Just spotted an issue that I thought I had cleared up with condensation in our wall build up.

[MikeSharp01]

Way back in 2016, during our design exercise, which you all helped with here: 

 

I thought the build up of the wall (inner to outer) was:

 

1. 12.5mm Platerboard
2. 35mm service void.
3. Air tight barrier.
4. 300mm I Joists filled with DRITHERM .032 insulation
5. 12mm OSB (4)
6. 40mm Kingspan K12
7. Tyvek UV breather membrain
8. 25mm Verticle battens (Ventilated)
9. 25mm Horizontal battens (Ventilated)
10. Marley Internit Thrutone cement tiles.

 

However I have now double checked the architects drawings, and those submitted to BC and the build up is now:

 

1. 12.5mm Platerboard
2. 35mm service void.
3. Air tight barrier.
4. 300mm I Joists filled with DRITHERM .032 insulation
5. 12mm OSB (4)
6. Tyvek UV breather membrain
7. 40mm Kingspan K12
8. 25mm Verticle battens (Ventilated)
9. 25mm Horizontal battens (Ventilated)
10. Marley Internit Thrutone cement tiles.

 

You can see that the Breather and KingSpan have swapped places. Simple question, does this make any sense putting the breathe membrane before the PU Kingspan, which is foil faced, as I don't get how the vapour can get out of the breather if it is covered by foil faced PU, or am I worrying about nothing?

 

 

 

 

Edited September 8, 2019 by MikeSharp01

[nod]

No it makes no sense 

I would check that it isn’t a Typo 

 

 

[Jeremy Harris]

I agree with @nod, looks like a simple error that's transposed those two elements.  Good thing to have spotted at this stage.  I'd also add that the airtight barrier might be better described as a vapour tight barrier, as it's primary purpose is control of vapour migration outwards through the structure.

[Mr Punter]

Did you look at having the Kingspan inside the I joists?  There was or is a rule of thumb that suggests that if you have more than one insulation type the less vapour permeable should go inside the more vapour permeable so that any vapour getting into the structure from the inside can easily diffuse out.

[MikeSharp01]

54 minutes ago, JSHarris said:

airtight barrier might be better described as a vapour tight barrier,

Yes it should say VCL really - it was a thread yesterday that prompted me to check as I was merrily working away on the principle that the housewrap was on the outside

 

I agree I think the architect got it wrong for some reason. I will drop him a note to check his thinking but it is on every section drawing in this form so his cad package, Archicad, just linked it through everywhere.

 

Just now, Mr Punter said:

Did you look at having the Kingspan inside the I joists?

 

No but there is a reason the PU is on the outside of the sheathing so as to ensure the cold bridges of the I Joists and the ends of things like (only) the ridge beams have insulation beyond them. You are right it seems a bit daft to have the permeability the wrong way around but I surmised that as long as the VCL is less permeable than the PU (Even though it is foil backed) and I don't foam the PU edges tight the water vapour will pass out of the structure.

[A_L]

On ‎08‎/‎09‎/‎2019 at 11:50, MikeSharp01 said:

but I surmised that as long as the VCL is less permeable than the PU (Even though it is foil backed) and I don't foam the PU edges tight the water vapour will pass out of the structure.

 

@MikeSharp01 except that the VCL is not less permeable than the PU layer. If the VCL (layer 3) is the one you say you used here

https://forum.buildhub.org.uk/topic/11282-alternative-vcl-to-dupont-airguard-control/?tab=comments#comment-189936

 

then it has a vapour resistance of 10-30 Mn.s/g. The foil layer of a board insulation has a vapour resistance of at least 70Mn.s/g

https://www.ribaproductselector.com/Docs/6/01636/external/AG953197.pdf

and 50mm of polyurethane a vapour resistance of about 5.75Mn.s/g giving a total of at least 75Mn.s/g.

I think you should also add about 8.5Mn.s/g for the OSB to the PU layer resistance

[MikeSharp01]

5 hours ago, A_L said:

@MikeSharp01 except that the VCL is not less permeable than the PU layer. If the VCL (layer 3) is the one you say you used here

Have not used it yet so still a chance to play with the details I guess.

[MikeSharp01]

5 hours ago, A_L said:

he foil layer of a board insulation has a vapour resistance of at least 70Mn.s/g

https://www.ribaproductselector.com/Docs/6/01636/external/AG953197.pdf

and 50mm of polyurethane a vapour resistance of about 5.75Mn.s/g giving a total of at least 75Mn.s/g.

I think you should also add about 8.5Mn.s/g for the OSB to the PU layer resistance

Hmmm... this is turning into something of a nightmare, we have all the insulation on site although we don't yet have the membranes - was on my list for this week. Anyhow the RIBA product selector sheet indicates this: "Joints between boards will facilitate the passage of water vapour under normal conditions of temperature and humidity." It is this that I had hoped would do the job for us and mean that water vapour could escape faster than the internal VCL. As I have not yet procured the VCL I could switch to 'AirGuard smart' which according to its blurb "lower sd-value of 0.05m (=25MNs/g) and an upper sd-value of 30m (=150MNs/g) for its adaptable AVCL DuPont™ Airguard® Smart."  and should push it through in winter conditions. Alternatively I could run a lawn pricker over the sheets to puncture the foils!

[Big Jimbo]

Sorry i can't help Mike, but whatever you do, don't do anything with haste. Defo nightmare if you get it wrong. I think that when i have been reading one of the biggest problems seems to be, Site 1 says one thing, and the other 5 sites say something else. Who do you trust ? Good luck

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Mr Punter]

22 minutes ago, the_r_sole said:

how much of a difference to the uvalue does moving the 40mm to the inner side of the timber frame make?

 

I will speculate that it makes none whatsoever and nor will it make any difference to thermal bridging if the I joists.  It will also be much easier to support plasterboard on this than it will the external tile cladding, which will be far more demanding. 

[Jeremy Harris]

I agree with the above, and would add that putting 40mm of PIR or similar on the inside may also make it easier to fix the VCL and tape it all up, plus it will decrease the overall internal vapour permeability, so looks to be a pretty good option all around, especially if you can arrange it so that there is minimal impact on internal space.  You can probably get away with slimming down the service void a bit, as most stuff will fit into a 25mm void OK.  We have a 50mm service void and all it does is waste a bit of internal space, IMHO, as the biggest thing running down it are runs of 15mm pipe.

 

Perhaps consider this build up, from the inside to outside:

 

1. 12.5mm Plasterboard
2. 25mm service void.
3. Air tight barrier.
4. 40mm Kingspan K12
5. 300mm I Joists filled with DRITHERM .032 insulation
6. 12mm OSB (4)
7. Tyvek UV breather membrane
8. 25mm Vertical battens (Ventilated)
9. 25mm Horizontal battens (Ventilated)
10. Marley Internit Thrutone cement tiles.

This would definitely be OK, as the vapour permeability gradient is all going the right way, i.e. least vapour permeable on the inside, most vapour permeable material on the outside.

[Mr Punter]

Is it too late to move the timber frame out by 40mm?  That way you could do the internal Kingspan with no loss of space.

[MikeSharp01]

9 hours ago, JSHarris said:

• 12.5mm Plasterboard
• 25mm service void.
• Air tight barrier.
• 40mm Kingspan K12
• 300mm I Joists filled with DRITHERM .032 insulation
• 12mm OSB (4)
• Tyvek UV breather membrane
• 25mm Vertical battens (Ventilated)
• 25mm Horizontal battens (Ventilated)
• Marley Internit Thrutone cement tiles.

Thanks for this Jeremy. Sadly we need the kingspan on the outside to deal with the cold bridges created by the large timbers in the structure extending to just inside the osb sheathing in several places. I have now sought out a firm to do a proper wufi analysis to check this out once and for all. 

[MikeSharp01]

7 hours ago, Mr Punter said:

Is it too late to move the timber frame out by 40mm?  That way you could do the internal Kingspan with no loss of space.

Yes the walls, well parts of them - I joists and sheathing, are up on the sole plate and that is seriously nailed down.

[MikeSharp01]

9 hours ago, Mr Punter said:

thermal bridging if the I joists. 

I am sure the U value will be the same but its not the I-joist bridges we are dealing with its the 220x310 gluelam and KERTO ends that butt up to the sheathing in a number of places it is designed to deal with. 

 

Today we have found evidence of a solution, used elsewhere, that involves drilling / punching holes in the foils. 

 

Given all this is helpful speculation I have engaged a WuFi professional to work it through.

[Mr Punter]

@MikeSharp01 I did not mean to shit you up and I think your solution as originally proposed will be fine.  Most condensation issues happen on flat roofs, not walls.

[Jeremy Harris]

How significant are these thermal bridges?

 

We have a fairly big glulam ridge beam that penetrates both 300mm thick gable end walls to the outer skin.  This doesn't create a significant thermal bridge, though, as it's surrounded by internal insulation, right out to the outer wall skin.  IIRC, the thermal conductivity of these beams along their "grain" is around 0.22 W/m.K, so their equivalent U value  for the ~300mm length through the wall is around 0.73 W/m².K, about the same as our windows.