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19 June 2015

To Improve a Thermal Envelope, Tackle the Easy Items First

My questioning in last month's blog generated a lot of worthwhile discussion but, apart from one general reference, no objective and specific data on the thermal degradation of polystyrene slab-edge insulation, as it gets wet, was produced.

As well as the comments written in the discussion panel, there were also comments sent directly to myself, including support for bringing awkward questions to the fore. I'm pleased that one product supplier was willing to send me data on their materials, but the others have been silent. Even so, the information related only to the materials themselves, and not to the performance of the installation — the same situation that allowed the 'weathertight homes' problems to develop.

It is not the supplied products themselves which are the problem, it is how they are detailed and installed, and how the whole construction degrades over time. If the slab-edge covering material was only being used below the cladding cavity to decoratively carry the wall face neatly down to ground level, then there would be no problem, (assuming durability is OK), but the difficulty I have is that the material is being promoted as a means of effective thermal insulation. If it is to give positive thermal results then it needs to continue performing for the whole life of the building, or else be easy to replace as it fails.

I have been told there were nearly 1,400 views of last month's blog. From the responses to my thoughts, it seems to me that there is an opportunity for useful research to be undertaken, given that this form of insulation is being promoted as an effective means of significantly improving the thermal performance of our housing. Perhaps there is a building science student out there looking for a study topic.

As I have written, my thoughts are specifically directed to the thermal insulation fitted to the exterior face of un-heated slab-on-ground concrete floors as this is the most common situation. Heated concrete floors, either by passive or active means, are a different problem which in my opinion must be detailed in completely different ways if the insulation is to be effective and efficient, and the floor is to give a net positive to the passive energy performance of the house.

While there is an emphasis on external slab-edge insulation there seems to be little consideration or discussion of other thermal bridging in the building fabric above the ground floor which have a far greater influence on the thermal performance of a house. It is relatively easy to make significant improvements to these weaknesses. In my August 2014 blog I comment on thermal bridging and refer to the deep full-wall thickness window lintels being fitted immediately below the second-storey floor joists resulting in structurally unnecessary thermal bridging.

As an industry we should be tackling the easiest areas first. Two of these are; insulating the perimeter of upper timber floors of multi-level houses; and using appropriate construction details at the exterior wall/window junction when thermally broken aluminium window frames are specified. Both involve little cost and thought, just good design.

In a room, heat rises so that the ceiling is much warmer than the floor, hence the sensible emphasis on ceiling insulation. With a multi-level house any heat that passes through the floor is not wasted because it warms the room above. This is why NZBC-H1 does not require insulation to floors which are contained within the thermal envelope. But, usually the upper floor is carpeted which slows the heat transfer. Heat easily passes through the plasterboard ceiling to accumulate within the timber floor void as it struggles through the more difficult, (more insulated), carpeted layer above. Usually this floor void will be warmer than the room below. Looked at in vertical section this is not a particular problem, but when viewed horizontally it can be seen that the floor voids usually extend to the perimeter wall where there is often only 45mm thick timber, plus the outer cladding, to restrict the outward heat flow.

I have seen and been told that it is standard practice to not install insulation material to this perimeter zone. There is little cost, and minimal work, involved to insulate the inner faces of the timber perimeter for a very substantial gain in thermal performance.

Considering just the unit, thermally broken window frames are better at insulating a house from external coldness than standard aluminium frames. While this is correct from a manufacturer’s point of view, it may not be as true for windows as they are fitted into an external wall, especially if the cladding is on a cavity — the usual form of construction these days. When designing the wall/window junction, and selecting the window frame profile, it become important to establish where the thermal break will be in relation to the zone of the wall insulation. With there being a significant distance from the outer face of the cladding, across its thickness, the cavity and RAB to the start of the insulation, a careful check must be made. If the thermal break is outside of the insulation zone then most of any benefit is lost. A little thoughtful design results in the maintenance of the desired thermal performance.

Why fuss about trying to make with slab-edge insulation work efficiently when there is more to gain for less effort by looking elsewhere.

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