Re-roofing Query

[djcsmith]

Hi, I am currently specifying a new roof covering for an old chapel building (1890's but not listed) which has been converted into office space. Since the conversion, no consideration was given to upgrading the thermal performance or the weatherproofing of the roof structure. The roof has never had insulation (it is a warm roof construction) and the current slate covering is letting in water.

Therefore, as part of the works we are adding insulation at rafter level and providing new natural slate covering. The problem is that we can only really add insulation between and above the rafters which comes with the added problem of raising the roof line and all the detailing issues that come from this around the eaves and verges.

On speaking with the local building control officer, he is willing to have a reduced amount of insulation (i.e it will not have to meet the current Part L U-Value standard of 0.18 W/m2/k).

My question therefore is, if I were to just provide insulation between the rafters (which would negate the issue of a raised roof line) and I then placed a breather membrane either directly on top of the insulation and rafters OR onto counter battens first, would I have issued with pattern staining on the new slate covering due to thermal bridging? In other words, I wil not have a continuous run of insulation across the tops of the ratfters.

Any advice would be appreciated.

Kind regards,

Dan Smith

 

[^woody^]

Yes you probably will get pattern staining, as the temperature differential is significant. The deeper the rafters the less the chance.

Choice of ceiling decoration may help

 

[djcsmith]

Even if I do lay a breather membrane across the entire run?

 

[^woody^]

Breather membrane has no insulation qualities

 

[djcsmith]

Ok. Do you know of any relatively thin insulation products I could use for the 'over rafters' insulation?

I know there are the multifoil products but the issue I have is that this is an old building and therefore its construction was built to breath anyway so I need to avoid anything which may increase condensation etc.

Many thanks

Dan

 

[^woody^]

IIRC, all the multi-foils are about 25mm thick and require a sealed air-gap of 25-50mm too

But the main thing is that they need to be on the warm side of the roof, as they probably wont do much insulating on the top of rafters. You need to check the specific product installation guides

 

[datarebal]

multi foils such as tri iso super 10 can be fitted either side of the rafter the only down side is required counter battening... so the roofline is still raised somewhat.