In a follow up to our previous blog about SAP in Scotland, we delve a little deeper into the key differences between SAP in Scotland compared to England and Wales.

A couple of interesting differences that weren’t mentioned are the opening orientation and thermal mass.

For England/Wales, specifying a greater proportion of south facing glazing increases the solar gain and therefore reduces the DER. However, the notional specification uses the same opening orientation and will have the same proportion of south facing glazing, therefore the TER is also reduced, so therefore this doesn’t particularly help with passing.

However, Scotland’s notional building assumes all openings are east/west facing, so adding more south facing glazing in the proposed building will benefit and reduce the DER because of solar gain, but will likely increase the TER (because more glazing means more heat loss through the fabric of the building), helping to pass compliance. This means that position of glazing is much more important in Scotland and south facing windows can be much more beneficial in meeting compliance.

Note that opening area (including doors) in the notional building is limited to 25% of the total floor area in both England/Wales and Scotland, so adding further openings past this in your proposed specification will not change the total glazing area, but the proportion of distribution between different orientations will still match that in the proposed dwelling.

The opposite situation happens with thermal mass. Low thermal mass buildings (such as timber frame) usually have a lower heating demand because they are not using energy to heat up the building fabric itself. The notional building in England/Wales assumes a ‘medium’ thermal mass. This means if the proposed building has a low thermal mass (such as timber frame), this will be beneficial for the SAP assessment. However, in Scotland, the thermal mass of the notional building is the same as the proposed dwelling. So if a dwelling is changed from ‘medium’ to ‘low’ thermal mass, this will reduce both the DER and TER which doesn’t really help.