May 8, 2016: Underslab Insulation

Before I get into this week’s discussion about insulating under concrete slabs I thought some readers might be interested in a short video  of a presentation I gave on the subject of ‘Shelter’ to an Economic Development Summit held late last year. In the video I talk about some of the problems that I see in current building and design practice in Australia. Then I explain how the Greeny Flat offers solutions to some of these problems and how those solutions can be applied to a wide range of building and design challenges to help us move towards more sustainable lifestyles, buildings and communities. (Click here to view the video if it does not appear below).

Insulation Under Floor Slabs. To do or not to do?

Styrofoam is commonly used to insulate under concrete floor slabs

In last week’s Newsletter I discussed the ‘Comfort’ results from our first two years in the Greeny Flat. In that discussion I made the following comment.

In these readings I have ignored a period when Cintia and I were away last winter and the indoor temperature dropped down to 10.6. Because we weren’t here to operate the blinds correctly no sun was coming in during the day. What is interesting about this is that the house still didn’t go any lower than 10.6 which tells me that we made the right decision in not putting insulation under the floor slab. The constant ground temperature under our house must be around 10.6 degrees. If we had insulated the floor slab we would have been disconnected from this ‘heat sink’ and I suspect that the house would have got a lot colder when we weren’t here for the winter.

This comment raised comments and questions from a number of readers about underslab insulation. A good friend of mine named Ran (we studied Architecture together and Ran now lives and works in Scotland) sent me the following:

Really enjoyed this one. The fact that the indoor temperature never got above 28 is amazing. But it was the comment on the lack of slab insulation which really pricked my interest. Over here we have to install it, and perhaps given the different climate and the heating rather than cooling requirements that is justified, but its always struck me as odd that we should insulate against the thermal mass of the earth.

Here is the response I emailed back to Ran:

Slab insulation depends entirely on the ground temperature. In Montana the native ground temp 8 feet down in the middle if winter was only 2deg C, and colder the closer to the surface. Definitely needed to insulate under floors in those conditions. I debated it here but glad I didn’t. Be careful with information about ground temperatures under houses. Most houses in Montana had a decent ground temp under them because they were constantly pumping heat into the ground. It’s the native ground temp that is important because that’s what you have to either be connected to or insulated from. It varies with depth. Colder closer to the surface in winter so slab on ground is more susceptible to fluctuations.

During the twenty years that I was designing and building in Montana I came to realise that insulating under floor slabs in that climate was absolutely essential. The ground freezes solid down to a depth of about 4 feet (1.2m). When we installed water pipes in the ground they had to be buried six feet deep to prevent them from freezing and bursting. In those conditions, if you don’t insulate underneath the slab, you end up pumping heat into the ground for the entire winter (which can last for 6-9 months). The laws of physics mean that heat will always move from warmer to colder. It is a common misconception to believe that ‘heat rises’. The fact is that hot air rises but heat moves in all directions from wherever it is warmer to wherever it is colder. So the ground beneath a house becomes an infinite ‘heat sink’ where, no matter how much heat moves into it from your home, that heat will dissipate further into the ground and ‘suck’ more and more heat out of your house.

The colder the ground, the more heat it will suck. So in Montana there was no question for me, we HAD to completely insulate under and around the edges of floor slabs. Here in Australia it is much more of a grey area, especially when we consider our different climate zones.

Our good friend Daniel (who helped us build the Greeny Flat and is currently completing a Masters Degree at the Sustainable Buildings Research Centre) emailed me the following:

I wanted to ask about your point on the slab, I think I might be missing something but these are my thoughts currently:

Having the slab insulated looks to have helped in the case of the house being unoccupied, but why would a home designer care about how a home performs when the occupants are on holidays?

In winter I feel that slab insulation would have improved temperatures for all days when you are home as the slab would have stayed closer to the average indoor temperature which is higher than the 10.6 degrees of the indoor temp when you were on holidays.

But summer is where I would assume the uninsulated slab becomes a benefit, as the lower temp of the ground makes the slab into a constant source of cooling, which would be a great benefit during a string of warm days.

In answer to Daniel’s first question; the reason I care about how the house performs when it is unoccupied is because it tells me something about the ‘steady state’ of the building. In winter, with no sun coming in, no heat on, and no other activity in the building, the temperature sat at a pretty stable 10.6degC. Many houses, especially ones that have uninsulated, suspended timber floors and very little thermal mass, would have been much colder than that, essentially fluctuating down to whatever was the outdoor temperature, which here means below zero at night in winter. In such a house, the heating system has to work much harder. To raise the temperature from below zero to, say, 18degC it may have to bring the indoor temperature up by 20degC from its steady state. When the Greeny Flat is occupied, we only have to heat the space by about 7 or 8 degrees from its steady state in order to achieve a reasonable level of comfort, so we are fundamentally more energy efficient. Add to that the fact that most of our heating is done by the sun using the principles of Passive Solar Design and we end up using MUCH less energy to heat and cool our home.

On the other hand, we don’t know what the temperature in the house might have been while we were away if we had insulation under the floor. My guess is that it would have got colder but the only way to know for sure would be to build two identical houses, one with underslab insulation and one without, subject them to exactly the same conditions and monitor the temperature to see what happens.

Insulating under the floor ‘disconnects’ the thermal mass of the floor from the thermal mass of the ground beneath. In effect this means that you have access to less thermal mass for stabilising temperatures (click here to read more about how thermal mass helps to regulate temperatures inside a Passive Solar home) but this is a tricky thing to get exactly right. Too little thermal mass and the temperature inside the house will fluctuate more than is desirable, too much and it will be difficult (if not impossible) to bring the interior temperature up to a comfortable level in winter or keep it down at a comfortable level in summer.

I will be very interested to see how my friend, Glenn Robinson’s, granny flat/studio performs through this coming winter because he used waffle pods under the floor slab and insulated around the edge of the slab as well (see photo above of a similar arrangement). In the house he built a few years ago he only insulated around the edge and not under the floor. So this will be a good test and I will be curious to hear which Glenn thinks is better.

So, to sum things up, I don’t have a definitive answer as to whether it is better to insulate or not under a concrete slab. I can think of five possible options;

1. No insulation under the floor or around the edges of the slab (as we did with the Greeny Flat).
2. No insulation under the slab but insulation around the edge (as Glenn did with his house. I would have liked to use this option on the Greeny Flat but I was not comfortable with the products available to make a termite barrier in this situation. We certainly lose a significant amount of heat through the edges of our slab).
3. Insulate around the perimeter and use waffle pods under the floor (Waffle pods are foam blocks that effectively insulate the majority of the slab but have spaces between where the concrete is in contact with the ground. This may prove to be an excellent compromise in our mild climate).
4. Use waffle pods under the slab but don’t insulate around the perimeter (this would make the termite barrier solution easier but, given the choice, I think insulation around the edge is more important than insulation under the slab).
5. FULLY insulate under the entire slab AND around the edges (this is the way I would build in Montana and I know of one house near here that has been built that way. It is certainly not common in Australia and I’m not convinced that it is worth the added complication and expense).

Having said all that, if you happen to have in-floor heat pipes then it is highly recommended that you insulate your slab as well as you can. When you’re paying to pump heat into your floor slab you want to do everything you can to make sure it stays there to heat the house and not the ground under it.

So what do you think? Here in Australia, should we insulate under floor slabs or not?