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Worth doing EWI with a 50mm cavity?


House man
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15 minutes ago, House man said:


It is possible but why not put the 50mm PIR between the battens and plasterboard screw straight infront of it saving the 25mm? 

 

I've done exactly that myself before, however, knowing what I know now...

 

You get cold bridging through the battens.

 

You've also got to dig into the 50mm pir to run cables in channels, or conduit etc. 

 

Easy to put extra sockets in with the service cavity (you can save cable too by going horizontal between sockets).

 

Plenty of benefit for losing that extra inch. 

 

 

Edited by Onoff
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On 30/12/2021 at 16:35, House man said:

 

Thank you, was more the answer I was looking for in the beginning but got a lot of insight from nod, now so I have made my mind up to go ahead with the 100mm XPS under the render, but would like to ask if I should fill that cavity at all? The roof isn't on yet so wondered if you would think of giving it a fill of some sort of leave it as it?


sorry I’m slightly lost here. 
 

If the roof isn’t on, do you mean this is a new build extension ..? If so, you can’t have met building regs with a 50/50 insulation and cavity build up so how is this being signed off ..??

 

Or are you saying the builder has cocked up with the insulation levels ..? 

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4 hours ago, PeterW said:


sorry I’m slightly lost here. 
 

If the roof isn’t on, do you mean this is a new build extension ..? If so, you can’t have met building regs with a 50/50 insulation and cavity build up so how is this being signed off ..??

 

Or are you saying the builder has cocked up with the insulation levels ..? 

Im saying I suspect the builders have not correctly installed the boards edge to edge and left gaps which were not filled, and am just wondering exploring further insulation options. It is a new extension currently ongoing.

 

 

5 hours ago, Onoff said:

 

I've done exactly that myself before, however, knowing what I know now...

 

You get cold bridging through the battens.

 

You've also got to dig into the 50mm pir to run cables in channels, or conduit etc. 

 

Easy to put extra sockets in with the service cavity (you can save cable too by going horizontal between sockets).

 

Plenty of benefit for losing that extra inch. 

 

 

 

That makes sense, at least we find our warm air gap in the inch. Can you state how much state how much of a difference the 50mm internally had?

Edited by House man
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7 hours ago, House man said:

That makes sense, at least we find our warm air gap in the inch. Can you state how much state how much of a difference the 50mm internally had?

 

I've not done a air gap / service cavity build up yet but am planning to.

 

As above there's two particular rooms / wall make ups in my house that I have retrofitted:

 

Bathroom:

 

Original: Render/4" brick/2" cavity/4" brick

I added: 2 - 4" pir between battens/vcl/12.5mm plasterboard or Aqua Panel/tiles.

 

Bloody freezing! Due I'm sure to the UFH not being connected, nor the towel radiator. On the North facing wall the pir  insulation layer is tapered from 2" one end to at least 4" (maybe 6" thinking some more) the other end of a 3m wall. Behind the illuminated pocket is only 1" of pir.  It tapers as I squared the room off by studding it out.

 

SAM_2641

 

20180426_205753

 

2018-07-09_07-20-16

 

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I've only got 100mm pir in the ceiling (at the moment) below a cold loft and loads of ceiling penetrations to make good. High hopes though when it's all done. Also got 150mm pir + 25mm EPS under the floor. 

 

This room though will be very near airtight when done. 

 

Study:

 

Original: Render/4" block

I added: 60mm cavity with 50mm Rockwool batts/4" block/50mm battens with 50mm Rockwool batts in between/12.5mm plasterboard.  

 

Warmest room in the house. 4" roll insulation under the floor between joists. 11" fluffy roll above the ceiling. 1 small double rad. 

 

Though there's no vcl anywhere in the study but my attention to detail at joints and junctions is pretty good so can only help.

Edited by Onoff
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8 hours ago, House man said:

Im saying I suspect the builders have not correctly installed the boards edge to edge and left gaps which were not filled,


But 50mm doesn’t meet building regs however well or badly it’s been fitted !! So there must be some drawings or spec that they are following to meet regs ..?

 

TBH I would be going down the internal insulated plasterboard route. 

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On 30/12/2021 at 11:52, House man said:

Hi guys,

 

Ongoing extension 

 

Have 50mm cavity insulation with 50mm air gap, suspect 50mm insulation boards not fitted properly

 

Just thinking wouldn't it be kind of useless to have 100m external insulation wall insulation  since I have a 50mm air gap between walls? 

Thoughts?

 

If the building is ongoing and the work is not done properly, get your builder to redo it to the required spec at his cost. Check your drawings. If it were me, I'd have the builder remove the boards, and pump cavity with platinum EPS beads. That will get you to about 0.22-0.25. Going down the route of external or internal wall insualtion is huge money in comparison.

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On 31/12/2021 at 20:26, House man said:

Having read the latest comments i assume the best action would be to have 50mm of insulation inside behind the plasterboard rather than 100mm on the outside, especially since im suspecting the builders of not taking the care to thoroughly execute the installation of the 50mm boards without gaps around the dozens and dozens of wall ties. The silicone K rend alone should probably more than suffice for providing the additional protection to reduce bridging and condensation.

 

Now would 50mm rockwool or 50mm celotex be the best option between the 50mm battens behind the plasterboard? Any better alternatives?

 

If you don’t use PIR or insulated plasterboard you’ll need a VCL on the warm side, so you may as well just go with insulated PB unless you want natural insulation. Avoid battens if you can, it’s just another thermal bridge 

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14 minutes ago, MikeGrahamT21 said:

Avoid battens if you can, it’s just another thermal bridge 

 

Unless the battens go on top of the pir to create a service cavity! 

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On 31/12/2021 at 15:06, MikeGrahamT21 said:

You’ll never get a cavity 100% sealed unless it’s built that way.

 

having the air gap will render EWI almost pointless, you’ll save a little on thermal bridging from the wall ties, but hardly any. Thermal bypass will occur in the gap.

 

filling with slow expanding foam is an option, but you MUST use fully breathable insulation on the outside if you do.

 

a better option is likely insulated plasterboard unless you have a good reason not to

 

Agreed. Every cavity ive ever had the misfortune of being involved in has LOTS of air movement in it. Even when its not that windy.

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On 30/12/2021 at 11:52, House man said:

Hi guys,

 

Ongoing extension 

 

Have 50mm cavity insulation with 50mm air gap, suspect 50mm insulation boards not fitted properly

 

Just thinking wouldn't it be kind of useless to have 100m external insulation wall insulation  since I have a 50mm air gap between walls? 

Thoughts?

Good thread this and learning all the time.

 

I have an ex council house two story terrace with cavity brick wall.. which has a howling draft in the cavity. Was contemplating EWI.  Problem is that the inner leaf of brickwork holds up the roof and first floor... and at ground floor level there is a 4" x 3.4" inch timber wall plate build into the load bearing brickwork, all be it it is pitch pine. In other words someone thought it would be a great idea to put in a bit of wood instead of a bed of mortar in the inner leaf which is carrying all the load from the house above! So not keen to fill the cavity as the timber could rot and the house settle.

 

@House man check you don't have the same dillema as I before progressing.

 

 

 

 

 

 

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6 minutes ago, Gus Potter said:

have an ex council house two story terrace with cavity brick wall.. which has a howling draft in the cavity. Was contemplating EWI.  Problem is that the inner leaf of brickwork holds up the roof and first floor... and at ground floor level there is a 4" x 3.4" inch timber wall plate build into the load bearing brickwork, all be it it is pitch pine. In other words someone thought it would be a great idea to put in a bit of wood instead of a bed of mortar in the inner leaf which is carrying all the load from the house above! So not keen to fill the cavity as the timber could rot and the house settle.

 

 

I wouldn't rule out cavity wall insulation. EPS beads are breathable so moisture won't get trapped in the timber. Also any insulation outboard will increase the temperature of the timber making it less likely to form condensation.

 

 

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37 minutes ago, Iceverge said:

 

I wouldn't rule out cavity wall insulation. EPS beads are breathable so moisture won't get trapped in the timber. Also any insulation outboard will increase the temperature of the timber making it less likely to form condensation.

 

 

Thanks, appreciate your help. Brain fade.. can now see how it shifts the dew point outwards.

 

ta for the heads up.

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11 hours ago, Iceverge said:

 

I wouldn't rule out cavity wall insulation. EPS beads are breathable so moisture won't get trapped in the timber. Also any insulation outboard will increase the temperature of the timber making it less likely to form condensation.

 

 

 

I presume then that EPS beads are acceptable in a cavity that has a fair bit of debris in the bottom and bridging, snots etc at the ties?

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2 minutes ago, Iceverge said:

I can't see how the EPS beads would make the situation any worse. 

 

AFAIK most problems with cavity fill are from blown mineral wool. 

 

Both systems work and problems are more down to workmanship or wall not being suitable for injection. More problems reported with mineral wool simply because this has the biggest part of the market, especially refurb.

 

injected cavity insulation will also reduce air movement in the cavity so getting the best from the EWI. The EWI will mitigate potential penetrating damp across the wall.

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