Thursday, 12 May 2016

Making Sense of Appendix K - Part 2

In the last post, we gave a summary of some of the main junction types found in a building and listed with SAP Appendix K. Here we continue down the list!

E10 - Eaves (insulation at ceiling level)

It is easy to design and build a good eaves detail, but it is equally easy to do a bad one. As with all things thermal bridging, the key is continuity of insulation, so make sure the ceiling insulation runs right over the wall insulation, both on the drawing and on site. Deeper rafters with the minimum 'bird's mouth' joint depth help to maintain insulation thickness over the wall plate, and may make installation a bit easier.

E11 - Eaves (insulation at rafter level)

Much like the E10 junction, deeper rafters may help, but will simultaneously make it more tricky to achieve a good U-value as there will be more repeating thermal bridging. Insulation over the rafters is better as it can run right over instead of stopping at the inside surface of the structure, but take care to avoid repeating thermal bridging via fixings of the roof finish.

E12 - Gable (insulation at ceiling level)

This is a classic overlooked thermal bridge, typically in a cavity masonry wall where the inner leaf bridges between the cavity wall insulation and the ceiling. Lightweight block inner leaves are better than heavyweight but its best to use a structural insulation block such as foamed glass.

E13 - Gable (insulation at rafter level)

This junction is easier to design for a minimal thermal bridge than E12, but requires some thought. Where possible, the inner leaf should be stopped short of the rafter insulation, which is then simply run out over the top edge of the wall insulation. If using a barge ladder, sit the 'rungs' on top of the inner leaf rather than building them in.

E14 - Flat Roof

This is generally assumed to mean a Flat roof verge (where the roof over-sails the wall). This presents a similar problem to the E13 junction. Where possible, roof joists should stop on the warm side of the wall insulation but this is often not possible, particularly if there is a deep overhang. Ensure the wall insulation buts right up to the deck (assuming you have a warm timber deck).

E15 - Flat roof parapet

This is another classic, often overlooked thermal bridge. It presents a similar problem to E12, and can be resolved the same way. An alternative is to use only the outer leaf of masonry as the parapet, and stop the inner leaf under the roof insulation which runs right out over the wall insulation.

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