Vapour Control Layer detail at wall/new flat roof junction

[Jilly]

Hi, we have a cold flat roof (because of height issues) and the builder did a lot of detailing to ensure the ventilation gap to the cold vaulted ceilings roof and the cold loft space. 
 

The builder says we are ok to rely on the PIR foil face to create the VCL, but I’m concerned about the best way to seal the gap at the wall/ ceiling before plaster boarding. Should it be taped? (Could anyone point me towards a YouTube video or other resource, if so, please?) . Nb The steel over the lintel can go in the thermal envelope, 

[crispy_wafer]

I'd prime the wall with something, i used some soudal airtight paint stuff to create a non dusty surface as that's what I had laying about, then a permanent flexible mastic sealant like Orcon or something and then a cut of membrane and tape it.

[Radian]

I think I can see a couple of problems:

So the void above the Xtratherm insulation is ventilated to the outside? If so, then so is the gap between the insulation and blockwork. You're right to want to extend the VCL to the wall but without continuous insulation through this gap, this junction will be a cold bridge.

Also, in that gap, all I can see is T&E cable that looks to be supported on screws?! AFAIK, If there are no wall mounted switches or other visible electrical fittings at this height then this horizontal cable run is not in a 'safe zone' unless it's 50mm above the finished ceiling level.

[JohnMo]

I can see a few issues that look like they need addressing first.

 

The screws going through the insulation will not support much for long, they all need washers to give some surface area. They also need to be taped over.

 

The gap between the wall and insulation needs to be filled that would be with foam or glass wool.

 

Wires supported on screws just looks wrong.

 

The white cover over what I assume are cables is held in place with cable clips, that's a bodge.

 

[Radian]

2 minutes ago, JohnMo said:

held in place with cable clips,

Although they're sometimes a handy source of small masonry nails if you take the trouble to pull them out of the plastic saddle 🙂

[andyscotland]

34 minutes ago, Radian said:

Also, in that gap, all I can see is T&E cable that looks to be supported on screws?! AFAIK, If there are no wall mounted switches or other visible electrical fittings at this height then this horizontal cable run is not in a 'safe zone' unless it's 50mm above the finished ceiling level.

 

There is also a "safe zone" (now called prescribed zone) horizontally along the top 15cm of wall where it meets the ceiling. Although arguably if the cable is above the line of the ceiling it is not in the zone.

 

Agree that supporting the cable on screws is not good if that's what's happening. I'd be concerned over time and with a bit of movement/expansion/contraction the screw threads cut into the sheath & insulation.

[Radian]

19 minutes ago, andyscotland said:

There is also a "safe zone" (now called prescribed zone) horizontally along the top 15cm of wall where it meets the ceiling. Although arguably if the cable is above the line of the ceiling it is not in the zone.

True.

 

Going back to the subject of the gap, i'm guessing the insulation in the photo is a continuous run under the roof joists and that there's more above inbetween the joists. Hopefully the upper layer extends over the top of the inner leaf and makes a continuous thermal envelope with the cavity wall insulation? This would reduce the demand on insulating the gap but how effectively it prevents a cold bridge depends on how well the hidden joins are linked together. Filling the gap would still be an easy win (if it wasn't for the wiring).

 

[TonyT]

47 minutes ago, JohnMo said:

I can see a few issues that look like they need addressing first.

 

The screws going through the insulation will not support much for long, they all need washers to give some surface area. They also need to be taped over.

 

The gap between the wall and insulation needs to be filled that would be with foam or glass wool.

 

Wires supported on screws just looks wrong.

 

The white cover over what I assume are cables is held in place with cable clips, that's a bodge.

 

 

Common around my area for capping to be fixed back with cable clips, it’s a masonry nail and the plastic clip helps with a larger surface area.

 

clout nails also common 

 

[JohnMo]

8 hours ago, TonyT said:

 

Common around my area for capping to be fixed back with cable clips, it’s a masonry nail and the plastic clip helps with a larger surface area.

 

clout nails also common 

 

That may be so, but still looks a bodge.

[Jilly]

Mmmm thank you folks, you've raised more issues than I realised, but not insurmountable. Building Control have seen it and not mentioned anything. I need to do it myself though, as the builder is busy. I don't know how the cavity wall insulation and the insulation at the ceiling is detailed.

[Radian]

2 hours ago, Jilly said:

Mmmm thank you folks, you've raised more issues than I realised

Almost inevitable when you post a photo on these forums I'm afraid - but you probably already knew that 😊

 

2 hours ago, Jilly said:

I don't know how the cavity wall insulation and the insulation at the ceiling is detailed.

If you have no drawings and the builder isn't able to confirm, I'd say it's important enough to warrant carefully cutting out a little section to reveal the junction with the inner leaf. If its another layer of rigid insulation, then you should be able to lift the insulation up off the block and with the end of a tape measure 'probe' for the end of the sheet. Hopefully it extends more than 100mm (to get over the block) and possibly some way past (to cap the cavity) or maybe butt up to the cavity insulation. Either way, if the fitter used foam to make a continuous envelope I'd be amazed and delighted - although it might make the investigation  a little bit harder.

 

To the top right of your photo the insulation (50mm?) appears to sail over the top of the concrete blocks so the insulation above might follow it but you need to be sure it meets the cavity insulation. Speaking of which - what is in the cavity?

[Jilly]

Thank you @Radian, the blockwork in the image, is solid, it's the old exterior wall of the converted stable. The extension build is cavity block and I have a horrid feeling the insulation does not go as you describe...

[Radian]

3 hours ago, Jilly said:

The extension build is cavity block and I have a horrid feeling the insulation does not go as you describe...

 

I read in another topic that your flat roof insulation has a 50mm ventilation gap that communicates with the eaves. Is this photo taken in the same area? I'm not familiar with your project so I don't know much more than this.

It sounds like there is no wall insulation - is there none planned to be installed?

[Jilly]

There's no insulation on the wall you are looking at as it will be an internal wall.

 

There's rockwool in the cavity of the new walls of the extension plus external wall insulation under featherboard.

 

Does that make sense?