Rafter Insulation and VCL

mac25

I'm looking for some advice on how to insulate my attic for a conversion. I have read through numerous posts and have not been able to find a definitive answer on the best method. I am worried about condensation which may lead to mould growth and an unhealthy house. I am also mindful of the need to achieve a .16 U value or better to meet with the current building regulations.


I have two attics to insulate. The first one has 150MM rafters and the second one has 225MM rafters. I have a breathable membrane with slates on top which are fixed to battens (not counter battened). I want to use a combination of Metac in between the rafters and a thermal laminate board on top. It is not practically possible to use a fully breathable insulation material like Metac and achieve .16 U value without sacrificing too much ceiling height. I have no heat recovery system and use natural ventilation in my house.


I am mindful of the need to maintain a 50mm gap between the breathable roof membrane and the insulation between the rafters. Lastly, I understand the importance of a vapour control layer on the warm side of the insulation. I have read from other posts that there should be no more than one third of your insulation on the warm side of the vapour control layer. I assume the vapour control layer on insulated plasterboard is on the plasterboard side so the one third rule is not a problem.


For the attic with 150MM rafters, I was planning on using 100MM Metac between the rafters and an 82.5MM Insulated plasterboard with an integrated vapour barrier. All joints would be taped and any gaps between the walls and floors sealed with expanding foam or silicone. I understand that I can achieve a .14 U value with this combination. I know I could increase the width of the battens by 50MM and go from 150MM Metac plus a 62.5mm insulated slab to get .15 U. This would take up 30MM more roof space and would not be as efficient. It is an option if the 82.5MM insulated slabs are hard to work with though.


For the attic with the 225MM rafters, I was planning on using either 150MM or 180MM Metac with a 62.5MM Insulated plasterboard with an integrated vapour barrier installed using the same method as above. This will give me a U value of .15 or .13. I know I would only have a 45MM gap between the top of the insulation and the breathable membrane if I used 180MM Metac. Is it acceptable to go with a 45MM gap in place of a 50MM gap to achieve the better U value?


I would like to know if the vapour control layer will be good enough by simply taping the insulated slabs together with an aluminium or similar tape? I understand any water vapour that gets through should hopefully evaporate with the help of the 50MM air gap under the slates. I have seen posts that recommend going with just foil backed laminate which is taped together followed by separate plaster board with no foil backing. Is this a better method than going with the insulated plasterboard?


Many Thanks

Calahonda52

Will solar gain be an issue here in the converted attics?
Are you having knee-walls and a flat ceiling area at apex?
You read this?
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx
Page 71 +/-
expanded foam not airtight
silicone will shrink
I prefer a separate vcl in roof spaces as well as separate rigid insulation and then tape all that up before applying plaster board

mac25

Thanks for your response Calahonda52. The attics will be subject to solar gain. There are 3 velux in each with 2 windows in each gable end. There are knee walls in both attics (127CM H & 166CMH) and there will be a flat ceiling at the apex of (157CM W & 162CM W). The ridges run north to south. I have just checked out the NSAI guide and there is good information in there. The ventilation for my cold roof attic will be provided by vents in the soffits. I will say the soffit vents are every few metres and not at every rafter as recommended in the NSAI guide. Also, I don't think there is a continuous 25mm gap at soffit. The soffits appear to be fitted very close to the side of the house. I am thinking about replacing the ridge tiles as they were spot bedded and keep blowing off, so some vented ridge tiles are an option. The attics are first fixed already with the cables run behind the knee walls and in the void in the apex.


Your advice to go with the separate rigid insulation followed by a VCL and plasterboard is good advice as I can see how you would get a better VCL seal. I assume you would recommend Kingspan or Xtratherm boards? As far as I know, all of the rigid boards have a built in VCL at each side. Would two VCL's (One on the rigid board and a separate VCL membrane) cause any issues? Can you recommend a separate VCL like Metac Vario?


I am attempting to finalise the specification for the builder and I don't want to take chances with a loose spec.


Cheers