Building Regulations and Window Energy Ratings
7) Give simulations room for scope
We advise you to always go for your WER simulations with the worst performing components that will achieve your required WER. This will give you and your IG supplier the biggest scope to exchange components as new and improved products enter the market.
Thermoseal Group urges sealed unit manufacturers to write to their customer base detailing the sealed unit component list that they use and suggest preferred unit constructions. In this correspondence the sealed unit manufacturer should include their declaration to EN1279 part 5 Annex ZA.
Building Regulations and WERs Page 13