Foam CWI for retrofit

I’m midway through a retrofit project. My house is 1970s detached in York. It has brick and block walls with blown fibre CWI that was installed about 13 years ago by the previous owners.

I initially based my plans on the assumption that the CWI was effective. As I’ve changed windows and exposed the reveals I’ve found more and more gaps in the insualtion.

As a last minute change I’ve insulated the downstairs walls with 25mm of kingspan to equal the insualtion value of what the CWI would have provided.

I don’t want IWI upstairs and am considering foam CWI. The airtightness improvement is the main selling point as the downstairs is very airtight and I’ve got an MVHR system so improving the airtightness upstairs and between the floors would be a good improvement.

My initial findings are that there aren’t many companies that install it and it’s very expensive. I’ve had a quote for 100sqm of walls at a 70mm cavity depth of £1750 to remove the old insualtion and £5500 to install the foam.

I’m wondering if anyone has had it done and if it is was worth it?

Hi can you track down the company that installed the cwi and bring a clam against then or grant body that supplied it, there are companys that will do it for you. It maytake some time

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The original company has gone bust and I’ve got an ongoing dispute with CIGA, the company that issues guarantees. The general jist of their argument is I’ve disturbed the insualtion by looking at it.

Can you get hold of the Carbon Coop thermal camera? If so, heat the house to more than uncomfortable over a winter’s night to give the walls time to heat up and then from outside in the cold before dawn take images of parts you haven’t “disturbed”. Any hot spots indicate holes in the insulation.

Edit: Can you see the height of the insulation by looking down into the cavity from the roof? A common place to be totally omitted is above top floor windows, but check anywhere you can.

Thanks for the suggestions. I’ve already borrowed the York Community Energy thermal camera.

I spotted a bright red patch under my bedroom window and when I removed the internal windowsill found that that part of the cavity was only about 25% full.

The cavity is closed with bricks where the rafters finish above the upper windows so i can’t see him. The gable end wall is reasonably well filled but some spots finish about 20cm from the top, less of an issue as the loft is outside of the thermal envelope.

Where a void is accessible from above it is relatively easy to top up with polystyrene beads.

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Foam is expensive but the benefit is that it reduces labor. If you’re removing the old cwi then you need very good access - I suppose it’ll be up at eves and they’ll need scaffolding to suck it out.

Which negates the whole point of the foam, which can be injected at the face of the wall.

I would instead replace it with a properly packed blown in solution. Use it as an opportunity to detail your attic vents and soffits to receive a good 300mm in the loft at the same time.

70mm wall insulation is good, but ideally you’d at another 70mm rigid EWI too. If you did, then you could use cellulose for the CWI any have a vapour open assembly.

I had a similar issue with no records from CIGA/ council. Polystyrene beads is a good top- up (silver or white) as you pour in and the space remains breathable without compressing what is currently there. I did not fancy removing/disturbing what was currently in place as there is no guarantee that the new stuff will be evenly distributed. Beads flow like water through any gaps and you can end up with beads on the outside or falling through small cracks (skirting boards-floor) or openings so best to include adhesive so they stay in place once filled.

You do need to remedy any flaws in the wall before adding CWI (or indeed any wall insulation). This doesn’t just apply to beads.

Thanks everyone, I’ll post an update once I’ve gone ahead with something.

@Jesper_Phillips - have you got any further with this? Sounds similar to us, we had mineral fibre CWI installed about 15 years ago (exposed site in Cumbria- so in retrospect maybe shouldn’t?)no real problems though I now find they didnt insulate the gable end, just front and rear elevations (I guess they couldn’t be bothered accessing it from the attached garage roof). About 13 years ago we extended over the garage so the gable end is now internal to the house (if that makes sense, but I’d still like the cavity). Struggling to get anyone to Cumbria (seems to be ongoing problem for everything- too far from anywhere)- but wondering if I can get beads put into the gable wall and also if it would be much better to get the mineral stuff taken out and do the whole house with beads?

Please give us some more info on the walls.
. Width of cavity
. External finish
. State of repair of external finish
. Composition of both skins of the wall
. Internal finish
. Type of ventilation
. Internal humidity adjacent to exposed walls over duration of a week after severe penetrating rain

Tim- thanks for your interest- answers that I have as follows:
Overall width of wall= 12"
I think both internal skin and external skin are brick-so assume cavity is 4"?
External finish is I think what people call ‘wet dash’?- ie there is a cover but its not smooth render- in good condition.
Internal finish is wet plaster to the brick,
Ventilation- at moment is just opening the windows- we know this needs addressed but are in an area at risk of high radon so doing the 3 month test to see results before we do anything.
Not sure re internal humidity-today its 16.7 degrees inside (heating off) and humidity is 79%, so highish. Not noticed that it goes any higher than that- and it feels like its been wet here for weeks.
No specific patches of damp on walls.
The mineral wool insulation probably was installed in 2008.

PS - we don’t have access to the top of the cavity.

A quick response, particularly regarding your PS:

Not having access to the top of the cavity will make inspection/top up/replacement of the insulation far more complex and therefore more expensive. It does change the equation of what you are prepared to spend for a hoped for but uncertain gain.

I think your ultimate decision will largely have to be based on thermography.

Borrow a thermal camera for mid winter, heat up the house as much as you dare and photograph the walls, inside and out, before dawn. The evening is not so good as the structure may have a little remaining thermal gain from the sun.

Any anomalies could be gaps or compromised insulation.

I’ll give your other answer some thought later in the day as there are fortunately a lot of details to consider.

My first consideration looking at your more detailed answer is the humidity. It is close to the danger klaxon sounding (80%). Particularly as the humidity is usually higher near the colder outside walls. I think that the first priority is ventilation. As you may have noticed from my posts in other threads, my preference is for whole house MVHR. Obviously that solution isn’t always best, depending on your circumstances and the ease of installation in your house. A temporary compromise could be a first floor system with scope for expansion as you do up the house. Ventilating the first floor well will have a beneficial effect on the rest of the house too. If you do have ducting in the roof it should be incorporated into ceiling height insulation. If you have a warm roof you don’t have to worry about that.

Now to the walls. It seems likely that your walls are pretty much waterproof, as you say the render is in good condition. Unfortunately “pretty much” isn’t necessarily good enough as any water that does get through can still damage your structure and/or insulation. If the thermographic survey shows anomalies it might be worth investigating the cavity by either drilling and using a borescope (available to borrow via CarbonCoop) or removing a brick. You might need to make a large hole in the render to find a brick to remove, so I would avoid that if possible. You know already that there may be gaps. Some or most of these could be edge effects where insulation has only been injected from one side (as by a window) rather than both and sufficient compensation was not made to the drilling and blowing pattern.

The cavity being about 4” you should see a noticeable benefit from fully filling it. However it is not enough to be assured of warm walls in winter or cool ones in summer. At some point you might want to consider either internal or external insulation but not before you have reason to believe that the cavity has dried out. Some drying can occur with the right insulation in place but it will be noticeably slower.

If you decide that you should leave the current insulation in place it might be worth treating the outside of the walls to reduce water penetration. This must be done with a treatment that doesn’t stop natural evaporation from occurring, leading to a drying process. I can recommend StormDry:

When you changed the windows, did you make good the defective insulation in any way? If you used foam that could hinder removal and/or replacement of the current insulation.

Returning to your original post on the topic: in your location there are 3 possible approaches to insulating the walls:

  • Free draining insulation
  • Closed cell foam (also acts an an airtightness layer)
  • Leave the cavity to its original task and install internal wall insulation.

Note that I have not included external wall installation. You cannot effectively insulate a house externally if there is an unfilled cavity.

Starting from scratch the free draining solution is the least cost and minimally intrusive during fitting. In practice polystyrene beads and resin are the best solution. The resin stops your insulation pouring out of any holes made in the wall at a later date.

Closed Cell foam is a highly specialised and therefore expensive solution. 99.998% of foam insulation fitters use open cell foam. Do not use this product except in sheltered locations. I used open cell foam in the London suburbs in the late 1970s and was very happy with it.

One of the advantages of foam, open or closed, is that it doesn’t need to start at the bottom of the cavity.

Internal insulation has its issues but they can be addressed. You are likely to find solutions to your particular problems elsewhere on this forum.

If I’ve overlooked anything please ask.

Edit: Above 60% humidity your timbers and woodwork will be increasingly at risk of insect and invertebrate attack even is it doesn’t go mouldy.

I got a couple of quotes both in the region of £5.5k for 100sqm of walls with a 70mm cavity. In my case I’d also need to separately arrange and pay for the removal of the mineral wool, probably for another £2k.
I’m going wait another year and see how I get on this winter.

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Don’t forget the thermal camera. And please show us the results.

Are you removing the mineral because of poor installation/moisture? I quite like mineral wool CWI, it’s the most likely to be installed well.

There are loads of voids in the mineral wool. I’m midway changing the windows myself and everytime I get chance to see into a cavity I find more and more places which haven’t been filled properly.

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