Felt or not

[jdt]

Hi, i`m upgrading an exsisting roof to building
regs insulation values, the rafters are 3"deep (built 1875)
the conservation officer has sugested not stripping
the roof or introducing breathable felt. Do insulation
systems need the roof to be felted. Also any preffered
methods with small rafters bearing in mind the property is
terraced. thanks.

 

[noseall]

There is argument to suggest that older (pre-felt) roofs are healthier than modern ones, being as they are allowed to 'breathe' freely.

'Cold' deck roofing systems i.e. those roofs that incorporate the insulation between and below the rafters all require ventilation. So, in answer to your question - no, felt is not a prerequisite and does not add or contribute towards the thermal layers.

Use foam board insulation e.g. celotex, kingspan, quintherm etc. You are likely to need something like 50mm between and 70mm underneath (under-draw) with a 25-50mm air gap on the cold side i.e. above the insulation or just beneath the tiles.

In order to achieve an air gap, whilst only having narrow rafters, is to add a batten to the underside of the rafter, thus deepening it.

 

[freddiemercurystwin]

In order to achieve an air gap, whilst only having narrow rafters, is to add a batten to the underside of the rafter, thus deepening it.

He can forget that if its listed!

 

[noseall]

In order to achieve an air gap, whilst only having narrow rafters, is to add a batten to the underside of the rafter, thus deepening it.

He can forget that if its listed!

Could you imagine the alternative - warm deck!

Those heritage bods would pass out if they saw a visible 6" step in the roof!!

 

[alittlerespect]

Hi

Use 50mm PIR insulation between the rafters which will maintain a 40mm air gap for ventilation between the insulation and the slates then add another 50mm or so PIR insulation to the underside face of the rafters. I say 50mm or so, what I mean is whatever residual thickness is required to meet the discretion of the BCO.

PIR is vapour resistant and you would not need to be worried about a vapour barrier, however, for good measure you could add the vapour barrier before fixing the second layer of insulation to the face of the rafters.

Regards

 

[A1chippy]

hi what about (tri iso10) foil insulation?. just a thought