We need to retile our Edwardian end of terrace house. We have about 100m2 of roof. Currently has concrete tiles over traditional roofing felt held down by 25mm battens. The top floor of the house is essentially in the roof (built that way) and gets very cold. I want to insulate as best as possible but there are some limitations. We are planning to replace the concrete tiles with slate.

1. Can't insulate internally (so no insulation under the rafters).
2. The rafters are only 70mm or 75mm deep (varies depending on which part of the roof - there are three pitches.
3. We are not allowed to alter the ridge height.

If we put insulation between the rafters with breathable felt on top, I understand from building control we will need to leave a 10mm gap above the insulation, thus limiting us to 60mm of insulation.

In checking with Kingspan about their Nilvent breathable membrane, they said the alternative is to lay the Nilvent taught and counterbatten above. As far as I can see, this will raise the height of the roof by about 25mm?

Another possibility I can see from looking at the product brochure for Nilvent, is to lay sarking boards over the rafters, membrane on top of this, and nail the slates directly down, without the need for battens. This would actually be 5mm less than the current set up. This, it seems to me will allow us to fully fill between the rafters with 70mm insulation and, if say we used 20mm Pavatex Isolair sarking boards, this would provide a (small?) contribution to the insulation itself.

Is it worth doing the latter (or is that even feasible?) or should we stick with 60mm insulation with a gap to allow the breather membrane to drape between the rafters? Anything else I haven't thought of?

Also, what insulation between the rafters? Building control say I have to use PIR or something else of equivalent thermal efficiency. Would phenolic board make a significant difference?

Thank you.

Krishna

Posted By: tonyPIR will be better,

Why do you say that, Tony?

you need to comply with building control so should take their advice

Indeed, and they apparently said krishna has "to use PIR or something else of equivalent thermal efficiency". Phenolic has better thermal efficiency and is safer in a fire, so I'm not sure what's not to like.

Posted By: djhWhy do you say that, Tony?

Available depth I would guess. 70-75mm is not a lot of depth.

That's a "desktop study" of course and we all know how realistic they can be... another consideration might be a spray solution which would also ensure near-perfect fitting (difficult with PIR) and also a level of air tightness.

It has a lower conductivity than most other insulants - typically 0.02n compared to 0.03n or 0.04n.

I know there are lots of other factors but I was just guessing.

I'm coming to the same conclusion for my own project. Enabling works to fit the "correct" (wood fibre, maybe EPS) solution are too expensive, so have to go for something else. Even though PIR is more expensive than other insulants this difference is dwarfed by the labour cost difference.

Sorry, yes.