Build options | Wide cavity vs narrower with drylining

tobdom

Have just had discussions with Engineer regarding our new self-build which we're hoping to commence in the very near future (have planning already).

Have read a lot here and elsewhere about the different construction options and have varying concerns around them, but was hoping to get some opinions on our options.

Engineer stated that 'most' people these days seem to be going with a 100mm outer leaf, 120 cavity, 100 inner leaf with drylining/insulation to the internal of the external walls. Not sure if the 120 cavity is filled with pumped bead or full-fill board board (or board with air gap - not sure that would meet regs). The other option is a wider cavity (200mm?) filled with pumped bead, but I understand the wider cavity brings it's own challenges from an Structural Engineering & construction point of view.

My main query is if there are any major disadvantages/drawbacks to the option of drylining the inside of the external walls? Is this a cheaper option than going with a wide-cavity option?

I've been reading about the option of going with the 'standard' or traditional 100-150-100 but using the full-fill board, which should achieve the required U-values and eliminate the need to dryline internally. That would seem like a no-brainer, any reason why you wouldn't go this route?

Our plan is to have a good level of airtightness, UFH with A2W HP and MHRV.

Appreciate any input anyone would like to provide!

mrsWhippy

After a lot of investigation and cost exercises (with the help of our QS), we decided on the 200mm cavity with eco-bead. We had to have an engineer on board anyway so he would have been signing off on structure - the only question really was over block on flat versus block on edge for the inner leaf as we will have a concrete FF. We didn't have huge expanses of glazing though, and the shape of the house is fairly simple so it probably had minimal impact anyway.

We were turned off the idea of a partial fill cavity as it's rarely done correctly, boards are not fitted straight, allowed to get damp etc. The eco-bead gave us best bang for our buck, and with the benefit of a thermal imaging analysis post build, we should be able to verify correct installation.

Hope that helps.

Edited to add: Not drylining will allow you benefit from the thermal mass of the walls esp when you'll be using UFH.

tobdom

Thanks mrsWhippy. You mention your cost exercises - can you give an idea of what the difference in cost was like for the 200mm option you went with, versus other options which were considered?

The more I'm reading about it, the more I'm liking the idea of a 100-150-100 (possibly might have to block on the flat inner leaf in some sections) using the full-fill insulation board option, like this: http://www.xtratherm.com/wp-content/uploads/2012/01/CavityTherm2011irl.pdf

mrsWhippy

I'm afraid I can't remember now, it was a year ago now when this decision was made! But I do remember that the wider cavity made the most sense (for us) when considering risk, cost and resulting u-values.

Can't comment on the full fill board - but I do remember both the QS and architect commenting on the risk of these boards getting damp as they're being fitted. Though some may beg to differ!

Best of luck, it's a minefield out there

Calahonda52

Internal dry lining in a new build is not the way to go: much has been penned on it here already.
Its the wrong answer period.
Rather than a no see eco-bead filled wide cavity, what about EWI on 215 blocks on flat, hang windows on outside of the blockwork and fit EWI around them.

Member SAS did that about 2007 here, go look here

As for TI afterwards to rectify any cockups with the no see eco-bead: I hope it stays fine...

mjp

Calahonda, What's the reason for going against internal dry lining?
Will be going for 150 mm cavity with inner and outer leaf of 100mm blocks with cavity pumped. Will be going with UFH and think engineer has spec'd for drylining on external walls but want to make sure we're going for best option. Floors will have 125mm insulation boards with 50mm liquid screed.

tobdom

Calahonda52 wrote: »

Internal dry lining in a new build is not the way to go: much has been penned on it here already.
Its the wrong answer period.
Rather than a no see eco-bead filled wide cavity, what about EWI on 215 blocks on flat, hang windows on outside of the blockwork and fit EWI around them.

Member SAS did that about 2007 here, go look here

As for TI afterwards to rectify any cockups with the no see eco-bead: I hope it stays fine...

Thanks Calahonda - not sure if you meant to include a link in your post, if so, it doesn't seem to have come through. EWI is not something I have considered yet as it seems to be more of an 'unknown' compared to other methods. My understanding also is that it can be quite expensive - we also have a plan for some stone facing sections, so not sure how that would work.

Calahonda52

OP: no link, meant you look here on boards: use advance G search with SAS and EWI and boards as the site domain.
http://www.boards.ie/vbulletin/showthread.php?t=2056432238&page=2

I disagree very much with the unknown comment, at least you get to see how it is fitted

mjp, have to head out now to earn a crust but two quick points:
you loose the thermal mass of the insulated walls and you create an unnecessary thermal bridge at the corner unless the inner walls are also insulated at least 1m into the room, there are tapered insulated panels that address this but are not popular here.

tobdom

Thanks for the link, appreciate your input.

From reading that thread, it would kind of make up my mind to not consider EWI, due to apparent cost and concerns around durability, correct execution/installation etc. (which I appreciate is a concern regardless of approach taken).

Will go back to the Engineer and see what he calculates for required wide-cavity as an alternative to the originally proposed 100-120-100 with internal drylining.

Calahonda52

The process has moved on and you are not just stuck with XPS or similar.
For example you can get a system with a rendered cement board finish which acts as a rain shield/barrier and it can be ventilated at the back.
Combined with insulated foundations, and properly done roof insulation, EWI can give you the complete tea cosy effect.

What's the rest of the spec of the house:
e.g. whats the design energy load
Whats the ventilation strategy, including the target ACH?

Dudda

mjp wrote: »

Calahonda, What's the reason for going against internal dry lining?
Will be going for 150 mm cavity with inner and outer leaf of 100mm blocks with cavity pumped. Will be going with UFH and think engineer has spec'd for drylining on external walls but want to make sure we're going for best option. Floors will have 125mm insulation boards with 50mm liquid screed.

Thermal bridging at internal walls and loss of thermal mass is the reason for not using insulated plasterboard internally. You'll also have to parge coat or plaster the blockwork wall to achieve good airtightness so you're plastering and then covering with insulated plasterboard which doesn't make sense.

As for the floor get more insulation. With only 125mm you'll just about if not struggle to get a 0.15 wkm2 u-value which is required for floors with underfloor heating. Get 150mm min. of insulation. Remember you'll be paying for pumping heat into the floor so you'll want that heat going into the room and not the ground. Also only a 50mm liquid screed isn't that deep. The underfloor heating pipes usually need 50mm above the top of the pipe to avoid cracking. If the pipes are 16mm thick you'll need 65-70mm liquid screed unless you go for the more expensive screeds which don't require the 50mm above the pipes.

Calahonda52

Thanks Dudda for that analysis.:)
In addition, the question of services behind the dry lining needs to be addressed.
What is the plan for lighting and power cables etc on the dry lined walls: just cut slots in the insulation.......

tobdom

Calahonda52 wrote: »

What's the rest of the spec of the house:
e.g. whats the design energy load
Whats the ventilation strategy, including the target ACH?

I don't think I'm going to be sold on the option of EWI to be honest, but I really do appreciate the insight & opinions.

In relation to the above, this is all currently with the Engineer, so I'm waiting on that. Ventilation strategy is MHRV.

Calahonda52

tobdom wrote: »

I don't think I'm going to be sold on the option of EWI to be honest, but I really do appreciate the insight & opinions.

In relation to the above, this is all currently with the Engineer, so I'm waiting on that. Ventilation strategy is MHRV.

No worries, its still a democracy here.....

Just on the MHVR, make sure the unit is within the conditioned air space and keep the extract and exhaust way from the road side of the house.

I have posted on this here before and well as some links to list's of do's and don't as well as design issue.

Am signing off this thread now

Keep well.

sako 85

mrsWhippy wrote: »

After a lot of investigation and cost exercises (with the help of our QS), we decided on the 200mm cavity with eco-bead. We had to have an engineer on board anyway so he would have been signing off on structure - the only question really was over block on flat versus block on edge for the inner leaf as we will have a concrete FF. We didn't have huge expanses of glazing though, and the shape of the house is fairly simple so it probably had minimal impact anyway.

We were turned off the idea of a partial fill cavity as it's rarely done correctly, boards are not fitted straight, allowed to get damp etc. The eco-bead gave us best bang for our buck, and with the benefit of a thermal imaging analysis post build, we should be able to verify correct installation.

Hope that helps.

Edited to add: Not drylining will allow you benefit from the thermal mass of the walls esp when you'll be using UFH.

Sorry to jump on here. Question for Mrs Wippy as I'm planning something very similar to what you have completed, I was wondering what you went for on your internal leaf after, block on edge or flat?

mrsWhippy

sako 85 wrote: »

Sorry to jump on here. Question for Mrs Wippy as I'm planning something very similar to what you have completed, I was wondering what you went for on your internal leaf after, block on edge or flat?

Block on edge was fine in our situation - as signed off by our engineer. Yours might be different!

adrianegan3

Could it be possible to go 200mm Cavity with 2 x 80mm cavity board Xtratherm etc with broken joints . Would this’s be a good option

sydthebeat

adrianegan3 wrote: »

Could it be possible to go 200mm Cavity with 2 x 80mm cavity board Xtratherm etc with broken joints . Would this’s be a good option

How would you work broken joints with wall ties