Old cold roof to Modern warm roof

[thompo5015]

Hello,

 

We currently have a roof from around 1950s consisting of Plasterboard, 45x100mm rafters, Sarking boards, felt then concrete tiles .

There has been an attic conversion carried out (no planning) around the 80s and we have zero insulation

 

Would like to convert it to a warm roof with decent insulation 2-300mm, but this wont be for a few years.

 

In order to insulate for now im wondering if this would have any detrimental effects and could be incorporated into the conversion to a warm roof.....

 

Full fill the 100mm with an open cell spray foam, I know some folk will shudder at this thought!.

Cost and U-value is comparable to EPS but install time is Hrs vs days because no 2 rafters are the same spacing or parallel to each other!

PIR obviously better but cost/install time again, sprayed equivalent closed cell foam is 4x the price for same thickness!

 

I know the rafters are meant to breathe, currently and probably since the roof went on, sofits and eaves are completely enclosed, standing in the attic rooms with no plasterboard and a howling wind, the only air movement comes from a poorly sealed velux. All timbers are sound and show no signs of degradation.

 

Would like to convert it to warm roof in  couple of years by

Remove tiles and felt.

Cross batten onto the sarking boards (to maintain airtight seal with spray foam)

Fit 2-300mm EPS, batten again and install a composite/lightweight tile rather than concrete to keep total weight comparable with existing, keeping main structure of roof intact.

This allows us to extend the eaves to tie in with planned EWI that should give us good airtightness and eliminate any thermal bridges.

 

Is it a viable option to effectively sandwhich the existing sarking in a warm roof conversion?

 

Any thoughts?

 

Cheers

Edited September 21, 2018 by thompo5015

[JamieL]

Seems a lot of work for a temporary fix. And what if when you get the tiles off you decide the sarking boards need replacing? Surely quite likely they will be rotten in a few spots. Also I would worry about gaps between the sarking boards, would the spray foam not squeeze out through any gaps? My go to temporary fix would be rockwool between the rafters. Better than nowt.

 

Thinking outside the box do you actually need the loft conversion in the short term? Or could you just use it for storage for now and do the whole job properly when you have the time/money? If so you could just put normal loft insulation in under the floor boards and leave it unheated. Depends how hard the floor is to lift of course.

[thompo5015]

Cheers for the reply,

 

I would hope to incorporate the sprayed foam into the proposed new roof structure when the time comes, so I view it as a 1st stage rather than temporary solution lol.

 

Upstairs currently is only heated when the stat calls for frost protection. Would prefer to keep the floor intact, as it is sound and I fear lifting it would leave it squeaky or needing replaced.

 

We had a lot of the tiles up when removing Chimney stacks at either end of the place and installing the solar panel. I had a roofer give structure a once over when resetting the hip ridges and he didn't see any problems and was surprised it was so solid with timbers half the size of a modern build, probably due to the sarking tying it all together.

 

No rot visible anywhere from the inside and its bone dry during/after storms. 

Sarking is a good fit, no gaps greater 5mm (odd knot that dropped out) and this is covered by felt so any bleed through of the spray would be stopped by the felt. Agreed we wouldn't know the full extent of any sarking issues until the felt is removed.

 

I had previously thought of lining the inside of the sarking with some paper to break the majority of the bond from foam to wood, allowing complete removal of sarking to get insulation to insulation contact when going to the next stage rather than encapsulating the existing sarking.