Where should I put a vapour membrane on IWI

[thetdog666]

Got a 1990s timber framed house with cavity walls. Finding it hard to find info on where the vapour control barrier goes or if its actually needed (taping 20mm layer over the top of the timber frame). Or if I should use foil backed plasterboard. Any advise would be greatly appreciated.

 

My setup will be from the inside = Plasterboard → Service Void using 40mm x 20mm battens → 20mm PIR on the face of the timber → 80mm PIR in-between the timber studs → OSB → Membrane → Cavity

 

Thanks

S

[Russell griffiths]

I think your terminology is a bit mixed up. 
I think you have a timberframe house, with a brick outer skin, this isn’t cavity construction, but a gap is left between the timber and the bricks leading to it looking like a cavity. 
are you planning on stripping all the internal plasterboard and upgrading the insulation levels. 
 

if this is the case I think a fair few on here would be interested to see the standard of the framing and insulation. 
 

I think we can answer this better when that is understood. 

[thetdog666]

11 hours ago, Russell griffiths said:

I think your terminology is a bit mixed up. 
I think you have a timberframe house, with a brick outer skin, this isn’t cavity construction, but a gap is left between the timber and the bricks leading to it looking like a cavity. 
are you planning on stripping all the internal plasterboard and upgrading the insulation levels. 
 

if this is the case I think a fair few on here would be interested to see the standard of the framing and insulation. 
 

I think we can answer this better when that is understood. 

Yeah I think your right about the make up of the house.

 

All plasterboard will be coming off of ceiling and external walls. I currently have 100mm wool(yellow itchy stuff) in the walls and floor.  I have attached a picture I have found on another page of what I am planning on doing. My frame is only 90mm deep so 80mm is the max I can go for. Space is also an issue so am only going for 20mm on the face of the battens. 

 

Im now starting to think that the vapour barrier goes over the battens (just behind the plasterboard if looking from inside). What do You think?

 

Thanks

S

[Redbeard]

 

8 minutes ago, thetdog666 said:

Space is also an issue so am only going for 20mm on the face of the battens. 

Not sure what you mean by 'on the face of the battens'. Do you mean between them? Have you found 20mm PIR?

8 minutes ago, thetdog666 said:

 

Im now starting to think that the vapour barrier goes over the battens (just behind the plasterboard if looking from inside). What do You think?

On the initial assumption that you were not having PIR between the battens I was going to suggest the VCL goes on the face of the 80mm, with the battens over, but if you have PIR between the battens that changes. 

[thetdog666]

27 minutes ago, Redbeard said:

 

Not sure what you mean by 'on the face of the battens'. Do you mean between them? Have you found 20mm PIR?

So the VCL is stapled to the 40mm x 20mm battens. So technically over the "top" of them. Celotex have 20mm PIR

 

 

27 minutes ago, Redbeard said:

On the initial assumption that you were not having PIR between the battens I was going to suggest the VCL goes on the face of the 80mm, with the battens over, but if you have PIR between the battens that changes. 

No pir between the battens. This is the service void where I will be running electrical cables. The illustration above is the exact way I would be doing it.

Edited January 7 by thetdog666

[mistake_not]

1 hour ago, thetdog666 said:

So the VCL is stapled to the 40mm x 20mm battens. So technically over the "top" of them. Celotex have 20mm PIR

If you do this then surely you have to breach the VCL multiple times when you run cables / pipes from your service void to your internal face.

 

You want your VCL behind your battens, I'm from of your first layer of insulation (working from inside). 

[thetdog666]

8 minutes ago, mistake_not said:

If you do this then surely you have to breach the VCL multiple times when you run cables / pipes from your service void to your internal face.

 

You want your VCL behind your battens, I'm from of your first layer of insulation (working from inside). 

yeah I would have to do that so maybe not the best.

 

How would I fix the vapour barrier though to the wall?.....Would staples into pir work? 

[Russell griffiths]

11 hours ago, thetdog666 said:

yeah I would have to do that so maybe not the best.

 

How would I fix the vapour barrier though to the wall?.....Would staples into pir work? 

If you are doing this it’s not cheap, so do it right. 

your VCL goes over the inner most pir and under the battens. 
 

it needs to be connected to the floor in some way and also needs to pass up through the first floor, your insulation also needs to pass up through the first floor joists. 
 

if you make a good job of the inner pir you can use the foil face of it as a VCL by taping it together, then you tape it to the floor and upper ceiling to form a continuous line. 
 

imagine drawing your house, then get a pen and drawer a line around your house as the VCL you must drawer this line continuously until you get to where you started without taking your pen off the paper. 

[sgt_woulds]

According to your drawing, there doesn't appear to be any racking board on the frame.  Is that correct? In 'traditional' UK timber frame I would expect to see OSB or plywood fitted external to the timber frame. 

 

(You also show a 120mm frame in the drawing but say it is only 90mm?  If it is 90mm why not fully fill the space - use two layers of 45mm or 90mm boards - both options available from Celotex?)

 

Is your frame open to the cavity? What protects the insulation and timber frame from the moisture in the external cavity - there should be a breathable membrane external to the frame? 

 

This is very important as it determines the 'breathability' of the structure and the type of VCL you need to fit internally.

 

This also creates follow-up questions regarding your insulation choice and fitting methods.  Detailing will be critical to ensure no long-term repercussions of 'upgrading' the insulation.

 

Are you insulating the whole house, or is this just a small section?  

[Redbeard]

I am very confused! Thanks, OP, for your response to my last post. The annotations (notably the dimensions) on your drawing do not seem to match up with the words in your description. The dwg suggests a total of 145 PIR (25 and 120). Your description below

On 07/01/2025 at 03:18, thetdog666 said:

20mm PIR on the face of the timber → 80mm PIR in-between the timber studs 

suggests a total of 100 (80 and 20).

 

Can you clarify?

 

@Russell griffiths above referred to terminology. Am I right in assuming that the reference to 'battens' below actually refers to the (vertical) structural Studs?

15 hours ago, thetdog666 said:

Space is also an issue so am only going for 20mm on the face of the battens. 

 

[Redbeard]

On 07/01/2025 at 03:18, thetdog666 said:

My setup will be from the inside = Plasterboard → Service Void using 40mm x 20mm battens → 20mm PIR on the face of the timber → 80mm PIR in-between the timber studs → OSB → Membrane → Cavity

 

 

@sgt_woulds, I think it's down to the difference (see my post above) between the dwg and the description. The OP's description (above) has OSB over the studs. 

 

 

[sgt_woulds]

So thetdog666, which build-up description is correct?

[thetdog666]

1 minute ago, sgt_woulds said:

So thetdog666, which build-up description is correct?

Hello,

 

The drawing above is missing the osb and membrane on the outer side of the OSB.

 

Thanks 

Edited January 8 by thetdog666

[sgt_woulds]

And the dimensions?

 

[Redbeard]

@thetdog666, noted the OSB and membrane missing, but apart from that is that *the* dwg of what you plan to build, with a total of 145mm PIR, as against your description, in your original post, of 100mm?

 

Edit: Cross-posted with @sgt_woulds

Edited January 8 by Redbeard

[thetdog666]

3 hours ago, sgt_woulds said:

According to your drawing, there doesn't appear to be any racking board on the frame.  Is that correct? In 'traditional' UK timber frame I would expect to see OSB or plywood fitted external to the timber frame. 

Sorry OSB is fitted on the outerside with membrane on the outside side of the OSB.

 

3 hours ago, sgt_woulds said:

(You also show a 120mm frame in the drawing but say it is only 90mm?  If it is 90mm why not fully fill the space - use two layers of 45mm or 90mm boards - both options available from Celotex?)

This is a drawing of one I found off the net. I didnt make it up so is slightly different to what i have. I heard there can be a tolerance with the timber frame studs and people have had trouble trying to fill it fully as the insulation sticks slightly past the timber frame.

 

3 hours ago, sgt_woulds said:

Is your frame open to the cavity? What protects the insulation and timber frame from the moisture in the external cavity - there should be a breathable membrane external to the frame? 

Answered above. OSB  with breathable membrane

 

3 hours ago, sgt_woulds said:

This is very important as it determines the 'breathability' of the structure and the type of VCL you need to fit internally.

 

This also creates follow-up questions regarding your insulation choice and fitting methods.  Detailing will be critical to ensure no long-term repercussions of 'upgrading' the insulation.

 

Are you insulating the whole house, or is this just a small section?  

Kitchen only just now but ot takes up 1/3rd of my downstairs. Livingroom and dining room next year (if there is a notable difference in temperature) which would be 80% of my downstairs done. 

[thetdog666]

2 minutes ago, sgt_woulds said:

And the dimensions?

 

Of the room? 6m x 3.4m

[thetdog666]

1 minute ago, Redbeard said:

@thetdog666, noted the OSB and membrane missing, but apart from that is that *the* dwg of what you plan to build, with a total of 145mm PIR, as against your description, in your original post, of 100mm?

 

Edit: Cross-posted with @sgt_woulds

That drawing I posted up is off another page. It is the most similar one iv come across to mine. My frames inside is only 90mm thick. 

[Redbeard]

OK. Got it! So your proposal is 100mm total PIR, as per your original description: *not* as per the drawing. Confusion over. Thanks!

[Iceverge]

Lots of confusion re vapor as usual here. Its like worrying about a mouse when there's an elephant in the room.

 

Airtightness is the real bogeyman. Poor air sealing allows hundreds of times more damaging vapour into the wall than vapour diffusion does. Not to mention the outsize effect airtightness has on building comfort and efficiency. 

 

I see you've been seduced by the headline K value of PIR to put it in-between the studs too. I wouldn't. It'll age differentially to the timber and you'll get gaps around it. You'll end up throwing lots and lots of offcuts in the skip too. Keep the mineral wool if it's still in good condition, perhaps add another 50mm to ensure the studs are full. ( I assume the studs are 140mm CLS at 600cc? 

 

Then an airtightness barrier. Then cross batten and insulate the service cavity.  

 

 

Very buildable, very little waste. A workable robust airtightness strategy. Much cheaper than what you proposed previously. 

 

Edited January 8 by Iceverge

[Iceverge]

Cross post I see the 90mm frame now and assume the centres are 400mm. 

 

 

 

Edited January 8 by Iceverge

[Iceverge]

 

 

Here's the above with the mineral wool and a 25mm layer of PIR and 20mm of a service cavity.  U value is much the same as the full fill mineral wool service cavity. 

 

 

 

Here's what you are proposing with the PIR in between the studs. 

 

A theoretical improvement of 0.043W/m2K. Say you have 100m2 of external wall area, that makes a difference of about 86W to your maximum heating load. A rough guestimate would equate that to about 120kWh of energy annually assuming the house was permanently heated to 21deg.  Assuming energy costs of 12p/kWh then you'd save £15 per year. 

 

Allowing for waste 100mm PIR is about £15/m2. At 100m2 it would take 100 years to payback, excluding the cost of your labour. 

 

 

 

Thats before you look at the real life situation with shrinkage and subsequent thermal bypass of the PIR in a timber frame.  You'd be better off setting fire to £20 notes than replacing the mineral wool in the wall with PIR.  

 

[thetdog666]

51 minutes ago, Redbeard said:

OK. Got it! So your proposal is 100mm total PIR, as per your original description: *not* as per the drawing. Confusion over. Thanks!

Yeah that's right hahaha. 

[thetdog666]

56 minutes ago, Iceverge said:

Lots of confusion re vapor as usual here. Its like worrying about a mouse when there's an elephant in the room.

 

Airtightness is the real bogeyman. Poor air sealing allows hundreds of times more damaging vapour into the wall than vapour diffusion does. Not to mention the outsize effect airtightness has on building comfort and efficiency. 

 

I see you've been seduced by the headline K value of PIR to put it in-between the studs too. I wouldn't. It'll age differentially to the timber and you'll get gaps around it. You'll end up throwing lots and lots of offcuts in the skip too. Keep the mineral wool if it's still in good condition, perhaps add another 50mm to ensure the studs are full. ( I assume the studs are 140mm CLS at 600cc? 

 

Then an airtightness barrier. Then cross batten and insulate the service cavity.  

 

 

Very buildable, very little waste. A workable robust airtightness strategy. Much cheaper than what you proposed previously. 

 

The studs are only 90mm thick and not sure if 600mm or 400mm  apart that's why I was going for PIR to get the u value up. My house loses temperature quite quick so trying to rectify that while I can.

 

I did price up 100mm insulation slabs but I'm sure the PIR actually came In cheaper.

 

I currently have netting and 100mm under the suspended floor. I'm going to take the floors up to. Do you recommend I keep with rock wool for the floor aswell?

Edited January 8 by thetdog666

[thetdog666]

51 minutes ago, Iceverge said:

 

 

Here's the above with the mineral wool and a 25mm layer of PIR and 20mm of a service cavity.  U value is much the same as the full fill mineral wool service cavity. 

 

 

 

Here's what you are proposing with the PIR in between the studs. 

 

A theoretical improvement of 0.043W/m2K. Say you have 100m2 of external wall area, that makes a difference of about 86W to your maximum heating load. A rough guestimate would equate that to about 120kWh of energy annually assuming the house was permanently heated to 21deg.  Assuming energy costs of 12p/kWh then you'd save £15 per year. 

 

Allowing for waste 100mm PIR is about £15/m2. At 100m2 it would take 100 years to payback, excluding the cost of your labour. 

 

 

 

Thats before you look at the real life situation with shrinkage and subsequent thermal bypass of the PIR in a timber frame.  You'd be better off setting fire to £20 notes than replacing the mineral wool in the wall with PIR.  

 

Wow thanks for putting that all Into perspective.  I thought PIR would be alot better than mineral wool.

 

Say I went with 100mm mineral wool and 20mm pir over the top would I need a vapour barrier?

 

Thanks

 

Steven

Edited January 8 by thetdog666