Situation: existing mid-terrace house with 100 mm rafters, battens and plain roof tiles.

Problem: 50 mm phenolic insulation between rafter and 50 mm under rafters is not going to be enough.

What we really need is 100 mm phenolic full-fill between rafters with 70 mm under rafters. But the problem with this detail is that it requires the addition of a breather membrane and counter batten.

I've seen examples around the neighbourhood where people have re-roofed and their tiles are now sitting proud of the neighbour's and simply sit at an angle over the party walls where they meet the neighbour's tiles.

I don't know if this is because they've introduced a counter batten or because they're simply using different tiles but is this really doable?

What do you think? Would it be possible to add a counter batten and be slightly proud of the neighbour's roofs? As long as we keep the ridge in the same place?

Seems to me it's reasonable on the basis that, given our CO2 reduction targets, the neighbours will likely be doing the same thing within the next decade.

speak to Celotex and get them to do a condensation risk analysis. Chances are if you tape seal all the joints in the 70mm underscoring and silicon all edges and any pipe or lighting penetrations then you will have formed a vapour barrier and be ok. Personally i would go with 75mm between and 70mm below and use nilvent which is BBA'd from memory to 25mm air gap.

Posted By: ShevekWhat do you think? Seems like a no brainer, there's gotta be catch no?

Head height. Installing in-line vents in existing tiling. (neither necessarily a problem).

J

Posted By: WillInAberdeenincreasing the depth of rafters just extends thermal bridge....? better to increase thkness of board below existing rafters?

The problem with this is that we'd need to use 50 mm between rafters (to achieve 50 mm airgap). And the insulation below rafters can't be thicker than the insulation between rafters (because of problems with dew point). So this would only give us 100 mm overall.

?75mm between rafters/75 below, and rely on the view of some that a 25 air gap will do? I prefer 50 myself, but it's worth a thought?

Posted By: Nick Parsons?75mm between rafters/75 below, and rely on the view of some that a 25 air gap will do? I prefer 50 myself, but it's worth a thought?

That is worth a thought definitely, but ideally the insulation below wants to be less than that between. So we'd be taking a risk on dew point and restricted ventilation. Also, that would be a u-value of 0.15 whereas the 100 between and 70 below will give us 0.14.

Ok, fair enough !

What do you think about

the insulation below rafters can't be thicker than the insulation between rafters (because of problems with dew point)

Is this something everyone should design to? I hadn't heard this
before.

Will (not a builder)

Ok thanks!!
I can see where 2 and 3 come from, but how does 1 make any difference? The timber is always going to be close to at external temp AFAICS, irrespective of between: below as it is half exposed to ventilation, and other half is quite conductive... ?

So timber will be colder than dewpoint, but not a problem with ventilation and Vcl...

What'm I missing??

Wood has a lambda value around 3-5 times that of insulation. So although its not a great insulator, its not a great conductor either.

Putting less than half the insulation below the rafter should be sufficient to keep most of the rafter above dew point. However, as mentioned on the other thread, I don't think it follows that you should increase the rafter depth to achieve this rule of thumb & it probably isn't relevant when ventilating directly above the insulation.

David