Only left 25mm gap above PIR

[23vc]

Hi
insulated my vaulted ceiling, 125mm rafters, 100mm PIR between rafters, another 50mm PIR across the undersides, gaps foamed and taped, then plasterboarded. Obviously only leaving a 25mm gap above the PIR, and where the underlay drapes, it gets pretty close. This was only a small room, tempted to just leave it and do the other ceiling with extra battens to give the required 50mm gap (I mistakenly thought it was 25mm) - reckon it'll be OK or actually likely to give me issues in future? the underlay is vent5 breather membrane
Cheers
John

 

[John D v2.0]

why do you need 50mm? the gap is so there is space between the batten and the breathable membrane so any water running down doesn't get stuck and end up getting through the nail holes etc.. 25mm will be absolutely ample.

 

[23vc]

Sorry I’m talking about the gap between the top of the pir board and the underlay...

 

[John D v2.0]

Yes me too! There's no need for any gap there, the gap is needed between the tile battens and the underlay for the reason i mentioned above

 

[23vc]

Interesting, hope you’re right but Interweb is full of references to a minimum 50mm gap in this situation, presumably to allow condensation out

 

[CJRatch]

25mm will be fine. The gap is there incase any warm air from your room passes the insulation and hits the cold of your membrane and causes condensation. The gap along with eaves vents or similar to allow air into the gap clears the condensation.

With breathable membrane this is less of a worry than it use to be using the none breathable membrane as warm air should pass through the membrane. Along with the fact that you've foil taped all joints warm air should not pass into your roof void anyway as the insulation taped acts as your vapour barrier.

 

[23vc]

Ok cheers guys, I won’t bother counterbattening the next one then and just crack on

 

[John D v2.0]

yes, as CJRatch mentioned, the 50mm continuious ventilated to outside is for where there is a vapour closed membrane e.g. flat roof weathertight layer or old style felt. In your case it's all going to pass straight through.
A condensation risk analysis would confirm the risk is zero, but unless you have a hot shower room and wool with no plasterboard in front, there's not much chance of issues

 

[freddiemercurystwin]

It's supposed to be a 50mm gap if the membrane is draped, 25mm if the membrane if pulled taught. The theory being that you end up with a 25mm clear gap either way, with the drape unlikely to exceed 25mm. The membranes are not designed to work with no gap at all. I wouldn't lose much sleep over though.

 

[John D v2.0]

The membranes are not designed to work with no gap at all.

maybe it depends on the manufacturer's spec, the OP would be advised to check that given the differing advice given here.

My expertise is purely because our builder installed a similar product (vent3) with no drape and the architect said that's only ok if counter battened. But it can be installed fully supported i.e. no space below. The builder just stripped and relaid with a small drape.

http://cromarbuildingproducts.com/wp/wp-content/uploads/2018/06/DataSheets/Vent3-TDS-Jan-10.pdf
"Vent3 when installed as a fully supported system, is laid over the support and secured with counter battens, alternatively the membrane can be installed over counter battens"

 

[23vc]

As said, I won’t lose any sleep over it. The thought of ripping down a decorated ceiling in winter if I get some horrible condensation issue, is still more appealing than redoing it in this weather lol

 

[geraldthehamster]

Assuming the membrane is newly-installed breathable membrane, and not old bituminous roofing felt

 

[23vc]

Assuming the membrane is newly-installed breathable membrane, and not old bituminous roofing felt

Yeah it’s new breather

 

[wwwebber]

I have no such membrane - just tiles and battens and I'm going to fill the existing 75mm depth with 75mm PIR and thus no gap. I'll also seal everything up with foam and foil tape. Correct ? or should I leave a gap ?

 

[freddiemercurystwin]

I would leave a 25mm gap, as when the tiles are eventually replaced by you or the next owner a membrane will be fitted.