william wrote
Hi William - this was the topic of my lengthy discussion with Hugo a while ago. I hope he replies as well as me, as he may well have a different view on this.
The method you describe for insulating roofs in Ireland is basically the same as the way timber-framed walls are constructed and insulated in England & Wales, except here it is normal practice to include a vapour control layer under the plasterboard and ply sheathing on the outer face of the timber framework, which is then covered by the breathable membrane. This construction for walls is recommended in British Standards and by the BRE document "Thermal Insulation: Avoiding Risks". However, AFAIK there is no actual Regulation about this - the ventilation of wall cavities or the risk of condensation in timber-framed walls is not covered by the Building Regulations. (It's possible I might be corrected here!)
But for roofs, Regulation F2 of the Building Regulations (England & Wales) requires "adequate provision to be made to prevent excessive condensation in a roof or in a roof void above an insulated ceiling". This applies only to the so-called "cold roof", where any part of the structure and any decking is not fully covered by the insulation. One acceptable method of complying with this is given in Approved Document F, and this is what your searches are finding. It requires a minimum 50mm deep ventilation gap above any insulation between the rafters, the idea being to allow air to circulate round the timbers and evaporate any condensation. The AD considers vapour control layers or barriers are not an acceptable alternative to this ventilation and there is no reference in the AD to breathable sarking membranes. (The current edition of AD F is 1995 and maybe technology has moved on since then). The above BRE document does refer to breathable membranes, but this applies only where the insulation covers the rafters (ie a warm roof) and an air gap of 50mm is still required, this time above the membrane. So, in simple terms, the recommended construction for timber-framed walls is not acceptable for roofs.
Personally, I cannot see why there should be any difference between the permitted constructions of timber framed walls and roofs, as the risks of damage by condensation are identical, but there you go. The really high levels of insulation required now are becoming unrealistic IMO, both in terms of the extra energy saved and the potential of causing damage to the structure and finishes. I think the 50mm air gap to allow condensation to evaporate is a good idea and should be retained as a minimum. I remain doubtful about the construction of timber-framed walls (and Irish roofs), where condensation can and will occur on cold and inadequately ventilated timber surfaces. Time will tell whether or not we are creating another major inherent defect similar to PRC houses.
Peter