We are currently modelling a dwelling whereby the client is looking to use custom skylights for the top level. The manufacturer has provided the following thermal performance values for the units (U = 2.0 , SHGC = 0.50). My understanding is that as the only way we can model this within energy rating software for NatHERS is to use the default double glazed skylight, which has the following thermal performance values (U = 4.22, SHGC = 0.72).
Our issue arises as there is quite a large jump in the performance values that can be modelled when compared with the values that would actually be used, and this is reflected in the rating. When the model is rated with the default double glazed skylight values we get a rating of 5.4-Stars. However in a test we completed with velux fixed skylights (U = 2.6 and SHGC = 0.24) the rating jumps to 6.6-Stars. We therefore believe that the actual rating using the preferred skylight would fall in the middle and would therefore meet the minimum 6.0-Star requirement. As there are limited options for skylights, is there anything we can do test/show that the proposed design compiles?
The options for roof windows in FirstRate5 software are very limited.
The best way of getting the closest match possible for the intended product will be to select a combination of 2 or more of the options available and allocate proportionate areas of each option to approximate the overall performance of U-2.0 SHGC 0.5. The result will necessarily be conservative for U-value and reasonably accurate for the SHGC number.
A summary of the procedure should be included in the additional information comments panel so that the NatHERS Certificate contains a clear explanation as to why the modelled values are different to the specified product.