Ceiling joists and condensation

[bissoejosh]

On our build we've done as others have and laid our first floor joists on an airtight membrane/ VCL which is turned up and lapped over the joist ends before tucking neatly into the first floor airtight layer. The underside will do the same at ground floor level.

 

My question is about condensation forming towards the joist ends where they protrude towards the edge of the insulation in both upper and lower walls. Does the airtight membrane stop all air movement and therefore eliminate the risk or will warmer air still find it's way in somehow and then condense when it finds a cooler surface? I know in the US a lot is made of insulating rim boards correctly but this seems to be in situations without an airtight barrier. 

 

This must apply to a lot of TF builds or any construction type where joists carry across the walls at the 1st floor junction and I'm really wondering if the airtight membrane is a complete solution or not? 

 

Thanks

Edited May 2, 2018 by bissoejosh
additional content

[MikeGrahamT21]

If its a breathable membrane, that means moisture can pass through, so I'm guessing in this instance it wouldn't be of any use at all. You'll need a full AVCL to stop moisture from passing.

[nod]

11 hours ago, bissoejosh said:

On our build we've done as others have and laid our first floor joists on an airtight, breathable membrane which is turned up and lapped over the joist ends before tucking neatly into the first floor airtight layer. The underside will do the same at ground floor level.

 

My question is about condensation forming towards the joist ends where they protrude towards the edge of the insulation in both upper and lower walls. Does the airtight membrane stop all air movement and therefore eliminate the risk or will warmer air still find it's way in somehow and then condense when it finds a cooler surface? I know in the US a lot is made of insulating rim boards correctly but this seems to be in situations without an airtight barrier. 

 

This must apply to a lot of TF builds or any construction type where joists carry across the walls at the 1st floor junction and I'm really wondering if the airtight membrane is a complete solution or not? 

 

Thanks

If it’s breathable 

it isnt airtight 

[MikeGrahamT21]

Just now, nod said:

If it’s breathable 

it isnt airtight 

Not strictly speaking true, but not truly airtight. Something like Tyvek Supro is airtight to 50pa

[bissoejosh]

1 hour ago, nod said:

If it’s breathable 

it isnt airtight 

 

Apologies, badly worded on my part, to be exact its a Gerband SD airtight membrane / VCL combined.

 

 https://passivehousesystems.co.uk/product/gerband-sd-control-airtight-membrane/

 

It is fitted to control air leakage around the joist wall interface and I can see it doing that fine. My issue is that in theory warm air could travel into the 'fold' as it is open where the joist goes in. I think the solution is to fill this area with a closed cell expanding foam to stop air reaching the timbers, or alternatively external wall insulation for this small area to keep the joist ends above dew point. 

 

I was curious to see if the issue has been raised before on timber frame constructions as it must happen a lot. Plenty of people have mentioned using a tony tray equivalent on their TF's.

[ProDave]

This detail has come to be known here as a "Tony Tray"

 

The joists are INSIDE the air tight layer of the building so moisture in the air inside the building can get to the joists including the joist ends.

 

The devil is in the detail. What es beyond the joist ends?  In my case the external wall insulation so they should not be that cold. But if it is just a "normal" timber frame with a cold vented cavity then a brick or block skin then there might be issues?

[bissoejosh]

5 minutes ago, ProDave said:

This detail has come to be known here as a "Tony Tray"

 

The joists are INSIDE the air tight layer of the building so moisture in the air inside the building can get to the joists including the joist ends.

  

 The devil is in the detail. What es beyond the joist ends?  In my case the external wall insulation so they should not be that cold. But if it is just a "normal" timber frame with a cold vented cavity then a brick or block skin then there might be issues?

 

Exactly what I mean. You've explained it much more clearly than I managed!

 

On my build there is currently nothing beyond the joist ends except the OSB sheathing (not yet fitted) but we have a large cavity before the rain screen cladding to allow for our hidden guttering so I could very easily add upto 50mm on external insulation for this problem zone. Alternatively I could use expanding foam in the pocket around the joists which should stop the air reaching the timber. Any thoughts on which is a safer bet?