Airtightness

[Nic]

Hi 

if you are looking to build in timber frame is vapour block osb the best option for internal airtightness control ? Obviously taping the joins etc or is there an alternative or better option? 

[G and J]

I’m building timber frame and I’ll be putting a vapour control layer inboard of my studs and that will be part of my  airtight strategy.   I’ll use Tony trays to maintain the layer between floors though I’ve yet to fully get my head round the precise detail, partly as I have an offset upstairs frame. 
 

Im not sure I’d get any benefit from vapour block OSB and it sounds more expensive. 

[Iceverge]

4 hours ago, Nic said:

Hi 

if you are looking to build in timber frame is vapour block osb the best option for internal airtightness control ? Obviously taping the joins etc or is there an alternative or better option? 

 

OSB is much more expensive than a membrane. 

 

If using a blown in cellulose then you can't check to see if all bays are fully filled like you can with a membrane. 

 

On the other hand OSB is far tougher. And less lightly to be punctured by wayward workers. 

 

18mm OSB is normally airtight although I would be confident in high quality 11mm stuff too. 

 

Crappy cheap OSB might not be ok.

 

 

[PNAmble]

We used standard 18mm OSB taped and purple paint.  Got 0.42 ACPH.  
 

it was applied with a spray.  

 

 

[G and J]

1 hour ago, PNAmble said:

We used standard 18mm OSB taped and purple paint.  Got 0.42 ACPH.  
 

it was applied with a spray.  

 

 

Was it difficult to find soft furnishings to coordinate? 😉 

 

Being serious why 18mm OSB?  Is that directly behind your service void just inboard of your frame?

[PNAmble]

24 minutes ago, G and J said:

Being serious why 18mm OSB?  Is that directly behind your service void just inboard of your frame?

Ooh.  You’ve made me go and check the details.  We only used 18mm in the garage which has no service void as everything is surface mounted and we wanted easy ‘fastenings’.   Main external walls are 11mm. 
 

[G and J]

38 minutes ago, PNAmble said:

Ooh.  You’ve made me go and check the details.  We only used 18mm in the garage which has no service void as everything is surface mounted and we wanted easy ‘fastenings’.   Main external walls are 11mm. 
 

So pretty much three potentially airtight layers.  Wow.  I’m impressed. 

[Nic]

This is a cross section of my build between floors sorted. I noticed that there was not a complete airtight barrier between first and ground floor floor. Would what I’ve drawn work ok ? Then obviously taped to the first and ground floor membranes 

[MikeSharp01]

We use the membrane approach and it worked out very well, using the 4m wide Intello stuff gets the coverage pretty quickly and with fewer joins, although building the internal structure for the service void afterwards / on top has been a pain. We had some section with PB skin on the inside as well as outside so I put inspection holes through that so I could see I had it packed correctly and then covered it in membrane. Don't forget that well packed cellulose fibre is pretty air tight on its own. We achieved 0.2 on the PH Scale (ACH n50) and .29 on the BR scale for our first test, pre Plaster boarding. So far I have not damaged it but I am about to push the MVHR inlet and outlet through it so we will see how we get on!

[G and J]

1 hour ago, Nic said:

This is a cross section of my build between floors sorted. I noticed that there was not a complete airtight barrier between first and ground floor floor. Would what I’ve drawn work ok ? Then obviously taped to the first and ground floor membranes 

That’s exactly what I’m planning.  Not a clue what to make the tray out of mind.

 

I’m also not clear yet how wall junctions are handled, but will study that nearer the time.

[ProDave]

2 hours ago, Nic said:

This is a cross section of my build between floors sorted. I noticed that there was not a complete airtight barrier between first and ground floor floor. Would what I’ve drawn work ok ? Then obviously taped to the first and ground floor membranes 

That earned the name "Tony Tray" on this forum a while back.  Our builders erecting the frame did that for us with Intello air tight membrane.  That paved the way to line the whole inside of the timber frame but using a cheaper membrane BarriAir (not sure how you spell it?

 

Here is a picture

 

[PNAmble]

3 hours ago, G and J said:

So pretty much three potentially airtight layers.  Wow.  I’m impressed. 

To be honest some of it ie the external taping was overkill.  But we and the builder were learning - it was his first airtight house.  Our contract had airtightness as an outcome that they signed up to, so he had a last minute wobble that the OSB wouldn’t be airtight so he decided to go with purple paint.  
 

the advantage is that it doesn’t get nicked and if you do need to screw anything into it you can dip the screw into the paint and it doesn’t impact the airtightness.   
 

there are loads of papers on whether OSB is airtight or not and how long it remains airtight but I think the scientific jury is still out.  
 

 

[G and J]

4 hours ago, MikeSharp01 said:

using the 4m wide Intello stuff

Given that we have 2.3m walls downstairs I could use that to be my downstairs VCL and Tony tray all in one go!  The fewer junctions the least taping and faffing perhaps….

[Nic]

2 hours ago, PNAmble said:

To be honest some of it ie the external taping was overkill.  But we and the builder were learning - it was his first airtight house.  Our contract had airtightness as an outcome that they signed up to, so he had a last minute wobble that the OSB wouldn’t be airtight so he decided to go with purple paint.  
 

the advantage is that it doesn’t get nicked and if you do need to screw anything into it you can dip the screw into the paint and it doesn’t impact the airtightness.   
 

there are loads of papers on whether OSB is airtight or not and how long it remains airtight but I think the scientific jury is still out.  
 

 

What exactly is this purple paint you speak of 😂

[MikeSharp01]

1 hour ago, G and J said:

Given that we have 2.3m walls downstairs I could use that to be my downstairs VCL and Tony tray all in one go!  The fewer junctions the least taping and faffing perhaps….

Yes can't see why not - sounds like a plan.

[Sparrowhawk]

1 hour ago, Nic said:

What exactly is this purple paint you speak of 😂

It's this one https://www.intelligentmembranes.com/ But you can get the same thing without distinctive features like the - ahem purple colour and breathless marketing - cheaper from other brands e.g. I used https://www.soudal.co.uk/pro/products/coatings/air-and-vapourtight-coatings/soudatight-lq

 

I believe the originator was https://www.blowerproof.co.uk/, they were certainly the first advert I saw.

[G and J]

5 minutes ago, Sparrowhawk said:

 https://www.soudal.co.uk/pro/products/coatings/air-and-vapourtight-coatings/soudatight-lq

Paint that changes colour when dry so you can see where to paint second coat - brilliant.

 

However, I’m guessing if my airtight strategy is principally based on taped membranes I’ll have little use for this?

[PNAmble]

4 hours ago, Nic said:

What exactly is this purple paint you speak of 😂

It’s a passive house certified airtightness layer. Doesn’t rip like a membrane.   Easy to look up 🤷🏻‍♂️

[PNAmble]

2 hours ago, Sparrowhawk said:

It's this one https://www.intelligentmembranes.com/ But you can get the same thing without distinctive features like the - ahem purple colour and breathless marketing - cheaper from other brands e.g. I used https://www.soudal.co.uk/pro/products/coatings/air-and-vapourtight-coatings/soudatight-lq

 

I believe the originator was https://www.blowerproof.co.uk/, they were certainly the first advert I saw.

Each to their own.  Unless is certified you can’t get passive house.  Happy you’ve got a cheaper solution.  Great thing about self build is there is no correct answer.  I’m sure your house is better than mine .  

[PNAmble]

2 hours ago, G and J said:

However, I’m guessing if my airtight strategy is principally based on taped membranes I’ll have little use for this?

If you have a membrane then yes you are correct you don’t need this.  The point is that there are different approaches.  We didn’t want a membrane, and had seen three passive houses during the design phase  which had cut into the membrane for various reasons and then taped the break and then stuggled on their first air test.   

[JohnMo]

3 minutes ago, PNAmble said:

Each to their own.  Unless is certified you can’t get passive house.

What rubbish. You don't have to use any certified materials, you just have to meet the specified standard. Sometimes it's easier to use certified products, like MVHR, but it's not manditory.

[Sparrowhawk]

1 hour ago, PNAmble said:

Unless is certified you can’t get passive house.

All 3 are certified:

• Blowerproof since 2019
• Soudatight LQ since 2020
• Passive Purple since 2020

An observation is that despite positioning as separate manufacturers Blowerproof and Passive Purple's certification both include the same "Primer „Primer 46“, Self-adhesive Tape „Butytape”" as part of the system.

[PNAmble]

Wow.  Purple paint isn’t like by this forum.  Sorry I mentioned it. House finished time to leave the forum. 

[Iceverge]

From experience the easiest and most robust way to make stuff airtight is to use tape to a solid non dusty surface like OSB or plastic. 

 

Membranes are pretty easy too so long as you take care with them and they're cheap. I designed out as many penetrations as possible in ours. 

 

The paints are messy and don't stick as well as the tapes in my experience. However they are good for irregular surfaces like blocks. It will need priming with a dilute coat first for concrete. I applied it neat and it pealed off too easily.

 

Parge coats are easy but labour intensive and don't deal with junctions. 

 

Airtight sealant is amazing stuff but my god is it messy sticky and impossible to clean. 

 

Airtight foam like FM330 is fine but quickly gets expensive and again requires lots of trimming and mess. 

 

If I had a choice I would opt for Tapes, OSB and membranes and avoid anything liquid/parge/foam altogether.

[Iceverge]

9 minutes ago, PNAmble said:

Wow.  Purple paint isn’t like by this forum.  Sorry I mentioned it. House finished time to leave the forum. 

 

I don't mind it. Like I say I used a couple of buckets of I think the Soudal stuff during our build. 

 

It's massive cheaper than any of the fancy tapes that stick to masonry. It can be used to do the back of wall chases etc too. Unlike other airtight measures it's forgiving to sloppyness, and the more you slather on the better.

 

I use it to paint the windows externally too to keep them windtight. 

 

With some Illbruck FM330 and some airtight paint over the top you can work wonders. Joining irregular block reveals to the OSB for example. 

 

We blew 0.31ACH by the way.