Making significant changes to thermal elements (walls, roofs or floors) would normally require Building Regulations approval and require the thermal insulation of the element to be upgraded to a reasonable standard. Walls are defined by Regulation 2(3) of the Building Regulations 2010 as being thermal elements.
The extent to which the work on the element is controlled and the amount of upgrading needed depends on the particular circumstances of the thermal element. Generally, when it is renovated then it should be upgraded, where it is cost effective to do so, to the standard set out in the Approved Document. See section 5 and Appendix A of Approved Document L1B.
Further guidance on this is available in Approved Document L1B covering:
- Guidance on thermal elements (Section 5 pages 17-18)
- Explanation of when renovation works trigger requirement for upgrading insulation and what additional work may be required.(see Appendix A and Table A1 pages 21-23)
You should fully consult the Regulations and the Approved Document and, if you are in any doubt, seek advice before commencing work. The definition of a thermal element does not include windows, doors, roof windows or rooflights.
Insulation can be placed between the ceiling joists. Again, the thickness will vary depending on the material you choose to use.
If the roof has no ceiling then the insulation can be placed between the rafters and ventilation maintained as described above - in which case the ridge should also have vent tiles installed to allow for through ventilation.
Further guidance on insulating roofs can be found in Approved Document L1B, Table A1.