Stick built wall build up

[G and J]

So, the answer is on site stick built.  Took me a while but I got there.  
 

I've been looking at ubakus and that’s been rather illuminating, though it’s raised as many questions as it’s answered.  
 

I seek a wall build up that is circa 300mm thick, has a sensible u value, has the required fire resistance due to proximity of boundary being less than 1m. 
 

Experiments show using lots of PIR makes for a thin and insulative wall.  Rockwool or cellulose increase u value quite a bit.  I note the comments on here about PIR alternatives being better for sound attenuation, etc. and I thought long and hard about this but then I realised SIPs were PIR and reports appear to say that they make good houses.  
 

So, I've come up with a build up which appears to hit the mark but there is a warning on the drying reserve.  Should I worry about that? 

[Thorfun]

Only 11mm service cavity? Are you sure that’s enough?

 

I presume your cladding is horizontal as well? If so can you reduce the battens to 25mm?

 

we did 140mm frametherm 32  equivalent glass wool between the studs and 80mm PIR internally to get similar U-values. 

[G and J]

4 minutes ago, Thorfun said:

Only 11mm service cavity? Are you sure that’s enough?

 

I presume your cladding is horizontal as well? If so can you reduce the battens to 25mm?

 

we did 140mm frametherm 32  equivalent glass wool between the studs and 80mm PIR internally to get similar U-values. 

With the plasterboard, OSB and 11mm service void that will accommodate 25mm back boxes.  There won’t be any pipes in them.  (Though I will have to cut the hole in the OSB oversize so the back box can grip the plasterboard.)

 

We're planning vertical cladding, hence the 40mm for counterbattens, but I don’t know how to do that on ubakus.  
 

I like the idea of Frametherm 32 @ 140mm to deaden sound, but not sure it’s worth 60mm off of the width of the house.  Will investigate, thank you. 

[Kelvin]

Where us the airtight layer going? 
 

You’re cutting through the airtight layer to accommodate the backboxes? 

Edited June 20 by Kelvin

[G and J]

Just now, Kelvin said:

Where is your air tight layer going? 

Excellent question, and the simple answer is I don’t yet know.

 

Peeps have referred to using the sheathing as the airtight layer which makes sense until I get to the upstairs ceiling (I’m assuming a cold loft). 

[Kelvin]

You can use the OSB sheathing some folk have achieved good results using it. However you’re cutting into it and fitting backboxes. 

[G and J]

Ah, sorry, I wasn’t clear.  The OSB behind the plasterboard is just for stiffening and making the walls super solid and able to take light fixings everywhere.

 

The sheathing on the outside of the frame is cement board, and that was the layer I was looking at.

 

 

[JohnMo]

8 minutes ago, G and J said:

10 minutes ago, Kelvin said:

Where is your air tight layer going? 

Excellent question, and the simple answer is I don’t yet know

Could you move to behind the 50mm PIR in the 140mm studs.

 

PIR between studs, all need to be perfectly cut, foamed in place with zero gaps. Frametherm is really easy

 

37 minutes ago, Thorfun said:

Only 11mm service cavity

can you get your cooker cables and clips in the space. What are the electricians clipping cables to? As there is nothing for horizontal runs to clip to.

[G and J]

2 minutes ago, JohnMo said:

Could you move to behind the 50mm PIR in the 140mm studs.

Actually that’s a brilliant idea, well done.   The 50mm PIR would keep the vapour barrier lovely and safe and snug, and stop it rattling (does that happen? I’m guessing it might if it was the airtight layer).  I’ll pop it in ubakus and see if it objects.  Still don’t know how to seal to the upstairs ceiling though.  

 

5 minutes ago, JohnMo said:

PIR between studs, all need to be perfectly cut, foamed in place with zero gaps. Frametherm is really easy

When I looked at rockwool generally it had half the R value of PIR, so in my case if I swap out PIR for frametherm my u value drops by circa 0.35.  I’ve not compared the cost difference.  Yet. (Or for that matter how much extra PIR is needed to compensate).

 

24 minutes ago, JohnMo said:

can you get your cooker cables and clips in the space. What are the electricians clipping cables to? As there is nothing for horizontal runs to clip to.

I think the oven will run off the ring main.  Won’t be running any cables horizontally except through the posi joists.  The kitchen service void depth is the exception.  That is on an outside wall, but the will have a larger service void to accommodate pipes.  All other pipes will be in internal walls.
 

I tend to fix kitchen units slightly further out to make life easy running drains and stuff behind the units, and I don’t mind a slightly larger worktop - I’m a messy cook and I like to cover acres with mess lol

[IanR]

2 hours ago, G and J said:

Experiments show using lots of PIR makes for a thin and insulative wall.  Rockwool or cellulose increase u value quite a bit.  I note the comments on here about PIR alternatives being better for sound attenuation, etc. and I thought long and hard about this but then I realised SIPs were PIR and reports appear to say that they make good houses. 

 

PIR/PUR has good insulating properties per mm of thickness, but on its own is poor for sound attenuation and doesn't provide the best decremant delay.

 

SIPs don't particularly make good houses, they're popular with volume builders as they reduce the skilled labour requirement on site. SIPs and PIR/PUR insulation are better when combined with a masonary skin as it improves the overall decrement delay and sound deadening, but less good with a lightweight structure.

 

SIPs/PIR/PUR do offer a U Value advantage in a given wall thickness. By comparrison, celulose and rockwool are less good insulators (per mm of thickness), but do offer a better decrement delay for a lightweight structure. There is more to thermal perfomance and comfort than U Value alone. ie. to make an exageration a 1m thick stone wall has a much poorer U value than a 25mm thick PIR board but the temp profile inside the room will be very different. The PIR will react very quickly to outside temp changes but the solid stone wall will hold the two-week average temperature.

 

The phase shift of your build up is just under 9 hours. I might accept a little compromise to the U Value (given that your side walls are width restricted) to swap out a material that gives a slightly improved phase-shift, to try and lift that to 12 or 15 hours. This would also likely come with improved sound performance.

 

Since your front and rear walls are not thickness compromised, perhaps they could have a different build-up? ie. only compromise where you feel you have to.

 

 

Edited June 20 by IanR

[Kelvin]

Have you spoken with your electrician to make sure they are happy putting the oven on the ring main. While some electric ovens can go on the ring a lot of manufacturers state in the specs they need their own protected circuit. What else will be on the ring main? 

[G and J]

Thank you for all that Ian - it’s access to knowledge like that that makes this place so fantastic.

 

37 minutes ago, IanR said:

Since your front and rear walls are not thickness compromised, perhaps they could have a different build-up? ie. only compromise where you feel you have to.

We may indeed thicken these up but I’ll leave that for now, partly because the area of our side walls dominate our situation and partly because I can only cope with so many hard sums at once lol. (Similarly the question of floor and loft insulation.)

 

45 minutes ago, IanR said:

SIPs don't particularly make good houses, they're

But it seems, even highly cynical, little old, round, bald fellas fall for the hype sometimes.  Noted.

 

Swapping the 140mm PIR for 140mm Rockwool increases the u value to 0.169, which I expect we could live with.  Certainly if it made the house feel nicer - and it’s not worth giving up 3.5” of room space to get back to 0.129 in my initial build up.

Oddly though, changing to Rockwool decreases the phase shift from 8.8hrs to 8.5hrs.  Can’t pretend I understand what that means, so I probably should read up on phase shift.

 

[G and J]

11 minutes ago, Kelvin said:

Have you spoken with your electrician to make sure they are happy putting the oven on the ring main. While some electric ovens can go on the ring a lot of manufacturers state in the specs they need their own protected circuit. What else will be on the ring main? 

I used to put built in ovens on a separate circuit till I was told not to bother by the sparky certifying a refurb that included full re wiring.  That’ll be part of the planning I’ll do when I engage with a sparky.  If needed it’ll be easy enough to run in at first fix.

 

Re what else is on the downstairs ring, we’re pretty light on electric around the house, so not much in amperage terms.  We will be having a quooker tap but from memory I think they’re lighter than my kettle.

[Kelvin]

Not all electricians agree on these issues. Ours wouldn’t have installed the ovens on the ring for example. 
 

The Quooker can be plugged into any free socket. Ours happens to have its own circuit just because it’s in the island so separate from the induction hob. 

[saveasteading]

25 minutes ago, G and J said:

Swapping the 140mm PIR for 140mm Rockwool

Within the stud space we experimented on the practicality. Because stud never provides an exact, squared shape, it leaves gaps. We settled on using half thickness in  pir and then dense mineral wool pressed in over it. The thinner pir can be fitted more precisely  and the gaps are small and sealed off, thus becoming small insulating pockets.

It's a compromise.

As a bonus, the vapour barrier can deflect a bit when services are going in the service void.

[G and J]

4 minutes ago, saveasteading said:

Within the stud space we experimented on the practicality. Because stud never provides an exact, squared shape, it leaves gaps. We settled on using half thickness in  pir and then dense mineral wool pressed in over it. The thinner pir can be fitted more precisely  and the gaps are small and sealed off, thus becoming small insulating pockets.

It's a compromise.

As a bonus, the vapour barrier can deflect a bit when services are going in the service void.

Now that is an interesting idea.  So on top of the PIR/Rockwool (which totals 140mm I presume) what other insulation layer did you use?  And is this inside wood cladding or a brick skin?

[G and J]

Also, I know I’m a pain but I worry - should I worry about “drying reserve”?

[joe90]

2 minutes ago, G and J said:

“drying reserve”?

Not heard of that before?

[Kelvin]

9 minutes ago, G and J said:

Also, I know I’m a pain but I worry - should I worry about “drying reserve”?

Well moisture risks and how buildings move it through the materials over the course of the year is something worth worrying about. Arguably it’s more important than the overall tightness. I was also concerned about this so used Intello Pro Clima as the humidity variable check layer. 

[saveasteading]

26 minutes ago, G and J said:

And is this inside wood cladding or a brick skin?

Stick built with osb outer skin and vcl, then a ventilation gap and treated wood cladding.

Frametherm, from memory. Density is  rigjt for compressing  into the space and staying there.

 

[IanR]

34 minutes ago, G and J said:

should I worry about “drying reserve”?

 

Yep, since Ubakus can't determine how much of an issue it is. If there's a potential issue you then need to move to something like Wufi to determine how significant.

 

Lack of drying reserve could be too high vapour resistance on both sides of the build up, so if moisture gets in it can't dry out quick enough.

[G and J]

26 minutes ago, IanR said:

 

Yep, since Ubakus can't determine how much of an issue it is. If there's a potential issue you then need to move to something like Wufi to determine how significant.

 

Lack of drying reserve could be too high vapour resistance on both sides of the build up, so if moisture gets in it can't dry out quick enough.

I wonder if I’ve specified an inappropriate membrane on layer 9 then?

[G and J]

33 minutes ago, saveasteading said:

Stick built with osb outer skin and vcl, then a ventilation gap and treated wood cladding.

Frametherm, from memory. Density is  rigjt for compressing  into the space and staying there.

 

Do you have further insulation within the frame?

[Thorfun]

5 hours ago, G and J said:

With the plasterboard, OSB and 11mm service void that will accommodate 25mm back boxes.  There won’t be any pipes in them.  (Though I will have to cut the hole in the OSB oversize so the back box can grip the plasterboard.)

just be aware that trying to fit drywall backboxes to that thickness will be tricky or require more expensive backboxes. you would be wise to cut bigger holes in the OSB before plaster boarding (or even just leave a strip empty of OSB at socket height. trust me, it's a pain to try and cut out OSB/ply after the fact without wrecking the plasterboard!

 

my other comment/concern is the benefit of the OSB? yes it will make the walls more "solid" but is 11mm OSB enough to hold shelves etc? also OSB is expensive! 25mm battens are cheap. and as @Kelvin said, if you're using the OSB as an airtight layer then you're penetrating it and making it pointless. personally, I would forgoe the OSB, use 25mm battens and use an AVCL as your airtight layer. can then also wrap that under the ceiling to maintain that airtight layer.

 

and finally, you don't need a cold loft! you can insulate between the rafters to make a warm loft and carry the airtight layer around that. means anything that's stored in the loft is nicely inside the insulated envelope and any MVHR or pipework running there won't be cold.

[Thorfun]

4 hours ago, G and J said:

The sheathing on the outside of the frame is cement board, and that was the layer I was looking at.

 

the outside membrane needs to be breathable. the layer inside the insulation needs to be airtight. think you've got them the wrong way round? at the very least get an interstitial condensation analysis done to ensure that moisture won't be trapped in the wall nor in the house!