Old Property Reroofing to Warm Roof Advice

[Lears]

Hi, as the title suggests, we're in the process of renovating an old stone wall property near Wales and looking for some friendly advice/ reassurance in all honesty.

 

The existing roof is partly vaulted in one section of the house and normal cold roof in the other. The vaulted section however has rafters approx 50mm thick with 160-200mm A frames sprinkled in and as a result, has barely any insulation at all. On top of this, we're removing a giant chimney stack that spans two floors so already signed up to do some fairly extensive roof rework so now thinking it's worth just redoing the entire thing.

 

Current thinking is to fit new 190mm rafters, woodfibre in between and below with a possible 40mm sarking above to get maximum breathability and efficiency but I'm getting a little lost of how the new rafters will affect the roof height and overall construction. I'm pretty sure the existing wall plate is on the exterior side of the wall which if we fit to, would increase the ridge height by c. 300mm which is considerable. So attached was my initial view that fits a new wall plate centrally resulting in about 120-150mm increase and much less infill on the interior side. The only issue is how the rafters then cut through the exterior wall to create a small overhang... unless we reduce the thickness of the rafters after the birdmouth?

 

Any advice on how to tackle a scenario like this?

[Redbeard]

A Green Building expert gave me a useful rule of thumb re 'hybrid Warm Roofs' (a Warm Roof being one which has all the insulation above the rafters). He suggests a condensation risk analysis for a 'some between and some on top' lay-up, but offered a quick possible alternative, which is that the insulation on the top of the rafters should have at least two-thirds of the R value, with the insulation between the rafters having a third. That reduces significantly the risk of interstitial condensation at the interface. Your '190 between and 40 on top' would not meet that rule of thumb. I realise that there are different approaches with fully-breathable materials (full-fill Warmcel, for example) and I think I may have heard of one of the wood-fibre providers endorsing a lay-up such as you suggest, but it's worth checking with your supplier. A quick calc says 230 of flexi WF with a lambda of 0.039W/mK (Pavaflex, for example) would have a gross U val of nearly 1.7W/m2K *making no allowance for the thermal bridging of the rafters, which will make the adjusted U value worse still. I thinnk Gutex's 'flexi' maybe has a lambda of 0.036W/mK, which would give you a U value of 0.156W/m2K at 230, but again, unadjusted.

 

I can't really answer re the roof height increase. Most importantly, is it detached? The neighbours/planners may not care or notice, or they may. Sorry, I don't knpow, but I am sure others will be along.

[Lears]

42 minutes ago, Redbeard said:

A Green Building expert gave me a useful rule of thumb re 'hybrid Warm Roofs' (a Warm Roof being one which has all the insulation above the rafters). He suggests a condensation risk analysis for a 'some between and some on top' lay-up, but offered a quick possible alternative, which is that the insulation on the top of the rafters should have at least two-thirds of the R value, with the insulation between the rafters having a third. That reduces significantly the risk of interstitial condensation at the interface. Your '190 between and 40 on top' would not meet that rule of thumb. I realise that there are different approaches with fully-breathable materials (full-fill Warmcel, for example) and I think I may have heard of one of the wood-fibre providers endorsing a lay-up such as you suggest, but it's worth checking with your supplier. A quick calc says 230 of flexi WF with a lambda of 0.039W/mK (Pavaflex, for example) would have a gross U val of nearly 1.7W/m2K *making no allowance for the thermal bridging of the rafters, which will make the adjusted U value worse still. I thinnk Gutex's 'flexi' maybe has a lambda of 0.036W/mK, which would give you a U value of 0.156W/m2K at 230, but again, unadjusted.

 

I can't really answer re the roof height increase. Most importantly, is it detached? The neighbours/planners may not care or notice, or they may. Sorry, I don't knpow, but I am sure others will be along.

Thank you for the response. The build up was basically from https://www.steico.com/eur/solutions/new-construction/roof-construction under there 'solid timber' section but I will confirm with them that the 190 to 40 ratio is acceptable as I am making an assumption at the moment. We are detached and out the way on a little narrow lane albeit, we do get traffic come up and down. There's a property up the hill from us but we aren't really in the line of sight for any of the views and we're removing two chimney stacks so I was sort hoping it might be fine but appreciate it's not the best strategy!

[Redbeard]

13 minutes ago, Lears said:

https://www.steico.com/eur/solutions/new-construction/roof-construction 

Got it!

 

OK, so as I understand it you are re-roofing from scratch including the whole timber structure. They show the roof effectively ventilated *over* the rigid sarking, which required the counter-battens which your current roof of course will not have (I assume). I see that Steico suggests the same lambda as Gutes, which makes you closer to complying with the 0.16 target. (I just checked that one. Look at table 4.2 in Part L, re 'new element in existing building', which your new roof would be - target goes up in stringency, down in U value, to 0.15.