Airtightness issue between double trusses

[Thorfun]

greetings. we're doing the airtightness at the moment and have come across and interesting/annoying detail around the attic trusses! we have 4 x 'double trusses' where two trusses are joined together. sadly they're not joined very tightly and it's become apparent that they are now a hole through our airtightness layer. 😞

 

 

so that gap will go to the outside where the cladding is already installed covering it so I can no longer get to it from the outside. that gap also spans the entire width of the roof so taping underneath and on top, although possible, is not ideal especially as so of the length of the joists are covered with chipboard flooring.

 

I was thinking of 'injecting' expanding foam into that gap to try and fill it as much as possible to reduce the air hole but the extension straw I have is too fat.

 

 

the gap is about 5mm on this particular joist but others are smaller. 

 

any ideas about how best to block this air gap?

 

ps. I do have a 3D printer so could, in theory, print myself a nozzle that's thin enough to fit in the gap but my 3D modelling skills are limited.

 

 

[markc]

Squirty foam is unlikely to travel right through or even to centre, you could try some 10mm(ish) foam sheet and push it in with a steel rule or similar.

this is one of those “I wish I had thought of it before” jobs

[Thorfun]

4 minutes ago, markc said:

Squirty foam is unlikely to travel right through or even to centre, you could try some 10mm(ish) foam sheet and push it in with a steel rule or similar.

this is one of those “I wish I had thought of it before” jobs

yeah, I definitely wish I'd thought of it before as I could've easily sealed from the outside! but at no point when the TF was being erected did it cross my mind that those double trusses would cause a hole in the airtightness layer.

 

foam sheet could be an option I guess. maybe I could get some compriband, or similar, and push that up with a steel ruler? then that would expand to fill the gaps?

 

any other possible products that could do this job?

[Conor]

Could you get a silicon tube nozel in there? That would work better than the foam and if you can get at it from the top is should work.

 

Or you could drill a 20mm hole right through, fully fill it with foam/silicone, then tape back to the membrane. Airtight paint is also a good job for this kind of unforseen detailing.

[Thorfun]

Just now, Conor said:

Could you get a silicon tube nozel in there? That would work better than the foam and if you can get at it from the top is should work.

 

Or you could drill a 20mm hole right through, fully fill it with foam/silicone, then tape back to the membrane. Airtight paint is also a good job for this kind of unforseen detailing.

it's tight to get any sort of nozzle in unless I can find/print a nozzle that's less than 5mm in diameter. drilling is an interesting option but I'd be concerned about taking too much off the roof trusses and it affecting their weight bearing capability. (yes I am paranoid!!). although, I guess a 10mm hole would allow a nozzle to get in there and would only strip a couple of mm off each joist. 

[jayc89]

A bead of airtight sealant followed by tape to join your existing airtight detail at either end?

Edited August 10, 2022 by jayc89

[Thorfun]

Just now, jayc89 said:

A bead of airtight sealant followed by tape to join your existing airtight detail at either end?

I do have some Orcon-F but, again, the issue is the gap is too small to get a nozzle up there. 

 

also, the joists run the width of the house so I can't just seal top and bottom where the AVCL is as air can get between the joists along the entire length of the double truss.

[Wumpus]

Ignore me, bad suggestion.

Edited August 10, 2022 by Wumpus

[Thorfun]

5 minutes ago, Wumpus said:

Ignore me, bad suggestion.

now I really want to know what it was! 😂

[Wumpus]

2 minutes ago, Thorfun said:

now I really want to know what it was! 😂

I had suggested Intello fabric strip to cover the whole double joist and tape to the adjacent fabric, but I missed your point about it running the entire width of the house so you will likely have a gap at the ends?

Edited August 10, 2022 by Wumpus

[markc]

2 minutes ago, Thorfun said:

now I really want to know what it was! 😂

Me too

Doh, too late

Edited August 10, 2022 by markc

[cwr]

Cross drill one truss from the side in a few places to get airtight foam in?

After first experimenting with some off-cuts to see what pitch you'd need for the foam to fully fill.

[Thorfun]

10 minutes ago, cwr said:

Cross drill one truss from the side in a few places to get airtight foam in?

After first experimenting with some off-cuts to see what pitch you'd need for the foam to fully fill.

this is an interesting idea and extends @Conor's suggestion. I presume cross drilling is less detrimental to the structural integrity of the trusses than drilling vertically between the 2 and 'taking some off the sides', so to speak? I could also drill alternate holes on each truss to halve the number of holes in a single truss?

[markc]

Cross drilling on the centreline of a cord will have virtually no impact of its integrity. Same with drilling holes in centreline of a beam

[Thorfun]

1 minute ago, markc said:

Cross drilling on the centreline of a cord will have virtually no impact of its integrity. Same with drilling holes in centreline of a beam

that's good to know. don't suppose you'd know if I drill vertically between the two cords taking a couple of mm off each one how that would affect the integrity do you?

 

something like the below:

 

 

then I could get a nozzle all the way and create a vertical line of foam/silicone/orcon-F and then tape underneath and above and that should seal it all.

 

something like this?

 

[ETC]

Screw the trusses closed?

[Thorfun]

2 minutes ago, ETC said:

Screw the trusses closed?

very interesting idea. although I guess I couldn't guarantee how airtight that will be so I'd like to use some form of foam/tape/silicone to ensure an airtight barrier. also, would that not make them 'twist' where screwed together when compared to the rest of the truss? and, any potential detrimental effect on the battens and tiles above? but could potentially think about doing both solutions, foam and screws!

[markc]

@Thorfun drilling down you are taking material from CSA however it is minimal and it is close to the truss heal. Drill down to around half depth and the effect will be negligible (removing material from compression area while maintaining tension material). Seems like a good plan

[Thorfun]

2 minutes ago, markc said:

@Thorfun drilling down you are taking material from CSA however it is minimal and it is close to the truss heal. Drill down to around half depth and the effect will be negligible (removing material from compression area while maintaining tension material). Seems like a good plan

I'm sorry but I don't understand the whole removing material from compression area while maintaining tension material bit!

 

I'd have to drill 'upwards' as I can't get a drill in above close to the end of the truss due to the angle of the roof (35°). so are you suggesting to drill at that point I circled in red to only the half way up the cord? that could potentially give me enough to get the nozzle up and then fire away with expanding foam which should, in theory, expand upwards to fill the gap between the cords. then as I pull the nozzle out I keep firing with foam which will fill the gap in the hole I'd just drilled. I can then tape top and bottom between that line of foam and the AVCL and job done! 🙂 

[markc]

1 minute ago, Thorfun said:

I'm sorry but I don't understand the whole removing material from compression area while maintaining tension material bit!

 

I'd have to drill 'upwards' as I can't get a drill in above close to the end of the truss due to the angle of the roof (35°). so are you suggesting to drill at that point I circled in red to only the half way up the cord? that could potentially give me enough to get the nozzle up and then fire away with expanding foam which should, in theory, expand upwards to fill the gap between the cords. then as I pull the nozzle out I keep firing with foam which will fill the gap in the hole I'd just drilled. I can then tape top and bottom between that line of foam and the AVCL and job done! 🙂 

No problem, just seen the red circle, keep the drill as small as possible and you will be fine. The amount you are removing is minimal and you are very close to heal anyway. Get on with it.

[Thorfun]

3 minutes ago, markc said:

No problem, just seen the red circle, keep the drill as small as possible and you will be fine. The amount you are removing is minimal and you are very close to heal anyway. Get on with it.

thanks. will definitely be minimal removal as I will use an 8mm bit which is enough for the foam gun tube to fit up, then I can fill while retracting the tube and it will fill down the gap.

 

now just need to decide whether to use expanding foam or Orcon-F!

[Thorfun]

and thank you to all the other suggestions. this forum, once again, proves how awesome it is. 🤘

[Dan F]

We have load of these and making the roof airtight was a nightmare.  This was MBC's problem though and they did a good job of making it airtight.. just took them a long time.  Using standard trusses was a bad idea, especially on our project which had a non-trivial roof.

 

The technique that MBC seemed to use on ours was to use tape one side, then feed the tape through the gap (bunching it up a bit) and then continue the tape on the other side.   A bit like the way I suppose a g-string works (not that i've ever worn one!).

 

Edited August 10, 2022 by Dan F

[Thorfun]

4 minutes ago, Dan F said:

We have load of these and making the roof airtight was a nightmare.  This was MBC's problem though and they did a good job of making it airtight.. just took them a long time.  Using standard trusses was a bad idea, especially on our project which had a non-trivial roof.

 

The technique that MBC seemed to use on ours was to use tape one side, then feed the tape through the gap (bunching it up a bit) and then continue the tape on the other side.   A bit like the way I suppose a g-string works (not that i've ever worn one!).

 

so, basically, stuff tape up there and hope for the best!? 😉

 

those cords seem quite shallow compared to ours though which is why that might've worked. ours are 197mm deep so would be really hard to try and force tape up there.

[markc]

The G-String technique … I like it.