Vaulted roof insulation condensation risk?

[LinearPancakes]

Should we be worried about condensation on the airtight membrane in the below setup?

 

* Vaulted roof with 200mm deep timbers supporting roof membrane, counter battens and slate tiles

* 120mm PIR between the timbers (80mm ventilation void above) (**foil taped to the timbers** which seemed like a good idea, but I'm wondering if it would have been better without since the intello plus membrane allows moisture to pass through)

* Airtight membrane underneath (Pro Clima Intello Plus)

* 50mm counter battens / PIR below that (inline with each other)

* Skimmed plasterboard

 

I believe more typical would be to have the airtightness membrane directly behind the plasterboard, meaning the membrane is at the temperature of the room, so no condensation risk. (And less cold bridging where the timber and counter battens intersect) But that would have meant lighting/power cabling going needlessly through and back out of the airtight membrane, which we wanted to avoid.

 

Hard to know what temperature difference the lower layer of insulation will create between the room and the membrane and whether or not that would be large enough to cause condensation to form.

 

We'll have an MVHR in place, so there shouldn't be too much buildup of humidity. Though I'm wondering it that could actually bring in more humidity from outside in the winter.

Edited January 19, 2023 by LinearPancakes

[Mr Punter]

I can't see an issue with that build-up.

[Iceverge]

Should work from a condensation point of view. 

 

 

 

It'll be a  pain cutting all that PIR however and getting it to fit snuggly. Not great from a noise perspective or heat protection aspect either. Running wiring behind the PIR will be tricky as you'll need to compress it or cut the PIR where it crosses the rafters. 

 

 

 

 

This setup would be better in my opinion. If you can't get cellulose then blow in mineral wool would have the same ease of labour, probably a bit cheaper. It wouldn't be as good for summer heat protection, airtightness, wind tightness or noise however. 

 

 

 

 

 

[Iceverge]

Play with Ubakus and cost up the materials on a m2 basis and you'll have an informed decision. 

 

By the way I couldn't show the battens perpendicular to the rafters so I just offset them to get the same thermal break effect. 

[LinearPancakes]

Thanks both. @Icevergethat looks like a great tool, wish I'd known about that earlier! Much of this is already set now unfortunately.

 

The PIR between the joists was foil taped to the timbers and to each other to try to make it airtight. With hindsight, if the moisture needs to vent to outside, this was probably a bad thing. Do you think I should ask our contractor to peel the airtight membrane back, slice through the foil tape where reachable then make good the membrane again?

[Iceverge]

No it'll be fine.

 

I still wouldn't run cables in the same layer as the PIR it'll be a pain. Attach the 50mm PIR to the rafters as a continuous layer using 70x22mm battens in line with the rafters below the 50mm PIR. Minimum 120mm screws. Foam all the boards where they are butt jointed with low expansion foam. This will create an unbroken layer of insulation and allow you 22mm to run cables etc. No worries about cable heating etc and there'll be space for 35mm downlighters if you want any. 

 

If you go for 15mm plasterboard you'll be able to use 600mm centres on the battens and it'll help with noise too. 

 

 

 

 

If you wanted to boost the heat protection aspect for summer you could swap out the 22mm gap for 50mm of woodfiber batts. The program has its limitations though and won't model the effect of the tiles on the roof accurately however so it isn't as bad as it looks in the above screenshot.

 

[LinearPancakes]

Many thanks guys. I'm sufficiently comforted to move on to the next challenge in the build. 😅

 

I also spoke with Ecological Building Solutions, who we purchased the membrane from, and they also said should be fine condensation-wise.

 

They did point out that when using foil faced rigid insulation the intelligent features of the intello plus membrane aren't really being used much anyway, which is interesting.

[Rishard]

I had the same feeling about the pir being taped acting as a vcl. My supplier of insulation did a condensation risk analysis of a similar build up to what you described. All seemed well. Does anyone know the benefits of an intello style membrane with pir if it’s being taped as a vcl? Is it double insurance for reducing moisture from the room? Or is it better targeted at more vapour open insulations? 

[Iceverge]

With the PIR there and a roof that breaths outwards there's much point in the variable permeability of Intello. 

 

It would come into its own if you had an impermeable layer outboard and the wall needed to dry inboard.

 

Here's an example.

 

 https://passivehouseplus.ie/magazine/new-build/longford-self-build-goes-certified-passive-on-a-budget

[Dave Jones]

On 19/01/2023 at 11:13, Iceverge said:

Should work from a condensation point of view. 

 

 

 

It'll be a  pain cutting all that PIR however and getting it to fit snuggly. Not great from a noise perspective or heat protection aspect either. Running wiring behind the PIR will be tricky as you'll need to compress it or cut the PIR where it crosses the rafters. 

 

 

 

 

This setup would be better in my opinion. If you can't get cellulose then blow in mineral wool would have the same ease of labour, probably a bit cheaper. It wouldn't be as good for summer heat protection, airtightness, wind tightness or noise however. 

 

 

 

 

 

 

that U value fails building regs need more insulation.

[Iceverge]

 

These are the limiting values for England I found in part L. Have I missed something? 

[Dave Jones]

yep

 

[ADLIan]

0.16 W/m2K is the worst acceptable for roofs under the 2021 Appr Doc L1 so 0.16/0.15 are both 'OK'. Probably will not help get compliance though if they are that high as the 'notional' house uses 0.11 W/m2K. Would need offsetting by more insulation in other elements.