Do I need to put foil tape on rafters between roof insulation?

[Graeme Marriott]

Hi,
We’re part way though an extension and are doing some bits ourselves. The builder did the construction, but we’re doing the roof insulation and the boarding.
We’ve used 10cm kingspan between the rafters, and will then cover the whole thing in 2.5cm Kingspan, which is what building control want.
Do we need to tape the rafters between the 10cm insulation with foil tape to meet bulidng regs?

Many thanks,

 

[Nozzle]

I believe this is the correct cold-bridging. For the sake of getting cold spots, just do it. I live in a chalet style house and the extension room (which has this type of hard insulation) feels noticably colder that the other bit. Though this could be attributed to being above an unheated garage.

Nozzle

 

[Ian H]

I did it with a 100mm wide foil tape.

 

[John D v2.0]

Do we need to tape the rafters between the 10cm insulation with foil tape to meet bulidng regs?

No need and not recommended.

The vapour barrier should always be on the warm side of the insulation. Otherwise you run the risk of warmer humid air getting trapped in a cooler layer of the insulation and building up condensation. In your case there's little chance of that, but there's no need.

So make sure you either tape the 2.5 or put a polythene sheet over it.

 

[Graeme Marriott]

Thanks for the replies,

John D v2.0, the 25mm is also a aluminum lined PIR board, so would it need another sheet?

I’ve attached some images of what each layer currently looks like...

 

[John D v2.0]

either tape the joints or a poly sheet. In your case tape as suggested by Ian would do it easiest.
I did both when I internally insulated our ensuite shower, as the old walls are solid uninsulated decorated walls i.e. no permeability on the cold side, and the shower would obviously be humid. But once you understasnd the principle you can make your own decision.

Also as an aside, make sure it's clear where the joists are above, as someone will need to put the plasterboard up.

 

[noseall]

Do we need to tape the rafters between the 10cm insulation with foil tape to meet bulidng regs

No but do it anyway. Robust is best.

Foil1

• noseall
• 2 Apr 2019

 

[noseall]

We foil tape both layers - i.e. those between the rafters and the next layer across the rafters. And we use foam in larger gaps....

Foil2

• noseall
• 2 Apr 2019

Foil1

• noseall
• 2 Apr 2019

Foil3

• noseall
• 2 Apr 2019

Foil4

• noseall
• 2 Apr 2019

 

[lc155]

We foil tape both layers - i.e. those between the rafters and the next layer across the rafters. And we use foam in larger gaps....

Foil2

• noseall
• 2 Apr 2019

Foil1

• noseall
• 2 Apr 2019

Foil3

• noseall
• 2 Apr 2019

Foil4

• noseall
• 2 Apr 2019

Hello.

This is an old post, but I'm bumping it to ask a relevant question: isn't taping both layers of PIR creating a double vapour barrier? Isn't this generally frowned upon because if moisture can cross the first layer, it could be trapped between both VCLs with no way of escaping?

 

[robinbanks]

If it can pass the first, it can't get trapped; if it could get trapped it wouldn't make it past the first.

The concern with vapour, and why we barrier against it, is of interstitial condensation. If there isn't anywhere for the vapour to condense (the dewpoint is in the middle of a big block of foam with no way for the vapour to reach it) then it's not a problem

Personally I wouldn't trust tape to carry on holding on or stay strong. A single layer of Tyvek AirGuard membrane in the inside for me, joints glued with a long life rated glue like Orcon, trapped under a batten that forms the service void and gives an air gap for the radiant heat control aspect of the membrane

Wall outer covered with something breathable like HouseWrap

 

[lc155]

If it can pass the first, it can't get trapped; if it could get trapped it wouldn't make it past the first.

The concern with vapour, and why we barrier against it, is of interstitial condensation. If there isn't anywhere for the vapour to condense (the dewpoint is in the middle of a big block of foam with no way for the vapour to reach it) then it's not a problem

Personally I wouldn't trust tape to carry on holding on or stay strong. A single layer of Tyvek AirGuard membrane in the inside for me, joints glued with a long life rated glue like Orcon, trapped under a batten that forms the service void and gives an air gap for the radiant heat control aspect of the membrane

Wall outer covered with something breathable like HouseWrap

To be honest, in my experiments with tape that stuff virtually bonds itself to whatever you stick it to, and when peeled off leaves a big gooey acrylic residue. I can't see that failing any more than the same sealing tape we use for the edges of sheets, etc. The main issue would be user application. I see plenty of people slap that tape on and crease the hell out of it, paying no real attention to ensuring that the whole width of the tape is slapped down nice and flat. I would imagine it is accepted within BC because there is little risk of failure with proper application (and with pretty much everything in BC, it comes down to building it properly).

I've also seen reports over the long term that the basic sheets most people use for this job (because who is paying over a hundred quid for tyvek sheeting if I'm seeing the right stuff) end up developing small holes over time, so no better than the tape really.

 

[robinbanks]

because who is paying over a hundred quid for tyvek sheeting if I'm seeing the right stuff

I'm fortunate to have built my house 10 years ago, when prices for AirGuard were still reasonable. It also meant I could use all sorts of wibbly wobbly unfoiled/tissue faced PIR seconds at an insanely cheap price and not worry about sealing every gap if both sides of the wall were wrapped..

But I take your point and if someone had sold me a house built like you describe I wouldn't set to and fit a membrane..