The inter-relationship of U-values & inter-stitial condensation
The location of insulation within a wall has a large bearing on how moisture and vapour move through it, and to what level it can be safely insulated. This is not evident in TGD L.
From the perspective of inter-stitial condensation we find that:
· for internal insulation, the lower the U-value the better
· for external insulation, the higher the U-value the better
Based on our hygrothermal simulations it appears that the risk of failure for internal insulation strategies where U-values are better than 0.50 W/m²K is unacceptable. It might take a few years but damaged walls, collapsed insulation and worse homeowner’s health could be at risk. Articles ‘Breaking the Mould IV’ and ‘V’ give a more detailed insight into this issue.
A Code of Practice for Retrofit needs to reflect this focus for existing buildings.
Given that the Government wants to move towards a 0.21 U-value for walls in new dwellings, this suggests that we’ve actually reached the end of the road for internal insulation as a new-build form of construction. It should therefore no longer be allowed.
Inter-stitial condensation assessment method used
In Section B.3 of Appendix B of the draft TGD L(2010), it says that inter-stitial condensation “full checks should be performed (…) in accordance with IS EN ISO 13788″, and it says that “IS EN ISO 15026 can also be used”. This is bad guidance.
IS EN ISO 13788 relates to the Glaser or dewpoint Method, and IS EN ISO 15026 relates to hygrothermal simulation. The first is a simple assessment created in 1958, is very limited in what it assesses and was designed for lightweight structures only (which is clear from reading the scope of the Standard). The second is a highly accurate computer simulation, with hour-by-hour inputs allowing for a range of vapour and moisture transport mechanisms and design-year weather. There is no comparison, only the second is suitable for assessing the issues facing highly insulated airtight buildings. For more information, see post ‘Condensation assessment: which approach?‘.