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Airtightnness taping externally and Posi joist junction


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My kit is currently going up and I've now started thinking about sealing the GF/FF junction where the posi-joists sit. I've considered the tony tray approach (see below) - but might be a little late as the Joists are sitting up on the wall plates. already to be spread out and nailed down. 

 

But is a better approach just to tape the outside of this joint with air tightness tape once they have fitted FF walls and prior to them wrapping the building? 

 

Also should I be taping all joints externally prior to them wrapping the building or is this overkill? 

 

As a third question - if you have a fully sealed area between the ceiling and floor above. Is there a possibility this could get condensation or mould growing with no air circulation? 

 

 

The junction is like this (but with SIPS below not TF):

 

image.png.32913e81041d05a855bc7168e429eb72.png

 

 

 

 

 

image.png.6dd123695dc9d74ea706bf82613f6b08.png 

 

 

@LA3222 I'm hoping you'll be along soon with the answer....

 

Edited by SuperJohnG
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Ha, I did the Tony tray bud but as you say, you've missed the boat on that one.

 

I'd go with tape now. The area to be covered will be the the thickness of the top chord of the joist, the floor deck, the soleplate for first floor walls, so approx 120mm. If you add some fudge factor in I'd probably go 200mm. 

 

The issue you will have is internally taping/sealing the wall all the way up to the underside of the first floor deck where the joists sit on the wall. There will be a few mm gap between joist and wall that you'll not be  able to get to.

 

Airtightness you'll be fine, but the VCL won't be continuous. Technically, according to kingspan a VCL isn't required with SIP?‍♂️

 

Maybe you could use some blowerproof paint in hard to reach areas? I've used some on an internal block wall, very easy to use.

 

The problem you have now is what can you get your hands in time as I expect the bods erecting it will be keen to crack on.

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Can you not get the Tony tray in quick? It sounds like the joists are just sat there at one end? You'll be in there way but sod it, get the membrane down where you can then pull it along as they move the joists to where they need to be. Distract them with food whilst you get it done?

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4 minutes ago, LA3222 said:

Can you not get the Tony tray in quick? It sounds like the joists are just sat there at one end? You'll be in there way but sod it, get the membrane down where you can then pull it along as they move the joists to where they need to be. Distract them with food whilst you get it done?

I possibly could but I'd need to be quick. What kind of membrane do I need? 

 

Do you just cut thick strips? Then just get it down below the joists inside, then put over where the joists are supported? Wrap around the ends the back up under the FF soleplate? 

 

Currently sitting like this...

20210521_212522.jpg

20210521_212511.jpg

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58 minutes ago, LA3222 said:

I'd go with tape now. The area to be covered will be the the thickness of the top chord of the joist, the floor deck, the soleplate for first floor walls, so approx 120mm. If you add some fudge factor in I'd probably go 200mm.

I think this is what I will have to do. Need to get some tape quickly and I can do it quickly before the guys wrap. Few 100mm wide rolls should do it? 

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Yeah you just need to get summat like Siga tape. You have plenty of time, they won't wrap that bit till the floor deck is down and walls up.

 

Looking at the pic you could get the membrane in. They working today? If they aren't working the weekend you could bob out and get some and do it quick time, will just need to lift each joist end up as you unroll.

 

Any VCL will do at this point so whatever you can source. 

 

As you say, I left it hanging down the outside/inside if the house, once the floor deck went down I folded it back into the house and the walls went down on top of it.

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4 hours ago, LA3222 said:

 

Any VCL will do at this point so whatever you can source

Typical hadn't really planned it. Is it just a VCL? I thought I seen somewhere it was a permeable membrane which wouldnt make sense. Guess it should hang lower than the joists internally so it can be taped to internal vcl?

 

Edit - I have two rolls of Capital plastics moisture barrier in the container which seems might be the job. 

 

So just lay it over the wall hanging below joists inside. Lay joists on top. Then flip it over onto the caberdeck before they fit the upstairs soleplate? 

 

(Hate rushing to do shit...and not knowing hownits to be done. ?)

Edited by SuperJohnG
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2 hours ago, SuperJohnG said:

Guess it should hang lower than the joists internally so it can be taped to internal vcl?

 

So just lay it over the wall hanging below joists inside. Lay joists on top. Then flip it over onto the caberdeck before they fit the upstairs soleplate? 

Yep and yep. The semi permeable bit is the 'intelligent' VCLs I believe - something like intello plus. The vapour resistivity changes dependent upon the RHI in/out during the different seasons. 

 

For the small section you are doing just wack a 'dumb' VCL in. Put something better in on the walls once its up. 

 

The VCL will.serve two functions for you, vapour barrier and air barrier.

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16 hours ago, LA3222 said:

The VCL will.serve two functions for you, vapour barrier and air barrier.

Thanks chief. 

 

I guess you don't VCL the GF ceiling. So how does moisture get out of there once you plasterboard? Or is it OK since it's in the envelope once you add the Tony tray? 

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If you're clamping down on air tightness then you need to put MVHR in or else the house will be like a swimming pool. My house was a nightmare over Xmas for condensation, the windows were drenched - I had to get some dehumidifiers in for 3 weeks to bring the humidity down. I've got my MVHR running now so I shouldn't have that issue again.

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6 minutes ago, LA3222 said:

I've got my MVHR running now so I shouldn't have that issue again.

No idea what your weather is like at the moment, but here (West Cornwall) it is is 9°C and 99% humidity.

Good test for MVHR (wish I had it, got window open as I am cooking).

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2 hours ago, SteamyTea said:

Won't the MVHR keep the condensation risk very low?

I was more thinking of the areas that it can access. I done the Tony tray today mostly (thanks @LA3222) but started thinking about well od you have the ceiling cavity sealed off after plasterboard but it is in the thermal envelope but has no MVHR then does that present an issue?

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1 hour ago, SuperJohnG said:

I was more thinking of the areas that it can access. I done the Tony tray today mostly (thanks @LA3222) but started thinking about well od you have the ceiling cavity sealed off after plasterboard but it is in the thermal envelope but has no MVHR then does that present an issue?

Nah, the use of MVHR for airtight house is a well beaten track. Moisture won't gather in the ground floor ceiling void if the rest of the house is only knocking at around 50%RHI. Dont overthink it?

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1 hour ago, SteamyTea said:

No idea what your weather is like at the moment, but here (West Cornwall) it is is 9°C and 99% humidity.

Good test for MVHR (wish I had it, got window open as I am cooking).

Supposedly knocking around 92% here, I hadn't thought about it until you mentioned, but I noticed the condensate has been dropping water into the bucket (I haven't done the drain connection yet!) and it is normally bone dry. The house is always knocking around 50-50% RHI so it seems to be working as intended?‍♂️

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