Full fill Rigid insulation in cavity

[SuperPav]

Hi all, long time lurker/reader, am just hoping to finalise the building drawings with a view to (finally) get the build started in the new year. 

 

It's currently a 1950's Cotswold bungalow, and we will be building up from current eaves height. For a number of various reasons, knocking down and starting again is a non-starter, so it's up up up and away... We have decided on masonry construction.

 

I am hoping to use Celcon/Thermalites with thin joint mortar for the upper storey inner leaf, exterior will be Cotswold stone to match existing. Due to the wall thickness we can't get a cavity much more than around 110mm (possibly 120mm if we can get the Cotswold stone supplied in a narrower bed width).

 

I know that normally Celotex board is a no-no for full-fill due to it being impossible to install without gaps etc. However, we are planning on building the inner leaf up completely to eaves height, which means we will be able to sheath the whole building with rigid boards, with minimal cuts (the only walls over 2.4m tall will be the gable end walls), and all joints can be butted/sealed and taped over before the outer leaf of stone goes up. I am hoping this combined with thin joint construction will reduce the presence of any gaps in the inner leaf, and there should be no mortar snots or other protrusions into the cavity preventing the sheets from sitting flat?

 

I would prefer to use 100m solid boards, rather than the T&G 85-90mm as the benefit of those is limited if erecting the inner leaf first, and they cost several times that of the normal boards.

 

My question is in this instance, would it be acceptable to full-fill with normal celotex boards (or leave 10mm residual cavity)?

Otherwise we'd have to reduce the insulation to 75mm which I don't really want to do, and we can't increase the cavity much more without the stone becoming prohibitively expensive due to being non-standard size.

 

I've searched on the topic but can't find any that refer specifically to a build where the inner leaf goes up first in its entirety...

 

Thanks!

 

[Russell griffiths]

I cannot see how this will work, unless you are getting all your stone cut to a regulated size how will you keep a nice regular face, the stone will be all different widths which you then lay to a nice line by having more or less poking backwards into the cavity. 

[Declan52]

The insulation boards with t&g are much easier it fit. Having a t&g means there should be no  gaps where the boards join. They are also the correct height for your wall ties as well. 

You might be better using beads blown in as with random size stone your not always at the right position for the wall tie to sit straight and will need moved up/down too suit the stone which will leave holes in the insulation.

[tonyshouse]

Sheet insulation is all but impossible to fit, there will likely be a LOT of thermal bypass going on - avoid like the plague 

 

mitigation is to fit it all yourself ?

[SuperPav]

Thanks all, I was hoping that as the inner leaf would be up and complete first, that fitting sheet insulation to it should be straightforward (and we *can* fit it ourselves, but we'll be on site to watch the builder to make sure the detailing is adhered to ).

 

I just wanted to know if as a result of putting the inner leaf up first I can get away with using normal sheet at £15/m2 rather than the T&G Cavitytherm (or equivalent) at double that, given that it's main advantage is its ability to go up easily when coursed with normal block and brick. I assumed that taped 2.4x1.2m sheets going on a smooth thin joint wall will be better in terms of performance than using 1200x450mm T&G panels. If the consensus is that the T&G stuff performs a lot better, I'm happy to go with that.

 

The ties I'm not too worried about as they're impact driven into the celcon blocks so can be inserted as appropriate as the outer leaf goes up, to match the stone coursing heights (the stone courses vary between 60-150mm tall)

[tonyshouse]

Big sheets are OK then, I would foam them un place and together others might even tape them 

[PeterW]

Large sheets aren’t certified for cavity insulation use. The BBA certificate doesn’t include this use case and some BCOs may not accept it as acceptable use. 

[Iceverge]

 

[MikeGrahamT21]

You have to use an approved product for full fill PIR, either cavitytherm or the kingspan product which they’ve just brought out. I should point out though that neither is compatible with thin joint as I discovered some years ago, as the boards are sized for the normal mortar beds. So if you want thin joint it’ll either have to be wool full fill or PIR partial fill. I know you said about using remedial wall ties, but they are seriously expensive and also void the BBA for the cavitytherm product as it has channels for the ties to sit in.

 

also worth pointing out in a 100mm cavity for example none are actually full fill, there is a small cavity of 5-10mm still left.

 

as others have said for it to get past BC you will have to adhere to the BBA documents.

[PeterW]

@SuperPav why not use EPS beads and then line the inside with insulated plasterboard or PIR..? What is the existing cavity below the new extension filled with ..?

[SuperPav]

 

9 minutes ago, PeterW said:

@SuperPav why not use EPS beads and then line the inside with insulated plasterboard or PIR..? What is the existing cavity below the new extension filled with ..?

 

Hi Peter, thanks for your input. I could use EPS beads, I reckoned even the full 100-110mm cavity with EPS beads the U-value will be lower than with PIR. I'm now reconsidering it. I would prefer to avoid insulated plasterboard inside upstairs due to the (fixed) layout of some of the corridor areas and bathrooms which are limited in space, but can put 35mm PB on if necessary.

 

The walls below (ground floor) are a solid wall with no cavity - the "middle" bit between the outer stone leaf and the internal brick leaf is loosefill rubble/mortar to tie the two skins together. The downstairs will most likely get 35mm insulated plasterboard on the external walls.

 

 

14 minutes ago, MikeGrahamT21 said:

You have to use an approved product for full fill PIR, either cavitytherm or the kingspan product which they’ve just brought out. I should point out though that neither is compatible with thin joint as I discovered some years ago, as the boards are sized for the normal mortar beds. So if you want thin joint it’ll either have to be wool full fill or PIR partial fill. I know you said about using remedial wall ties, but they are seriously expensive and also void the BBA for the cavitytherm product as it has channels for the ties to sit in.

 

also worth pointing out in a 100mm cavity for example none are actually full fill, there is a small cavity of 5-10mm still left.

 

as others have said for it to get past BC you will have to adhere to the BBA documents.

 

Yes this is what I was worried about with regards to approved use. On the initial pre-visits BCO here seemed fairly practical and reasonable but doesn't seem overly enthusiastic on new stuff/methods! And if all the suppliers only warrant the foil faced PIR board for use with a residual 25mm cavity, the U-values vs EPS beads start to look a lot more comparable, so you've now got me re-thinking!

[A_L]

1 hour ago, SuperPav said:

I reckoned even the full 100-110mm cavity with EPS beads the U-value will be lower than with PIR.

 

Only if you intended to use 70mm or less of PIR and you use silver/graphite EPS beads.

[SuperPav]

22 minutes ago, A_L said:

 

Only if you intended to use 70mm or less of PIR and you use silver/graphite EPS beads.

 

Sorry, that should've said values HIGHER. In essence for me it's now a choice of either 80mm PIR with approx 25mm residual cavity, or approx 105mm filled with EPS beads. Using the full-fill cavity (e.g. Celotex CF5000 at 97mm for 100mm cavity) is too expensive, and I'm now concerned that using it with remedial ties will invalidate the certificate anyway! Although if the full fill PIR negates the need for insulated PB on the inside, then maybe the costs even out a bit.

 

The WALLTITE stuff posted above looks impressive but for some reason I would be worried that it doesn't actually fill the full void everywhere, and its benefit and cost only really applies when access to the cavity is difficult, which isn't the case here..

[Mr Punter]

Celotex CF5000 is no longer available post Grenfell.

 

How about stonework, a 75mm cavity, filled with 75mm Dritherm, inner 7.3N Celcon blockwork, 50mm PIR joins taped, 25mm vertical battens for service void, 15mm plasterboard.

 

Where you have heavy loads, fix 25mm x 150mm timbers.

 

If your SAP person counts the 25mm as a low e gap, you should get a u value of 0.18 W/m².K.

 

Edited January 1, 2021 by Mr Punter

[Iceverge]

What is your priority @SuperPav, low cost or maximum insulation? 

 

 

[Roger440]

11 hours ago, SuperPav said:

 

Sorry, that should've said values HIGHER. In essence for me it's now a choice of either 80mm PIR with approx 25mm residual cavity, or approx 105mm filled with EPS beads. Using the full-fill cavity (e.g. Celotex CF5000 at 97mm for 100mm cavity) is too expensive, and I'm now concerned that using it with remedial ties will invalidate the certificate anyway! Although if the full fill PIR negates the need for insulated PB on the inside, then maybe the costs even out a bit.

 

The WALLTITE stuff posted above looks impressive but for some reason I would be worried that it doesn't actually fill the full void everywhere, and its benefit and cost only really applies when access to the cavity is difficult, which isn't the case here..

If you check some other walltite videos, you will see it goes in as a liquid. there wont be voids. It can be used to crate flood proof walls.

[SuperPav]

10 hours ago, Iceverge said:

What is your priority @SuperPav, low cost or maximum insulation? 

 

 

 

Great question, probably should have started with that!  

 

The priority is to get a good fabric within the dimensional and planning constraints we have, at a reasonable (but not necessarily lowest) cost - in other words maximum efficiency is not a priority (or indeed a possibility) for us but happy to specify and do it right as much as possible.

 

The downstairs will always be poor, so we're being realistic. The downstairs walls are probably currently somewhere around 1.50W/m2 so even a value of 0.25 upstairs will be a big improvement. The house is also currently very drafty due to the floors (mixture of suspended timber and uninsulated slab on loosefill hardcore) so if we sort the floor hopefully it'll feel warmer.

[Russell griffiths]

Have you considered a timberframe up the top. 

Have you got the room to put a timberframe inside the stone walls downstairs. 

[MikeGrahamT21]

Timber frame is a good solution, which may also be required...have you spoken to a structural engineer about your existing foundations and have they been checked to ensure they are up to the job?

[Iceverge]

10 hours ago, SuperPav said:

 

Great question, probably should have started with that!  

 

The priority is to get a good fabric within the dimensional and planning constraints we have, at a reasonable (but not necessarily lowest) cost - in other words maximum efficiency is not a priority (or indeed a possibility) for us but happy to specify and do it right as much as possible.

 

The downstairs will always be poor, so we're being realistic. The downstairs walls are probably currently somewhere around 1.50W/m2 so even a value of 0.25 upstairs will be a big improvement. The house is also currently very drafty due to the floors (mixture of suspended timber and uninsulated slab on loosefill hardcore) so if we sort the floor hopefully it'll feel warmer.

 

In that case I'd just use Dritherm 32 fullfill cavity batts or EPS graphite blown beads. You'll end up with a U Value of about 0.3 but it sounds like you've got bigger fish to fry with airtightness and uninsulated floors and walls. 

 

Are you removing the joists currently in place to make a new first floor? What level of demolition are you going to have to do on the current house? 

[SuperPav]

I'll try and get a build thread up in one of the other forums, as I'm sure there'll be a lot more questions and changes!

 

Foundations have been assessed with trial pits by SE and calculated to be OK for upper storey in block work with stone outer (worst case scenario), so foundation loading shouldn't be a problem (groundbearing pressure limit 125kN/m2, current foundations are approx. 700mm wide at 1100mm depth, max loading calculated around 65kN/linear metre). Timber frame upstairs was always an option, but because of 3 structural ridge beams (the house is a U shape with infill in the middle), blockwork is easier to the gable walls to support the beams, otherwise we're into a lot of quite complicated goalposts and other elements to spread the load onto the downstairs. So definitely sticking with blockwork on the upstairs.

 

Can't afford to lose much space downstairs either so that will most likely get existing plaster removed, parge coat to the internal brick, and then dry-lined with insulated plasterboard. 

 

We're essentially going to be taking the "hat" off the current house, so rafters and ceiling off, wall plate off, the upper storey built up from that level, keeping downstairs ceiling height similar to where it is now. I'll probably put the first course of inner leaf blocks on marmox thermoboard to remove the thermal bridge. First floor joists will be metal webbed posi joists on hangers cut into the celcon blocks.

 

Having removed a few stones from the current wall to have another look following this thread, I now thing EPS or PU injection will give the best result upstairs since the outer stone leaf will be so uneven in the cavity, the cavity will range from 110-140mm in some cases due to the differing bed depth of the stones, if I use batts or sheets, those additional voids will be pointless whereas with EPS or PU they'll get insulated.

[Iceverge]

Either would would work fine. 

 

The house won't every feel warm however unless you get a descent air tightness strategy and all your hard work is null if wind can blow through the insulation. 

 

Building joists into the wall and is a tricky detail to get right. Do you have any drawings of this?

[Russell griffiths]

I’m not sure what sort of budget you have for this project, but let’s say for instance your going to be spending £150,000 plus for this, then I think there are better ways to go about it. 

 

No doubt it will look stunning, but it’s never going to be well insulated or a very airtight house. 

Just depends what you are after. 

I think it will be similar to a 1990s barn conversion, nice to look at but will still cost a bundle to heat.