Timber frame extension to existing house - vapour barrier?

[Rick734]

Afternoon gents,

 

Been racking my brains on this one a bit and thought to turn to the helpful people on here. Who probably could run the country a lot better than the current shower. Anyway...

 

Stick-built TF extension on the side of my house, over the existing garage walls. Everything up, roof slated, external walls currently being rendered when weather permits. Now working on the inside on the first floor and my understanding is a VCL around the walls is a good idea on TF to prevent moisture moving into the timber frame from the inside - fine. I have myself some silver reflective VCL from Tradestore ready to go - but a couple of questions I've been thinking about:

1) How to seal the edges of it, where it meets the existing house - how much do I need to worry on this front. Can I just lap it over onto the render, run a bead of silicone down & tape over?

2) At floor level, is it just a case of taping it down onto the chipboard? Or is there some technical way to seal it? My brain says just seal it down and be done with it, but at the same time there is the ground floor below, so surely moisture could rise through the ceiling structure and get up into the wall structure...or is that just overthinking it.

 

I should add, I am more about speed and getting the job done at this point, rather than perfecting it. Definitely not a forever home and I'm in a hurry so not trying to spend time achieving microscopic increases in building performance...if that makes sense - quickest solutions that are acceptable, ideally!

 

Over to you! Image below of one of the vertical junctions where the TF meets the masonry...

 

 

Edited Tuesday at 15:26 by Rick734
Adding detail

[Rick734]

Further addition to the post...decided to have a bit of a chat with ChatGPT to get his views on the subject. The interesting bit was that when I mentioned I would be lining the internal face of the external walls with 18mm OSB (for fixing purposes), he/she/they/it decided that OSB can replace the whole vapour barrier/air tightness thing and is fine by itself...?! Sounds great, but I am cynical unless someone can weigh in on this please?

Edited Tuesday at 16:18 by Rick734

[JohnMo]

9 minutes ago, Rick734 said:

he/she/they/it decided that OSB can replace the whole vapour barrier/air tightness thing and is fine by itself

I wouldn't, OSB is vapour open.

 

Do it correctly install vapour barrier, tape all joints, 

1 hour ago, Rick734 said:

How to seal the edges of it,

Use something like this

https://passivehousesystems.co.uk/shop/adhesives-primers-sealants/fortax-6400/?utm_term=&utm_campaign=P-Max+Test&utm_source=adwords&utm_medium=ppc&hsa_acc=4901615203&hsa_cam=20355723828&hsa_grp=&hsa_ad=&hsa_src=x&hsa_tgt=&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gad_source=1&gad_campaignid=20364888406&gbraid=0AAAAApUFUNC26rUQyD2HwqFlOulMhVQWS&gclid=CjwKCAiAmKnKBhBrEiwAaqAnZ5CPWGBFVVio4GUfqzpKS1479NcoHwQrQVgTtExyEM110NUvopZ9HBoCl6AQAvD_BwE

 

1 hour ago, Rick734 said:

should add, I am more about speed and getting the job done at this point, rather than perfecting it.

Only one chance to do it right - slow down a bit.

 

1 hour ago, Rick734 said:

Definitely not a forever home

You never know what's ahead of you, you maybe their longer than you think and wish you had done a proper job.

[Mr Punter]

I understood that the OSB vapour openness can vary significantly by brand / batch, so I would polythene over this.  Quick to do with a stapler.  Run it down to the floor and trap it with the skirting.  Where are electrics going to be?

[Redbeard]

52 minutes ago, Rick734 said:

he/she/they/it decided that OSB can replace the whole vapour barrier/air tightness thing and is fine by itself...?! Sounds great, but I am cynical unless someone can weigh in on this please?

Exactly what I had in mind for my post-and-beam extn, but at a late stage the variability in quality of OSB in terms of air-tightness made me think again. I used Pro Clima Intello and taped all joints and perimeters to an obsessive degree.

[Tony L]

59 minutes ago, JohnMo said:

You never know what's ahead of you, you maybe their longer than you think and wish you had done a proper job.

 

This is my way of thinking, despite the missus wanting to be out as soon as we're safe from the hefty CIL fine.

[Iceverge]

It's gaping air leaks to worry about not diffusion. 

 

OSB will be adequate for your purposes if you tape it diligently, however a vcl may be faster. 

 

Beware the difference between a vapour barrier and a vapour control layer. With a barrier you'll always run a higher risk of Interstatial condensation. 

[ADLIan]

1 hour ago, Iceverge said:

It's gaping air leaks to worry about not diffusion. 

 

OSB will be adequate for your purposes if you tape it diligently, however a vcl may be faster. 

 

Beware the difference between a vapour barrier and a vapour control layer. With a barrier you'll always run a higher risk of Interstatial condensation. 

Can you please explain the difference between a vapour barrier and vapour control layer and why one gives a higher condensation risk?

[Rick734]

14 hours ago, JohnMo said:

Do it correctly install vapour barrier, tape all joints,

Thanks @JohnMo - any particular vapour barrier? Are the smart/intelligent ones e.g. Pro Clima Intello / Isover Vario KM Duplex worth the money?

 

Re timing - yes I don't want to bodge it, but I do see people on here looking at getting the ultimate performance from a project as it is their forever home and they are prepared to put significant effort in to achieve that. As long as it isn't going to cause damage or serious building issues in the long term, I'm not super worried about getting the highest possible specs out of it. I guess I'm looking for more of a balance of performance of practicality, speed and performance, rather than just performance, if that makes sense?

[Rick734]

14 hours ago, Mr Punter said:

Run it down to the floor and trap it with the skirting.  Where are electrics going to be?

Thanks Mr Punter - just tossing the coin on whether to do a 25mm service void or not at the moment...due to the layout of the room it may be the case that I do it in some areas but not others. Any recommendation?

The construction is a 145mm C47 timber, 120mm PIR with 9/11mm OSB on the outside plus renderboard and Fassa render system.

[Rick734]

14 hours ago, Redbeard said:

Exactly what I had in mind for my post-and-beam extn, but at a late stage the variability in quality of OSB in terms of air-tightness made me think again. I used Pro Clima Intello and taped all joints and perimeters to an obsessive degree.

@Redbeard this is super helpful as the Pro Clima is one of the options I was looking at. Any comments on performance (or I guess there's probably not much to tell)?

 

Did you end up doing the OSB at all or leave it off altogether? Just wondering now if it's best to put the membrane inside or outside of my OSB...

[Rick734]

8 hours ago, ADLIan said:

Can you please explain the difference between a vapour barrier and vapour control layer and why one gives a higher condensation risk?

@Iceverge can give us chapter and verse on this but my understanding is that a vapour barrier completely blocks moisture, which means that if any does get behind (e.g. leaks, perforations etc) it makes it impossible for it to escape again, potentially leading to interstitial condensation (between surfaces inside the wall) and therefore mould etc.

 

Whereas a vapour control layer slows down the rate of moisture transfer, but is still technically slightly moisture permeable so trapped moisture can escape.

 

Shoot me down if I'm wrong...

 

If anyone's interested I have a link to my conversation with ChatGPT...seems to have a strong opinion on this subject: https://chatgpt.com/share/e/694b98bc-d6a8-8000-a4b4-5b08ee8d0ea9

[Iceverge]

19 hours ago, ADLIan said:

Can you please explain the difference between a vapour barrier and vapour control layer

 

Sd value above 100m is a barrier. 

 

19 hours ago, ADLIan said:

and why one gives a higher condensation risk?

 

Barrier will be worse. It won't allow construction moisture to dry back to the inside. In practice there will be holes and imperfect sealing of any membrane. Any moisture ingress will dry over time through a lower Sd material and won't through a Vapour Barrier. 

 

 

 

 

[Iceverge]

12 hours ago, Rick734 said:

@Iceverge can give us chapter and verse on this but my understanding is that a vapour barrier completely blocks moisture, which means that if any does get behind (e.g. leaks, perforations etc) it makes it impossible for it to escape again, potentially leading to interstitial condensation (between surfaces inside the wall) and therefore mould etc.

 

Whereas a vapour control layer slows down the rate of moisture transfer, but is still technically slightly moisture permeable so trapped moisture can escape

 

Got it in one. 

 

In layman's terms all that matters is that "drying exceeds wetting".

 

Vapour barriers will in theory have less diffused moisture making its way into a structure than Vapour control layers. However they have such terrible drying qualities that in practice the trade off is they result in wetter walls than VCLs. 

 

In any case moisture diffusion is the hill that too many people die on unnecessarily. Holes in imperfect airtight layers are massively more significant. Often carrying hundreds if not thousands of times more moisture into a structure. 

 

 

[Iceverge]

14 minutes ago, Iceverge said:

In any case moisture diffusion is the hill that too many people die on unnecessarily. Holes in imperfect airtight layers are massively more significant. Often carrying hundreds if not thousands of times more moisture into a structure. 

 

 

Try playing with the numbers, there's always truth there. I'm sure @SteamyTea will concur.  All the formulas are online or there's also chatgpt. 

 

Take your 11mm OSB. It has an Sd value of 2.5m which is quite vapour open. A house with an internal RH of 60% at 20deg will exert a vapour pressure towards the outside on a winter's day. Assume it's 5deg and 90% outside. Over an hour about 0.18g will diffuse through 1m² into your wall. 

 

Now put a 1cm x 1cm hole in the OSB and exert a 50Pa pressure difference on the wall representing a strong breeze blowing on the wall of the house. Over an hour an extra 7g of moisture will end up in the wall. 

 

In short a 1cm² hole will loose as much moisture into the structure as 40m² of very vapour open wall on a typical February day. 

 

 

 

 

[Iceverge]

TLDR:

 

1. DRYING MUST EXCEED WETTING

 

 

2. Forget about the microscopic holes (diffusion)!!.Worry about the big ones (airtightness) !! 

 

3. Some moisture will always get in, just ensure it can get out again. Low Sd materials are king here. 

 

Merry Christmas! 

[ADLIan]

What is the ‘sd’ value of a vapour control layer? Also where are these numbers defined?

[Iceverge]

That number I used was from Siga's website. I don't think BS defines it unless you have more insight than me? 

 

To @Rick734s question. Something like this would be fine. 

 

https://insulation4less.co.uk/products/vc2-air-leakage-vapour-control-layer-1-5m-x-50m-75m2-roll

 

To connect the membrane to the brick I would use an airtightsealant. Orcan F is the one that popped up first.

 

https://www.earthwiseconstruction.co.uk/product/airtightness/adhesives-primers/orcon-f-airtight-sealant-adhesive/?attribute_pa_orcon-f-size=310ml-cartridge&gad_source=1&gad_campaignid=21761028231&gbraid=0AAAAADqL4zA7HZs48ZWRJyFGMUuzu-U3k&gclid=CjwKCAiAu67KBhAkEiwAY0jAlZBJ-cvv1xqU3kk-2xm2BrsDsiiC-IiArhamJDFKawbitr-HRg56cRoCRbgQAvD_BwE

 

I would always be in favor of a service cavity to avoid puncturing the airtight layer. If you have enough space 50*50mm battens work nicely for 50mm mineral wool to insulate and boost the U value a bit. 

 

Maybe Santa will bring some, I think I can hear him rattling around up on the roof.🎅

 

[ADLIan]

So no technical justification for the numbers just one manufacturer’s sales line - I’ve never seen those definitions or limiting values used elsewhere. I’ll stick with the info in relevant British Standards.

[Redbeard]

On 24/12/2025 at 07:33, Rick734 said:

@Redbeard this is super helpful as the Pro Clima is one of the options I was looking at. Any comments on performance (or I guess there's probably not much to tell)?

 

Did you end up doing the OSB at all or leave it off altogether? Just wondering now if it's best to put the membrane inside or outside of my OSB...

 

Sorry, did not spot this till now. I kept the OSB - it was anti-racking, too, and stapled the Intello directly to it. A/T tape over each staple...