Vaulted roof with no ventilation below roof membrane - how best to insulate?

[oranjeboom]

I have a vaulted roof, and wondering how best to insulate it as there is a section of the roof that won’t have any ventilation at the bottom of the rafters as there are no eaves. Difficult to explain, so let’s for the sake of keeping things on topic assume it’s a case of semi-detached houses – one has vaulted roof as in the pic below. No eaves therefore along the central valley.

 

My construction so far is:

 

• Slates
• battens (50x 25mm)
• Breather membrane
• 175mm rafters

 

My plan was to:

• fill in between the rafters with insulation (full fill with 200mm), leaving no ventilation gap as I have no ventilation pathway at the bottom of the rafters (i.e. valley). Taped joins.
• VCL
• Insulated Plasterboard

 

NB: I don’t need a service void (will be using uplighters)

 

If I fully insulate between rafters and ensure the VCL is carefully taped up along all joins, should there be much condensation risk? I have breather membrane on top of rafters and the ridge is ventilated, so warm air beneath the tiles has an ‘outflow’ at the top at least. Internally, I will have MHVR.

 

As for insulation, I need to check u-values but was thinking of going for PIR as my rafter spacings are irregular in any case so would have to cut whatever I use. May as well improve u-values and use PIR rather than Frametherm. Probably a bit more of a pain cutting PIR to size, but would foam in every crevice/gap, then add VCL carefully.

 

I have considered using foam (icynene) but concerned contractor will bodge it up with too much uplift of breather membrane. And also the cost is an issue!

 

TIA!

[Temp]

If the breather membrane is vapour permeable you can usually get away without ventilation below it (eg no need for the normal 50mm ventilated void). However you should allow a small void to allow the membrane to "drape" between the rafters. This is to avoid pressing the membrane to the underside of the tile/slate battens. The drape allows water blown under the slates to run down the membrane into the gutter. If you don't do this it can pool above the batten causing the batten to rot. (Edit: I'm not clear how the spray foam companies avoid this issue or even if they try to).

 

The plastic sheet VCL stops water vapour caused by people inside the house getting through gaps in the insulation to the cold membrane where it might condense. So essential to do good job of the VCL. Not so essential to tape the joints in the insulation as well. Seal any big gaps with squirty cream foam.

 

If you fit insulated plasterboard the VCL will be _in_ the insulation layer rather than entirely on the warm side of the insulation layer. That's should be OK provided the VCL isn't too far towards the cold side of the combined insulation. I think the usual rule of thumb is to ensure that 2/3rds of the insulation is outside the VCL with only 1/3rd on the inside.  Ideally you would get an interstitial condensation risk analysis done on the proposed make up to ensure the VCL wasn't too far towards the cold side. Perhaps run your make up past the company that will be supplying the insulation.  Some insulated plasterboard may also include a VCL?

 

 

 

Edited April 14, 2017 by Temp

[oranjeboom]

17 hours ago, Temp said:

If the breather membrane is vapour permeable you can usually get away without ventilation below it (eg no need for the normal 50mm ventilated void). However you should allow a small void to allow the membrane to "drape" between the rafters. This is to avoid pressing the membrane to the underside of the tile/slate battens. The drape allows water blown under the slates to run down the membrane into the gutter. If you don't do this it can pool above the batten causing the batten to rot. (Edit: I'm not clear how the spray foam companies avoid this issue or even if they try to).

 

Thanks @Temp.  Yes, the membrane is vapour permeable, so that extra 50mm will help with additional insulation, though will leave maybe 10mm void. Probably bang in a row of nails 10mm from the top of the rafters to stop me pushing the PIR all the way against the membrane.

 

17 hours ago, Temp said:

The plastic sheet VCL stops water vapour caused by people inside the house getting through gaps in the insulation to the cold membrane where it might condense. So essential to do good job of the VCL. Not so essential to tape the joints in the insulation as well. Seal any big gaps with squirty cream foam.

Just a bit anal with my insulation. I've seen how the cowboys have done it elsewhere. And if there's any 'leaks' in the VCL the foiled PIR can act as a secondary VCL with minimum airflow through gaps. So will probably get my shaving foam out for every gap and also tape all joins. This is a relatively small area of 14sqm.

 

18 hours ago, Temp said:

I think the usual rule of thumb is to ensure that 2/3rds of the insulation is outside the VCL with only 1/3rd on the inside.  Ideally you would get an interstitial condensation risk analysis done on the proposed make up to ensure the VCL wasn't too far towards the cold side. Perhaps run your make up past the company that will be supplying the insulation.  Some insulated plasterboard may also include a VCL?

 

Yes, i think the more expensive ones have foil, so I could even end up with 3 VCLs. I'll probably end up going with your 2/3 rule

[Declan52]

I think you can only fully fill with no ventilation gap if you cross batten the tiles. Check with your BC guy before you start.

[Crofter]

It's very little work/cost really to add a counterbatten. I think TRADA specify a minimum of 12.5mm, although I used 25mm.

I had to do this because I had sarking so you really can't skip the counterbatten if you are going that route.

 

Also in the back of my mind was my parents' house- sarking, tiles, no counterbatten = rotten battens where water cannot run off. For the sake of a few pounds worth of wood it is going to create an awful lot of work when they finally have to rip all the tiles off and replace the rotting battens.

[Cpd]

 

Dont read this unless you are totally mad and suffer from overly attentive to detail when dealing with insulation.......

really dont read it........

I have done something similar was working with 220mm rafters, my build up so far is

slates 

breather membrane

22mm sarking board

i then put 50x25mm battens up against the inside of the sarking boards nailed to the rafters.

i then put in 50mm calotex and held each bit in tightly against the batten  with multiple bits of wood.... i then foamed around the 5mm gap i had left round each board, the foam could not go passed the batten as the calotex was held tightly up against the batten. I then cut the foam away and removed all the bits of wood and then foamed any gaps where the bits of wood had been..... then cut that back and taped everything....... this gave me my 50mm gap between sarking and insulation and a very tightly sealed layer between inside and outside. I felt that there was just no way of properly sealing the gaps around the insulation if i had put in 170mm thick sheets, it would be a bit like spraying and praying with the foam. I then cut 120mm blocks of insulation to bring it out level with the inside of the rafter faces, temporally cross battened it on the inside at 400mm intervals to pull the 120 insulation down tight against the 50mm, foamed it all and removed battens, foamed any gaps that were hidden behind battens and then cut it all back and taped the whole lot..... my next job is to put large sheets of insulation up to mitigate the cold rafter ends, foam and tape. I am then thinking of OSB over the whole lot followed by two layers of acoustic plaster board.  Well i told you not to read it........ honestly it was a horrid  job and if i could have seen how horrid it was going to be i would have ripped all the slates of and put all the insulation outside of the rafters..... shear madness. If you have no choice, dont  believe me or are simply  a bit mad like me..... then fill your boots and send your reply on a postcard.