Cavity wall

Moggaman · 08-11-2019 12:50am #1

Hi
I have a few queries re Cavity Walls.
1. I want to do a 100mm block inner and outer leaf with a 250mm beaded insulation cavity. Can I just get longer wall ties and ensure decent embedment ?.. surely this detail is fairly standard now and I don’t need the ok of a structural engineer.
2. The roof is bearing on the wall plate which is on the inner leaf. Do the wall ties connecting the inner leaf to outer leaf act as a conduit to take some of the roof load? Or is the outer leaf separate in its entirety to the roof?
3. What is yer view on hollowcore suppliers who say there slab can bear on a 100 mm inner leaf rather than a block on flat?

BryanF · 08-11-2019 1:37am

even if you opt out of BCAR, structural compliance with TGD A (structure) and all other building regs apply.

We can’t discuss structural maters in this forum, but at a 250 cavity you need an engineers design and sign off.

Moggaman · 08-11-2019 10:46am

BryanF wrote: »

even if you opt out of BCAR, structural compliance with TGD A (structure) and all other building regs apply.

We can’t discuss structural maters in this forum, but at a 250 cavity you need an engineers design and sign off.

Thanks. Yes I will need an engineer to sign off on the build but my question is this?...if I was building a normal cavity , the engineer would not be consulted because it’s a standard detail, I am not too familiar (yet) with TGD A but it doesn’t list any cavity widths at all, just says good practice is to use the right type of blocks, mortar making sure wall ties are in etc ( I could be wrong)...therefore , when does it become a non standard detail. I am not trying to cut any corner, I am genuinely wondering if wider cavities as being built now in probably every cavity masonry structure require more analysis than the normal cavity?

Gumbo · 08-11-2019 10:52am

Moggaman wrote: »

Thanks. Yes I will need an engineer to sign off on the build but my question is this?...if I was building a normal cavity , the engineer would not be consulted because it’s a standard detail, I am not too familiar (yet) with TGD A but it doesn’t list any cavity widths at all, just says good practice is to use the right type of blocks, mortar making sure wall ties are in etc ( I could be wrong)...therefore , when does it become a non standard detail. I am not trying to cut any corner, I am genuinely wondering if wider cavities as being built now in probably every cavity masonry structure require more analysis than the normal cavity?

You need a Structural Engineer to sign off on the wider cavity.
Believe it or not, its very uncommon in Dublin to go with this wide cavity.

I've only seen it once or twice on housing projects.

Basically after 150mm the 2 wall structures stop behaving as a system and are moreso a 2 individual walls now that have to be tied together correctly. theres also specific wall ties that have to be used.

Moggaman · 08-11-2019 11:23am

kceire wrote: »

You need a Structural Engineer to sign off on the wider cavity.
Believe it or not, its very uncommon in Dublin to go with this wide cavity.

I've only seen it once or twice on housing projects.

Basically after 150mm the 2 wall structures stop behaving as a system and are moreso a 2 individual walls now that have to be tied together correctly. theres also specific wall ties that have to be used.

Thanks, so the outer leaf has a structural job to do in relation to holding up the roof.?

Gumbo · 08-11-2019 12:25pm

Moggaman wrote: »

Thanks, so the outer leaf has a structural job to do in relation to holding up the roof.?

Depends on the Structural design, and only your Engineer can answer that.

Calahonda52 · 08-11-2019 9:11pm

For a 250 cavity I would be going on the flat for the HC

Moggaman · 29-11-2019 12:44am

I did some U Value Calcs and its seems that the PIR Board Insulation like CavityTherm or Xthratherm is the best option (In theory). With a thermal conductivity of 0.033 ish for bead, u would need to go to 200mm wide to make up the difference. What worries me though is that I read an article this evening saying that if the boards are not flush with the inner leaf, that the effectiveness of the insulation is reduced MASSIVELY.... I think a 5mm gaps is allowed. I am not in the house building area but I was wondering for those of u out there that watch insulation boards get installed regularly...is it possible to get them done near perfectly?. The issue i believe if they are not is that thermal loops will occur and suck the heat from inside out...
Also on a slightly different note, I saw a post by someone condemning insulated plasterboard on the inside face of the external wall. I thought there was only positive with that stuff but maybe there isnt...hanging stuff for example...will chasing for wires reduce the effectiveness..
Lots of questions ...
Thanks

MicktheMan · 29-11-2019 7:13pm

Moggaman wrote: »

1. I was wondering for those of u out there that watch insulation boards get installed regularly...is it possible to get them done near perfectly?
2. Also on a slightly different note, I saw a post by someone condemning insulated plasterboard on the inside face of the external wall. I thought there was only positive with that stuff

1. Possible? yes; Probable? no
2. You thought wrong for a number of reasons (dew point of wall in possible wrong place, air tightness?, increases thermal bridging, reduces thermal mass, reduces internal area ... has been discussed to death here on boards)

Moggaman · 30-11-2019 1:04am

MicktheMan wrote: »

1. Possible? yes; Probable? no
2. You thought wrong for a number of reasons (dew point of wall in possible wrong place, air tightness?, increases thermal bridging, reduces thermal mass, reduces internal area ... has been discussed to death here on boards)

Thanks Mick . I know it’s discussed a lot .... so much I get confused. I find it remarkable that professionals are specing insulates plasterboard on new builds if it’s negatives outweigh the positives .
Also, can I ask ur opinion in relation to Pir board in the cavity , u answered possible but not probable in relation to the board being fitted snugly against the inner leaf . If that is the case , are you suggesting not to use it and if not , are u not then looking at a huge cavity to bring the wall to approx 0.15 u value.....
Thanks

MicktheMan · 30-11-2019 9:05pm

Moggaman wrote: »

Thanks Mick . I know it’s discussed a lot .... so much I get confused. I find it remarkable that professionals are specing insulates plasterboard on new builds if it’s negatives outweigh the positives .
Also, can I ask ur opinion in relation to Pir board in the cavity , u answered possible but not probable in relation to the board being fitted snugly against the inner leaf . If that is the case , are you suggesting not to use it and if not , are u not then looking at a huge cavity to bring the wall to approx 0.15 u value.....
Thanks

On your first point, perhaps the wrong professionals are specifying the fabric detail. Imo, too many vested interests with their snout in the trough. Take the following scenario; A certain elemental Uvalue required to satisfy bregs. Professional has attended a CPD day sponsored by a product manufacturer who promotes at the CPD a certain elemental buildup to satisfy the Bregs using their product. The professional, who's qualification is not in building technology, subsequently uses this off the shelf solution for their client's project without a second thought. Box ticked. Everybody is happy:rolleyes:

Have you considered that the current method of cavity wall construction may be no longer fit for purpose given the current part L requirements and that a major rethink on our build methods and training might be an idea.

Moggaman · 30-11-2019 11:07pm

MicktheMan wrote: »

On your first point, perhaps the wrong professionals are specifying the fabric detail. Imo, too many vested interests with their snout in the trough. Take the following scenario; A certain elemental Uvalue required to satisfy bregs. Professional has attended a CPD day sponsored by a product manufacturer who promotes at the CPD a certain elemental buildup to satisfy the Bregs using their product. The professional, who's qualification is not in building technology, subsequently uses this off the shelf solution for their client's project without a second thought. Box ticked. Everybody is happy:rolleyes:

Have you considered that the current method of cavity wall construction may be no longer fit for purpose given the current part L requirements and that a major rethink on our build methods and training might be an idea.

Well I get that timber frame is probably the way to go but I am not satisfied to go with it , nor am
I satisfied that external insulation with acrylic plaster is the way to go(unproven long term plaster ) in my view and u can’t hit a sliothar off it.
I want the tried and test sand cement render on the outside, that much I know but not with a timber frame inside it....so I’m left with a cavity....anyway part L can be achieved with cavity and good workmanship?

Cavity wall

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