problems are and having a look around the house, I climbed up into the attic to have a look. It was a cool and overcast day outside but the attic was probably 50degC. As I suspected, there was very little ventilation in the attic, just a few cracks around the edge of the roof. So, in spite of the fact that there was a reflective foil sarking installed directly under the roof tiles, there was a great deal of heat radiating down from the black roof tiles above and accumulating in the attic space. On a really hot day I would not be surprised if that attic got up to 70degC or hotter.

As I also suspected, there was only a thin layer of insulation batts on the ceiling (probably about R1.5) and not very well installed. This means that the heat from the attic can readily radiate DOWN into the house below (hence the science lesson above). So, in effect, her entire ceiling becomes a radiator during the day and well into the night (due to the tendency for massive materials like tile to hold heat). This is why it wasn’t working for her to close the house up during the day. That technique only works if the ceiling is well insulated, the attic well ventilated, and the walls and windows well shaded. Her’s aren’t.

My first recommendation to her was to get quotes for a) installing some sort of ventilation through the roof (either the Whirlybird type of some sort solar powered fan) and b) to add another, thicker layer of insulation to her ceiling (at least R2.0 over the top and laid in the opposite direction to the batts that are already there).

I also suggested that, as a temporary measure she attach some shade cloth to the gutters on the north and west sides of the house and angle it out away from the building to some tent pegs in the lawn. This will serve to shade the north and west facing walls, windows and doors which will further help to reduce the heat gain to the interior during hot days. This is cheap and easy to do and, if it works well, we can look at ways of doing the same job in a more permanent way such as awnings, porches or trellises.

‘Draftstoppa” ceiling fan cap

One further recommendation was that she look at installing something like the ‘Draftstoppa’ pictured onto the bathroom exhaust fans that currently vent directly into her attic. In a poorly ventilated attic like hers this can create serious condensation problems in the attic in winter time. For this reason, ideally, all bathroom and kitchen exhaust fans should be vented directly to the outdoors and NOT into attics or under houses. The Draftstoppa doesn’t correct that problem but adding ventilation to the attic will help to alleviate it. What the Draftstoppa’s will do is to help prevent cold air from the attic dropping down into the house in the winter time. And the extra insulation that she adds for summer cooling will also help to keep the warm air in the house where it belongs in winter.

Case 2: The Steel Framed Floor

Mittagong granny flat with walls and roof of steel-clad foam-core SIPS panels. Note the steel framed, off-the-ground floor structure.

Another project that I’ve been helping with lately is a little granny flat that a friend of mine is building in the back yard of his Mittagong home. The owner is on a very tight budget and doesn’t have a lot of building experience so he has chosen to use a SIPS panel building system which promises to be quick and easy to erect while still providing exemplary thermal performance. Right now, as you can see from the photo above, it looks a bit like he’s building a big esky because the windows and doors will mostly be cut out of the panels after the roof is on. I’ll be very interested to see what it looks like when it’s all finished. Meanwhile he’s finding it relatively simple to cut and fit the panels into place.

These 200mm-thick steel-clad SIPS panels make up the granny flat’s walls and roof.

As you can see from the photo above, there is a LOT of foam in those walls and the roof will be at least as thick. He also will be using double-glazed windows. So five sides of his simple six-sided structure will be very well insulated. The tricky part for him is going to be the steel-framed floor.

As discussed above, heat can travel by radiation and conduction equally well in all directions and steel is a VERY good conductor of heat. So it’s going to be very interesting to see if we can come up with a system that effectively reduces the amount of heat loss through his floor in the winter time. It will also need to be relatively inexpensive, easy to install, and durable. I’m glad he took my advice to raise the floor far enough off the ground to make it fairly easy to move around under the house because all the work of insulating the floor is going to have to be done from underneath. If anyone has any brilliant ideas for how to easily, durably and effectively insulate this floor please let me know.

The other problem with this type of construction is that it doesn’t incorporate any sort of thermal mass inside the thermal boundary of the building. This means that he’s going to have trouble maintaining a relatively constant temperature unless we can figure out a way to store heat inside the house. One possibility that I’ve been exploring for some time now is the use of Phase Change Materials for heat storage. More on that soon.