thermally broken wall ties and other questions re a new external wall construction

[low_and_there]

Hi folks, looking for help on a couple of questions re a new external wall we're building in the extension. For context, the wall is being built within cms of another wall so we can't render externally. Have been advised to apply a tanking slurry to the inside (a bit like you would in a basement) to acheive weatherproofing.

 

Two questions:

1) In this context, is it worth using thermally broken wall ties or is the fact that the tie will be on a Timber stud effectively act as a thermal break? Looking to achieve a high thermal performance.

 

2) Is it safe to put the PIR straight onto the coated blockwork or should we leave a void? If so, how many mm for that void?

 

Thanks!

 

---

Proposed fabric build up from outside to in:

External air (50mm max)
Dense fairfaced conc block (100mm)
Coated with a tanking slurry
PIR - 125mm
Timber studs 47mm filled with 25mm PIR
lined with fireproof board
plaster finish

 

[Redbeard]

Sounds like a potential recipe for interstitial condensation, whatever the Glaser Method (BS condensation risk analysis tool) says. Glaser method will only spit out results according to the gist of what you tell it. If you say it's got a VCL it will assume *perfect* VCL. I'd get it modelled at least, if you have not already done so, preferably in a dynamic model such as WUFI, but practitioners are hard to come by, in my experience.

 

The Glaser method, if I remember rightly, covers itself in saying that the outside of the 'sandwich' should be permeable in case water vapour *does* get through. Your tanking slurry would/could mean that the wall is no longer permeable.

 

Why do you, or your advisor,  think you will not be able to achieve waterproof-ness building 'blind' anyway? If it's 50mm from its neighbour you'd be pretty unlucky if it was classed as exposed. With careful use of a pull-up batten I would think you could get the joints pretty good, just not pointed.

[saveasteading]

If its not built yet, I'd be very fussy about build quality. You will never get another chance to inspect or repair it.

The blocks must not be cracked and the pointing must be perfect.

 

Add a vcl.

 

I'd use the strongest ties, and not worry about the thermal breaks.

 

Why do you need fire rated plasterboard?

 

The gap to the next wall is tiny. Likely to get damp and fill with leaves and litter. Will you be able to clear it?

Have you had professional advice or just the builder?

[joe90]

Just now, saveasteading said:

I'd use the strongest ties, and not worry about the thermal breaks.

+1, I didn’t use basalt ties on my passive esk build as I worked out the losses were minimal.

[low_and_there]

3 hours ago, Redbeard said:

With careful use of a pull-up batten I would think you could get the joints pretty good, just not pointed.

This sounds interesting - is this a 'method'? The bricklayer has said he'll do his best and also try to keep the gap clear of mortar but he was keen to flag that it won't be perfect - if there's a method he's perhaps not heard of for making the joints better then that would be interesting! Thanks @Redbeard

 

[low_and_there]

2 hours ago, saveasteading said:

If its not built yet, I'd be very fussy about build quality. You will never get another chance to inspect or repair it.

The blocks must not be cracked and the pointing must be perfect.

 

Add a vcl.

 

I'd use the strongest ties, and not worry about the thermal breaks.

 

Why do you need fire rated plasterboard?

 

The gap to the next wall is tiny. Likely to get damp and fill with leaves and litter. Will you be able to clear it?

Have you had professional advice or just the builder?

 

• Good point about the blocks not being cracked. Will inspect!
• Yes forgot to call out that there will be a VCL on the inside of the PIR.
• Fire-rated pb is stipulated by BC.

 

Yes the gap is tiny, hence the proposal to use a tanking solution to reflect that it is going to be impossible to guarantee it won't get stuff in it. It's not perfect. I don't think anything in this situation is perfect. It's not been suggested by the Builder but rather building control have indicated they've seen this used in similar situations successfully. Other suggestions welcome but anything involving leaving a bigger gap is not an option.

[joe90]

Perhaps a plank of wood on the floor of the gap with ropes so it can be lifted and cleared of any crap that falls when the wall is being built. The wall will be built “overhand” so pointing as they go should be possible IMO.

[Redbeard]

Yes, @joe90 exactly what I meant.  That way you can get the joints good and full, even if you can't get in to 'tickle up' the pointing. If it is so close, and thus so sheltered, I should have thought just striking them off flush would suffice.

[Redbeard]

Still bothered about the tanking. It seems to be an impermeable barrier at the very wrong point in the thermal 'sandwich'.

[Roger440]

7 hours ago, Redbeard said:

Still bothered about the tanking. It seems to be an impermeable barrier at the very wrong point in the thermal 'sandwich'.

 

Agree.

[low_and_there]

 

20 hours ago, Redbeard said:

Still bothered about the tanking. It seems to be an impermeable barrier at the very wrong point in the thermal 'sandwich'.

 

 

Where do you think the right place is in the sandwich? Or are you proposing simply trusting that the overhand pointing will be decent enough? Appreciate other suggestions if you have any. Not keen on the tanking idea myself but seems better than not having a weatherproof outer skin

[saveasteading]

A membrane will keep water out but allow the building to breathe.  You also know that it is in place, whereas tanking might flake off over time.

 

[Redbeard]

4 hours ago, low_and_there said:

Where do you think the right place is in the sandwich? Or are you proposing simply trusting that the overhand pointing will be decent enough? Appreciate other suggestions if you have any. Not keen on the tanking idea myself but seems better than not having a weatherproof outer skin

I had previously written:

 

"Sounds like a potential recipe for interstitial condensation, whatever the Glaser Method (BS condensation risk analysis tool) says. Glaser method will only spit out results according to the gist of what you tell it. If you say it's got a VCL it will assume *perfect* VCL. I'd get it modelled at least, if you have not already done so, preferably in a dynamic model such as WUFI, but practitioners are hard to come by, in my experience.

 

The Glaser method, if I remember rightly, covers itself in saying that the outside of the 'sandwich' should be permeable in case water vapour *does* get through. Your tanking slurry would/could mean that the wall is no longer permeable."

 

The 'right place in the sandwich' would be on the warm side of the insulation. Theoretically the foil on the PIR (the 'room side' of the 125 layer) should be the VCL, but it needs detailing *really* tightly, and I would suggest using air-tightness tape on joints and perimeters, not foil tape which, at the corners, is not strong under tension.. Normally you'd go for VCL immediately next to the plasterboard, but at that point your VCL is incomplete because you have battens. You could use a separate VCL in this position.

 

Is it possible to send us a picture or diagram of how this will look? How sheltered is it? How likely is wind-blown rain, etc.?

 

 

[saveasteading]

Part of the problem here, i think, is that the design is flawed in itself. The external blocks are porous and difficult to point.. 

Is there time to change to brick? Choosing an engineering brick perhaps, because they are waterproof and, inexpensive and the wall won't be visible. then the bricklaying will be a thin course, allowing better control of the mortar, and it can have waterproofing added to the mix.

That plus dress a dpc up the inner timber to catch any drips, and lap the vcl over that. leave a gap to the timber.  Fixing a vcl on the back of a timber isn't easy but is possible.