ECOBuild 2013: Fixing through EWI

External wall insulation is a great idea in that it wraps the entire house and eliminates almost all of the thermal bridges associated with traditional cavity builds. It does present some challenges that don’t exist for cavity builds, in particular securing things to the wall.

The most common items to be attached are probably rainwater downpipes, satellite & aerial poles and external lights.

In any case the fittings will need screws right through the insulation into the block work itself. While these are a minor thermal bridge in their own right the overall impact is probably small, almost certainly due to the relatively low number of fixings.

There could be a concern over creating cold spots within the structure, either at the joint between the structure and the insulation or even on the internal surface. These in turn could lead to condensation and the risk of mould or structural damage. It’s probably getting into the realms of paranoia, but is something to be aware of.

The second issue is that the fixing needs to be done in a way that doesn’t rely on the compressive strength of the insulation. This could damage the insulation or render leading to premature failure.

Thirdly the fixing needs to allow for some differential movement, particularly in the case of large satellite dishes where there is a lot of force caused by the wind.

Finally it needs to be relatively straightforward to seal around the fixing to ensure it won’t let water into the insulation and either impact the performance or even cause structural damage.

It seems that the standard solution to avoid insulation compressions is to use pieces of timber replacing the insulation in small sections to tighten against. This introduces a small area with effectively no insulation and while timber is a relatively poor thermal conductor it will still be a bridge. It has the advantage of being simple and uses readily available materials.

I’ve seen a similar idea using some thick wall plastic pipe to act as a collar around the fixing, the advantage being that the inside of the pipe can be filled with a flexible foam insulation to limit the cross-section of the thermal bridge while still allowing for some movement. Obviously, this doesn’t have the same contact area with the substrate and may not be as stable as the timber block.

I can’t help thinking that there is an opportunity here for a product that really breaks the thermal bridge, possibly made out of a tough plastic material. I have some ideas here which could work. I’ll need to give it some thought.

I did see one product at ECOBuild aimed at fitting satellite dishes through EWI. This consists of a large steel bracket bolted to the wall under the insulation through a thin sheet of plastic with a piece of angled steel coming out to support the bracket for the dish. The supposedly clever bit was the use of a short box section sticking thoroughly the top of the insulation with a custom made plate through which the steel angle protrudes. The selling point seemed to be the mechanical isolation of the bracket and the render meaning improved tolerance to movement. While they weren’t doing so you could fill the box with a compressible foam insulation to reduce the losses. However, a steel angle like that rigidly fixed to the blockwork still seems like a large thermal bridge, although perhaps not much more than a couple of screws. Again, I can’t help feeling that there should be a better solution.

In all cases the watertight seal is provided by the use of silicon sealant/mastic at the penetration. It works to hold my fishtank together, so if well applied it should work here too.