insulation & air tightness & dormer window

YourAverageJoe

I have finished my air tightness. The dormer windows have wooden structure. I have put 50mm of rockwoll between the studs with 50mm air gap behind it. I then put the membrane on and intended putting 50mm rockwoll over the membrane. Now I am thinking that 100mm is not enough. Does anyone have any opinions on this or does anyone know if I could put 100mm over membrane but would that not cause condensation problems on inside of membrane due to majority of insulation on inside of membrane.

gman2k

Your membrane should be on the warm side of the insulation.

YourAverageJoe

What actually happened was i had put 150mm extratherm throughout the sloped roof in the living areas. I had intended putting an inulsated plaster board slab next so the mebrane was put in between the two layers. I then decided not to use the insulated plaster board but a layer of rockwool instead covered by a sheet of standard plasterboard. So, in 95% of the roof the membrane is on the warm side (all be it behind the rockwool).

It is just the small dormer window area that i have found myself in this position. Is 50mm rockwool, then membrane, then 50mm rockwoll very bad for condensation? Is it very bad thermally? Could i use 100mm rockwool in that small area just around the dormer? Any thoughts?


Very long reply, sorry.

beyondpassive

oops posted twice

beyondpassive

Can you line the entire roof and dormers in softboard and do away with the ventilation gap as your dew point is outside the timber zone. Te 50mm gap will result in your rockwool being ineffective. If you use 100mm rockwool at the dormer, the surface temperature of the wall here will be 5 degrees below air temperature in the room and if humidity gets over 80% you will get condensation and mould.

Who is certifying compliance with Part L building regs for you?

YourAverageJoe

I have an architect/engineer who is over seeing it. I will give him a call this evening and ask him to take a look at it so. Sounds like i have a problem! Its a pain as I only have a very small area like this, approx 300m x 300mm on each side of three windows and aprox 1m x 1m on each side of one window.

Thanks for the advice

beyondpassive

Is he/she qualified as both architect and engineer?

Dormers have been built the last 15 years with a bit of quilt and nothing at all wher the cutesy cottagey window yokes pop out. Don't expect to get the right answer. Get back to the drawing board and do it right, oil aint gettin any cheaper and carbon taxes are only going one way.

YourAverageJoe

No, he is not but there are different guys in the office so am covered.

I will have to think about it so and will post up my solution as it must be a problem others are having.

YourAverageJoe

Passive,

What if i was to counter batten on the room side of the membrane and put on 100 or 150 mm rockwool. is there a problem then because of the position of the membrane, i.e. more insulation inside than outside the membrane?

beyondpassive

That would work better, be carefull that the rockwool does not slump. You also need to thermally break the studs so your battens could run counter to the studs. You could line the inside with osb board to get an airtight barrier and then create a services cavity, which again is insulated. But that might be getting a little too complicated.

YourAverageJoe

spoke with the architect/engineer.

He advised me not to interfere with the membrane, to put the 50mm rock wool on like planned but then put a 50mm insulated slab over the whole lot.