Vapour barrier

[deuce22]

Hi.
I am looking for some advice on an issue I have with my roof.
The roofers have fitted a warm pitched roof and told me that the vapour barrier gets fitted on the underside of the rafters. I have vaulted ceilings on the top floor and a lot of spots and sprinkler heads.
I’m now finding out that the vapour barrier should have been fitted to the boards above the rafters.
Is there a way for me to fit the vapour barrier underneath, without having a lot of areas where condensation can pass through?
Thanks.

[Redbeard]

Not easily, no, if I follow your description correctly. I assume the ceiling is not up yet, although the 'lots of spots.. etc' seems to imply otherwise, since you need a ceiling to fit the spotlights in. If the ceiling is not in you could *hope* to achieve vapour-tightness IF you can get vapour-tight shrouds for every 'intrusion'. This will effectively be a 'spotlight condom' with a grommet to fir tightly over the cable. And remember that if you pull an EPDM grommet over more than one cable it will not be air- or vapour-tight. 2 wires, 2 grommets. I hope you can make it work.

[deuce22]

The ceiling is not up yet. I've spoken with a company that has advised to do the following.

Fit a VCL to the underside of the rafters, board and skim, use sealed downlights and seal them to the ceiling with silicone, then use a paint on VCL to the whole ceiling before emulsion. I like the idea of the spotlight shrouds, where do you get these from?

Starting to get fed up with tradesmen, just had to go around re-fiting all the door straps into the concrete infill (ICF construction), the fitter used wood screws into the foam only, an almost 1/2 tonned bifold, being held in place by bits of spray foam.

Thanks for the help.

 

[Redbeard]

  On 13/11/2024 at 16:59, deuce22 said:

Starting to get fed up with tradesmen, just had to go around re-fiting all the door straps into the concrete infill (ICF construction), the fitter used wood screws into the foam only, an almost 1/2 tonned bifold, being held in place by bits of spray foam.

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Wow! Think I'd be fed up too!

 

I regret my patented (not) 'Spotlight condom' does not exist (though I think it should!). However Partel make this: https://www.partel.co.uk/product/airtight-downlight-cover/ which might work (???) (Actually, now I have looked at it I don't think it would work, as it's obviously made to sit down on a ceiling, whereas your lights will, I think, be pushed up into holes in the ceiling). I feel you are going to have to fabricate a version of the 'spotlight Condom' which can be fitted with a/t tape from below and then pushed up with the spotlight.

[Iceverge]

  On 13/11/2024 at 12:00, deuce22 said:

roofers have fitted a warm pitched roof

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Define this please, especially where the insulation is.

 

What is the buildup exactly from top to bottom.

 

 

 

 

[Firsttimer]

The contractors suggestions sounds decent. You could create a 'dropped ceilng'. So a VCL onto the underside of the rafters and then a frame onto the rafters for spotlights to sit in.

Edited November 13, 2024 by Firsttimer

[deuce22]

  On 13/11/2024 at 17:40, Iceverge said:

 

Define this please, especially where the insulation is.

 

What is the buildup exactly from top to bottom.

 

 

 

 

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roof tile

counter batten with breather membrane between

150mm PIR insulation

8x2 rafter with mineral wool between

[Iceverge]

Ok. 

 

Just to be sure. 

 

 

Tile.

Tile batten,

Membrane

Counter battens running along the line for the rafters. 

150mm PIR. 

200mm Rafters with  200mm mineral wool in between? 

 

Any further layers of OSB or membranes in there?

 

 

 

[Redbeard]

Good Qs, @Iceverge. @deuce22, you had said: "

  On 13/11/2024 at 12:00, deuce22 said:

I’m now finding out that the vapour barrier should have been fitted to the boards above the rafters.

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Merely so my mental picture can be complete, are those boards there?

 

You had referred to this as a warm roof. Strictly it's a hybrid warm roof, as (in my view) it is generally accepted that a Warm Roof has all the insulation above the roof structure. Precautions need to apply where some of the insulation is above the rafters and some between, lest the 'between' insulation should cool the 'interface' (the boards or 'not-boards' referred to above) between itself and the insulation above the rafters. Has an interstitial condensation risk assessment been done on this basis? 

 

If a CRA has not been done then a 'rule-of-thumb' from a highly-respected colleague is that two-thirds of the R value should be over the rafters and on-third between. (Reverse that, for example and the interface gets potentially too cool). Taking PIR as 0.022W/mK lambda value the R value of the PIR is 6.81m2K/W. Min wool lambda values vary but if we take a worst-case at 0.044W/mK that gives us an R value of 3.41m2K/W BUT we have rafters 'interrupting, which I am not calculating at this point. With no real justification let's say the 'timber fraction' lowers the R value to 3.0.  That about fits the rule of thumb, but there's still the outstanding question-mark - the subject of your thread - vapour control.

 

@Firsttimer I like the dropped ceiling idea. Still needs excellent attention to detail on the VCL above, though - a job for an obsessive! Even the fixings of the dropped ceiling become potentially-leaky penetrations.

[deuce22]

  On 14/11/2024 at 10:24, Iceverge said:

Ok. 

 

Just to be sure. 

 

 

Tile.

Tile batten,

Membrane

Counter battens running along the line for the rafters. 

150mm PIR. 

200mm Rafters with  200mm mineral wool in between? 

 

Any further layers of OSB or membranes in there?

 

 

 

Expand  

No.

 

Im not a roofer, but have an idea of how it all works. I'm now realising that the OSB should have been laid on top of the rafters, with the VCL fitted to this.

 

[deuce22]

  On 14/11/2024 at 11:02, Redbeard said:

Good Qs, @Iceverge. @deuce22, you had said: "

Merely so my mental picture can be complete, are those boards there?

 

You had referred to this as a warm roof. Strictly it's a hybrid warm roof, as (in my view) it is generally accepted that a Warm Roof has all the insulation above the roof structure. Precautions need to apply where some of the insulation is above the rafters and some between, lest the 'between' insulation should cool the 'interface' (the boards or 'not-boards' referred to above) between itself and the insulation above the rafters. Has an interstitial condensation risk assessment been done on this basis? 

 

If a CRA has not been done then a 'rule-of-thumb' from a highly-respected colleague is that two-thirds of the R value should be over the rafters and on-third between. (Reverse that, for example and the interface gets potentially too cool). Taking PIR as 0.022W/mK lambda value the R value of the PIR is 6.81m2K/W. Min wool lambda values vary but if we take a worst-case at 0.044W/mK that gives us an R value of 3.41m2K/W BUT we have rafters 'interrupting, which I am not calculating at this point. With no real justification let's say the 'timber fraction' lowers the R value to 3.0.  That about fits the rule of thumb, but there's still the outstanding question-mark - the subject of your thread - vapour control.

 

@Firsttimer I like the dropped ceiling idea. Still needs excellent attention to detail on the VCL above, though - a job for an obsessive! Even the fixings of the dropped ceiling become potentially-leaky penetrations.

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No, those boards are not there. They've used 200mm+ screws and fixed vertical battens, through the PIR and into the rafter.

[SteamyTea]

Check with the light manufactures, or a proper electrician, about ventilation/cooling for the spot lights.  Many cannot be simply buried into insulation.

[Firsttimer]

  On 14/11/2024 at 11:02, Redbeard said:

Good Qs, @Iceverge. @deuce22, you had said: "

Merely so my mental picture can be complete, are those boards there?

 

You had referred to this as a warm roof. Strictly it's a hybrid warm roof, as (in my view) it is generally accepted that a Warm Roof has all the insulation above the roof structure. Precautions need to apply where some of the insulation is above the rafters and some between, lest the 'between' insulation should cool the 'interface' (the boards or 'not-boards' referred to above) between itself and the insulation above the rafters. Has an interstitial condensation risk assessment been done on this basis? 

 

If a CRA has not been done then a 'rule-of-thumb' from a highly-respected colleague is that two-thirds of the R value should be over the rafters and on-third between. (Reverse that, for example and the interface gets potentially too cool). Taking PIR as 0.022W/mK lambda value the R value of the PIR is 6.81m2K/W. Min wool lambda values vary but if we take a worst-case at 0.044W/mK that gives us an R value of 3.41m2K/W BUT we have rafters 'interrupting, which I am not calculating at this point. With no real justification let's say the 'timber fraction' lowers the R value to 3.0.  That about fits the rule of thumb, but there's still the outstanding question-mark - the subject of your thread - vapour control.

 

@Firsttimer I like the dropped ceiling idea. Still needs excellent attention to detail on the VCL above, though - a job for an obsessive! Even the fixings of the dropped ceiling become potentially-leaky penetrations.

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True. However you build you will end up piercing the VCL.