You are using an outdated browser version not supported by this website.
Click here to upgrade your browser

22 May 2015

Slab Edge Insulation: An Interesting Experiment

From this 'Thought Experiment' I decided to move to a simple unscientific physical experiment.

By writing as I did in November I wanted to stimulate some discussion on the long-term value of external slab edge insulation, when considered alongside the hassle and difficulty of first installing it, and then maintaining it in a manner so that it remains fully effective for the life-time of the building. I got my wish; there was substantial discussion and comments through EBOSS, but I also received a lot of comments from Architects and Designers directly to myself. Most of these were along the lines of "I've always thought that", or else wanting to discuss my questioning as they had not given it any thought.

Due to this interest I made a point of often raising the topic at gatherings of building people. I have been curious to observe that the primary argument in favour is that "we are told that it should be done", or "80% of the slab heat is lost through the exposed edge so insulation must be fitted." There is never any objective scientific evidence presented. On the other hand there is also much strongly expressed opinion that there is little value, especially when considered alongside the cost and difficulty of installing it. Again the views are expressed without objective evidence.

In my November 20014 blog I set out some of my 'Thought Experiment' considerations and so won't repeat them here, except for the principle that everyone has experienced. On a cool winter's day you can be outside wearing cotton clothing and feel reasonable comfortable, but if there is a shower of rain to dampen the fabric you immediately feel cold and focus on finding warmth. It is well established by building science that insulation must be kept dry to perform at its best, and that thermal performance falls off quickly as moisture is introduced into the material. Everyone in the building industry details and constructs the thermal elements of a building so that the insulation remains dry for the long term — so why is the same attention not applied to the slab-edge insulation?

The common detail for slab edge insulation is to fit a 'rigid board' form of insulation material, (rather than fibre pads), against the concrete face gapped from the bottom edge of the wall cladding down to just below ground level or deeper. The exposed outer face is then usually coated with a waterproof material as a water and physical protection surface. The most common material for this is expanded polystyrene (EPS). Extruded polystyrene (XPS) sheet is also available but there is a cost penalty. If I recall correctly, one response to the November blog stated that they were quoted well over $4,000 to edge insulate a slab, but I have no idea what material was to be used or the extent of the work.

When speaking with building product manufacturers and suppliers, they say that their materials and systems are sufficient to keep the insulation dry so that the thermal performance does not degrade over time. I have no problem with accepting this if I look at the product narrowly, (as the laboratory water penetration test does), but if I stand back to take a wider view of a real building, and the factor of time is added, then I have yet to be convinced. When applied to a building how is the top surface, and especially the bottom edge, completely and permanently sealed so that no water will reach the insulation. If moisture should enter how does it escape, and if not is it acceptable to allow an accumulation over time and does this degrade performance to such an extent that the money could have been better spent elsewhere, e.g. for reducing thermal bridges in the framing above?

In theory a fully sealed slab edge insulation system will remain dry, but that is not the reality. Apart from the problem of achieving 100% perfection during the installation, how is full protection from damage guaranteed until handed-over, and then continued for the life of the building? Gardening, lawn-mowing, accidental impacts, etc., etc., can cause hair-line breaches of the protective outer coating, let alone obvious damage resulting in water entering and then not being able to escape. Isn't this the scenario of the 'weathertight homes' problem — we were assured then that the system was waterproof.

Recently I was finally stimulated to undertake the very unscientific experiment which I have been thinking of doing for years. I submerged blocks of expanded polystyrene (EPS-S), graphite infused EPS, and extruded polystyrene (XPS) under water for 24 hours, weighting the blocks before and after the dunking. I wanted to get a rough measure of the water uptake which was as listed below. Submerging the blocks is similar to the situation where slab edge insulation is encased in a waterproof coating, so that should any water enter, it will be held against the polystyrene for a period.

  • For EPS the dry weight was 21gms; after 24 hours under water the weight rose to 36gms
  • For graphite infused EPS the dry weight was 22gms; after 24 hours under water the weight rose to 48gms
  • For XPS the dry weight was 23gms; after 24 hours under water the weight rose to 26gms

Maybe there is someone out there with a mathematical bent who can do the calculation of the increase in thermal conductivity due to the water content, and the consequential lowering of the R-value. I do not present myself as a scientist — just someone exploring their curiosity — so I encourage others to try the experiment to see if they get similar results.

Why do I think this matters? For a start most concrete floors only have a few hundred millimetres of slab edge exposed to the air and the floor surface is usually carpeted in most of the perimeter rooms. Assuming 80% heat loss at the edge, this is 80% of a little so is even less. Insulation becomes more important as the temperature difference between the two sides increase. The difference is much less at slab level below carpet to the exterior, than at the ceiling/exterior wall junction. The cost and difficulty of achieving a properly constructed and robust installation appears to be way out of proportion to the gains made. Maybe there are situations, especially in the north-west quadrant, where slab-edge insulation actually inhibits net passive thermal gains within a sun exposed concrete floor edge.

In November and now, I am questioning the promotion of slab edge insulation for the whole perimeter of all houses, without distinction, because I have not seen any supporting independent objective evidence from 'in-the-field' studies to justify the expense. I'm happy to be persuaded otherwise.

comments powered by Disqus

Posts by Keith Huntington

See All

Get a free weekly digest of essential news

New and updated architectural products, design solutions, inspiration, technical advice and more when you sign up for EBOSS.