How much extra EWI might I need on top of cavity wall insulation?

[Cornish]

Really hoping for some help.

 

We live on the south coast in Dorset, almost on the cliffs in very exposed spot. We have cavity walls, which we were told by our surveyor 10 years ago never to fill. The walls are part rendered (mainly rendered) and part brick. 1950s house. Brick built. The walls are quite thick - 315mm - with open cavities at the top inside the roof space: The house was built very much for its exposed location. Not sure what size cavities that gives me.. haven’t been up to measure the gap in the roof.

 

I was hoping to to fill the cavities with bead insulation, if I can get the front wall of the house watertight. I’m assuming it would be a very bad idea otherwise. I have been looking into costs of doing that either with cedral cladding, or with a full EWI insulation and render. And then once I’ve got that, I can add the cavity wall insulation (or at the same time). 

 

I know it will cost a lot. We have a big house. But it’s run on oil central heating, we’ll be here at least another 20 years I reckon, and we need to do something. I was thinking of doing it a face at a time over a few years to spread the cost.

 

The cedral cladding people would put in a layer of superquilt, or 50mm celotex underneath the cladding. Only problem is that they only seem to want to do the top half of the wall, not the bottom half (too fiddly they say), so I’d need to at least get equivalent EWI insulation and render on the bottom half. And I’m beginning to think EWI might end up cheaper than the cedral cladding option based on initial cedral quote.

 

My questions are:

 

1. Does anyone see any problems here, like condensation or whatnot? Another recent post discussion has confused me a bit on this, talking about interstitial condensation.

2. Assuming I fill the cavities, how much extra insulation do you think is necessary on the outside to get decent insulation. I read somewhere that if I was filling the cavities, just an extra 50cm of celotex on the outside would achieve a good end result (but I don’t know what the cavity size was). But do I even need the outside wall insulation as long as I get the walls watertight - is the width of my cavities big enough to be sufficient with bead insulation?

3. Any other ideas on how to achieve an insulated house given my high exposure to wind driven rain and external walls that are certainly currently not watertight (brickwork, constantly eroding gaps around verandas and French windows and the like). I had come across a ‘Corksol” product for the outside walls, but not looked further into that yet. Has anyone had any dealing with that - it’s supposed to help waterproof the walls of your house I think, and add a bit of extra insulation too. [already got double glazed windows and extra roof insulation. Underfloor is not an option].

 

I’m just trying to save on some heating bills, but seem to be creating a mountain of debt instead…. I have a pretty house, so if it were  not for the insulation issue, I would not be bothered about cladding or re-rendering or anything like that.

 

Thank you!

[Iceverge]

In a costal location it'll never get very cold but it'll be very windy. Ventilation losses are lightly a higher than average percentage of heat losses. It's important to deal with this. 

 

Closed cell foam in the cavity like Walltite would give you a U value of about 0.4-0.5 but importantly would provide a moisture proof barrier to prevent any wind driven rain crossing the insulation and an airtight layer too. 

 

It's not cheap and needs to be installed correctly. It would allow your external facade to remain unaltered. 

Edited November 4, 2022 by Iceverge

[SimonD]

2 hours ago, Cornish said:

Does anyone see any problems here, like condensation or whatnot? Another recent post discussion has confused me a bit on this, talking about interstitial condensation.

 

As you're in an exposed location, I wouldn't be considering ewi with a thin coat render but ewi with a 50mm cavity and then a carrier board which is rendered.

[Cornish]

Thanks for these two comments.

 

I had no idea there was now a cavity wall option for cavities in my type of location . That would be perfect. You’re right it is extremely windy here. I will look into Walltite - hopefully it doesn’t create problems with mortgage lenders as I’ve heard some horror stories with banks refusing to lend on properties with spray foam in roofs, due blocking up the rafters I think.
 

And the comment about thin cost render on top of EWI is worrying - I thought EWI with thin fender would be the expensive gold standard. I will ask questions about that … they told me K Rend is bullet proof.

[Radian]

3 minutes ago, Cornish said:

I had no idea there was now a cavity wall option for cavities in my type of location . That would be perfect. You’re right it is extremely windy here. I will look into Walltite - hopefully it doesn’t create problems with mortgage lenders as I’ve heard some horror stories with banks refusing to lend on properties with spray foam in roofs, due blocking up the rafters I think.

 

This is what worries me. There's very little experience being shared about this system. Everything I found when I looked into it a year or so ago seemed to be positive spin from vested interests. I'm not saying to avoid it, but I wish it was in more widespread use. We know plenty about mineral wool fill and the kind of applications it isn't suited to. When we finally settled on EPS bead fill I had an interesting conversation with the installer when I asked him about the types of fill they're removed (it's another service the company offers emblazoned on the side of the van). I half joked that they wouldn't be removing spray foam but he said it was done and described a 'powered whip' that is introduced into holes and used to break the foam into granules that are then sucked out. This suggests it's not all sweetness and light.

[SimonD]

1 hour ago, Cornish said:

they told me K Rend is bullet proof.

 

The question is: who told you? There's a lot more research now about the vulnerabilities of thin coat render ewi systems. Norway, for example, now has a specific chart to indicate when exposure to wind driven rain makes it unsuitable for thin coat render and ewi. This is due to premature failures of this system which highlights a weakness of the testing regime. Its recommendation is the include a cavity in the buildup. Likewise, Finland has developed its own testing system because its climate has resulted in significant premature failures due to freeze-thaw cycles not sufficiently tested for in the wider European technical assessments.

 

Like @Radian says about retrofit cavity wall insulation, buyer beware and choose your supplier/installer carefully. It would be interesting to ask K-Rend technical department whether they have any information on suitability for exposed locations.

[Cornish]

Yes OK, thanks,  if I’m minded to stick with the EWI route I will contact K Rend technical dept. 
 

im crossing my fingers on wall tite, but agree I can’t find a lot about it on the internet in the U.K. 
 

the CWI/EWI company I spoke to, based in Christchurch I think, tried to persuade me that EPS bead CWI would be adequate by itself and is “very forgiving” of wind driven rain but I don’t think I’d be persuaded of that suggestion without some sort of independent paid for advice. The rain is like horizontal bullets where we live.

[Cornish]

I cam across this discussion on green building forum whilst researching this walltite. The author said he had spoken to BBA about it too and considered that BBA’s certification testing was inadequate to identify the problems caused in corners. Any thoughts on this issue he identified? Obviously I’ve already got wall ties as I am retrofitting, unlike this guy, and I’m not looking for wall tie support … but  I don’t really know how concerned to be by this anecdote, solely for retrofitting for insulation in a highly exposed location. Does this just take us back to needing an experienced and careful professional fitter?
 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15392&page=1

 

1. When I was researching the different methods for insulating the building I settled upon BASF walltite, which was supposed to insulate all cavities. However I did not trust that the foam could reliably provide a replacement for wall ties, so I used Telplo ties. The way we have put the foam in is to build a lift of wall up to a max of about 2m then clean the cavitiy full and fill it with PU foam. However after I started, I found that the PU foam shrinks when it cools and can cause significant damage to the masonry. BASF walltite seems a bit worse than other foams, which I also tried. The only solution to prevent all damage was to line the inside face of the outer leaf with polythene sheet to stop the foam bonding to it. This has allowed it to shrink back a little without causing any damage. However this does mean that wall ties are mandatory as the foam is providing no alternative to the ties. My installer has said he has seen many examples of retrofit foam causing cracking in the same way, but it was only when he saw it happen in an open cavity where everything was visible when it became clear what causes the problem.
2. • tony
   •  
   • Nov 21st 2017

    
   but presumably leaves a gap for potential draughts. are there any lateral cracks?
3. • mikael
   •  
   • Nov 21st 2017

    

   Posted By: tonybut presumably leaves a gap for potential draughts. are there any lateral cracks?

   
   
   The gap left is about 10mm between the foam and the inside face of the outer leaf. As long as the entire cavity is filled properly, then the gap left would not result in any draughts, as the inner leaf is still fully covered and sealed. The cracks that occurred were predominantly in external corners, where the foam pulled in, but the force on one side is always slightly higher than the other and one side ends up having to move.