Options to reduce insulation thinkness in loft conversion
Discussion
We are soon starting a loft conversion and the architect has spec'd 150mm of rigid insulation (kingspan etc) between joists and 37.5mm Kingspan k18 or similar (plasterboard + rigid insulation) as the finishing over the top of this.
Space is a bit tight are there any options other than the kingspan k18 that will offer the same u value but not as thick?
Space is a bit tight are there any options other than the kingspan k18 that will offer the same u value but not as thick?
It's been a real war of attrition this whole area.
The original spec was 100mm in between the rafters with 50mm air gap at sarking, however as the roof joists terminate in the wall head there is no eaves gap or air flow. The BC first requirement with this was to fit a roof vent to every joist space - this would have made the roof look ridiculous with 20 vents in a 9m run.
Anyway we found a new solution and this is the last issue I want to address so yes I am carefully considering it as I do worry about interstitial condensation!
The original spec was 100mm in between the rafters with 50mm air gap at sarking, however as the roof joists terminate in the wall head there is no eaves gap or air flow. The BC first requirement with this was to fit a roof vent to every joist space - this would have made the roof look ridiculous with 20 vents in a 9m run.
Anyway we found a new solution and this is the last issue I want to address so yes I am carefully considering it as I do worry about interstitial condensation!
strath44 said:
It's been a real war of attrition this whole area.
The original spec was 100mm in between the rafters with 50mm air gap at sarking, however as the roof joists terminate in the wall head there is no eaves gap or air flow. The BC first requirement with this was to fit a roof vent to every joist space - this would have made the roof look ridiculous with 20 vents in a 9m run.
Anyway we found a new solution and this is the last issue I want to address so yes I am carefully considering it as I do worry about interstitial condensation!
Breather membrane and vented ridge negates this. Your BCO is a dick.The original spec was 100mm in between the rafters with 50mm air gap at sarking, however as the roof joists terminate in the wall head there is no eaves gap or air flow. The BC first requirement with this was to fit a roof vent to every joist space - this would have made the roof look ridiculous with 20 vents in a 9m run.
Anyway we found a new solution and this is the last issue I want to address so yes I am carefully considering it as I do worry about interstitial condensation!
roofer said:
Breather membrane and vented ridge negates this. Your BCO is a dick.
Agreed, I done dozens of older properties without soffit ventilation, high and low level vents were always accepted and worked a treat. I'm currently upgrading an existing loft (never had regs) BCO has agreed to 80mm in between rafters and 30mm below, this allows for a decent air gap and thermals far beyond what the rockwool was providing, it's not up to current levels but he has allowed a bit of give and take.roofer said:
TLX gold over rafter, 140 mm under will give .16
120 mm will give .18 with the TLX. No need for insulated plasterboard.
How does the TLX Gold work ? Do you still have an air gap below the TLX Gold and the rigid foam between the rafters for circulation, or block that completely and just rely on an air gap around the tiles and battens ? If it's the former, surely the TLX is on the cold side both sides of it ?!120 mm will give .18 with the TLX. No need for insulated plasterboard.
strath44 said:
We are soon starting a loft conversion and the architect has spec'd 150mm of rigid insulation (kingspan etc) between joists and 37.5mm Kingspan k18 or similar (plasterboard + rigid insulation) as the finishing over the top of this.
Space is a bit tight are there any options other than the kingspan k18 that will offer the same u value but not as thick?
Wouldnt 100m king span be sufficient? Sounds like overkill? Space is a bit tight are there any options other than the kingspan k18 that will offer the same u value but not as thick?
I don't know if you'll get away with anything thinner than the k18s; even the multifoils require their own thickness + minimum 25mm air gap then plaster board: The k18s do have their own integral vapour barrier. Re. the ventilation, I'm assuming this is a mono pitch roof, you can use these if you do need to re-think and have to vent over the top of the Kingspan Abutment vent BTW shouldn't your architect have sorted all this out on your behalf?
brrapp said:
roofer said:
No need for insulated plasterboard.
The insulated plasterboard won't be in the spec for overall u-value, it'll be there to stop any cold bridging through the rafters into the room.http://www.tlxinsulation.co.uk/assets/Uploads/gold...
ks building regs regarding U-values etc are.Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff