1970's Renovation: If/how to bring existing walls insulation up-to building regs standards?

[ambient]

We're about to embark on a significant renovation of our 1970's bungalow and are considering options for improving the insulation of the existing exterior walls that will ultimately make up about 50% of the final external walls.

 

The existing walls are 115mm facing brick, 50mm cavity, 100m block. The cavity was filled with "Megafoam" (Urea formaldehyde) in 1977 but I suspect that's largely useless now due to age and ideally needs removing, although I've not yet had a survey in that regard.

 

We will be...

- replacing the entire roof (giving us access to top of the cavity)
- replacing all windows and services (so an expansion of the exterior wouldn't be a problem, were we to install EWI)
- rendering practically the whole facade (so no concerns about retaining existing brick)

 

We're not looking specifically to achieve eco or passive haus standards. The new walls and new roof will be built to latest regs only (walls will be typical brick and block to match existing unless someone is able to convince us and the builder of something better)

 

Presumably we would ideally bring the existing walls to regs standards to match the new walls, otherwise heat is going to leak out of the poorer half of the exterior?  Is my thinking correct here?

 

Originally we were thinking of just replacing the CWI, this is obviously an affordable approach but I doubt this will cut it. IWI is a possibility but we'd rather not lose any floor space. So I suspect EWI is the way to go but I'm a little in the dark on pricing and also have some concerns about about condensation, dew point issues etc. We do love the fact EWI might provide an acoustic barrier and the contained thermal mass will help keep things cool in the summer, so that might make any additional cost more bearable.
 

Would love to hear your thoughts, opinions and/or experiences. What would you do if you were in our shoes?

 

[Conor]

Have you considered demolition and building a new house? Sounds like you're at about the tipping point where it's potentially a more cost effective option. Even if it costs more, the end product will be superior.

 

If not, a 50mm cavity is next to useless. If you are rendering, a ~100mm of EWI then a silicon/acrylic render system would be the way to go.

Edited January 14 by Conor

[ambient]

Thanks for your input. Yes we gave strong consideration to a full rebuild waay back at the start of our journey but it wasn't going to be cost effective from our own calculations.

 

When you say a 50mm cavity is useless, presumably you mean from a cavity wall insulation perspective?

 

Approx 100mm of EWI with silicone render certainly looks like good option, thanks for the reassurance. Does air flow need any consideration? Would I need to remove the existing CWI? etc

For the insulation type, I was eye-ing up mineral wool for its accoustic properties, I don't think there's a major u-value difference or other benefits if going with EPS/XPS?

[ambient]

Would love to hear some more thoughts on this. We are really struggling to make a decision and are perhaps leaning towards doing nothing at all.

[Iceverge]

On 14/01/2024 at 21:38, ambient said:

Thanks for your input. Yes we gave strong consideration to a full rebuild waay back at the start of our journey but it wasn't going to be cost effective from our own calculations.

 

Perhaps this requires more digging into. Are you planning on doing much work yourself and have you allowed a reasonable price for your time. Say £20/HR. 

 

 

[ambient]

23 minutes ago, Iceverge said:

 

Perhaps this requires more digging into. Are you planning on doing much work yourself and have you allowed a reasonable price for your time. Say £20/HR. 

 

 

The structural work will all be done by a builder.

Edited February 3 by ambient

[Iceverge]

11 minutes ago, ambient said:

The structural work will all be done by a builder.

 

Have you had them price it both ways including taking down the original house?

[Russell griffiths]

Build the extensions out of an eps icf block, then fit Ewi in eps all on the old bit, this will give a uniform exterior material to render onto. 
 

probably eps beads in the old cavity, but I’ve never used that so someone else will need to comment on that. 
 

isn’t fitting Ewi pointless without sorting the cavity insulation ?.

[Iceverge]

Yup although solid concrete blocks walls with EWI would be cheaper I'd wager. 

 

 

[ambient]

1 hour ago, Iceverge said:

 

Have you had them price it both ways including taking down the original house?

I've not had a detailed quote yet, we should be get the steels design from the structral engineer this week and then the builder will complete his quote with a better degree of accuracy.  I will ask the builder to price up demolition and rebuld of the existing exterior wall.

FYI there is 23m of wall standing 2.5m high that we had intended to keep. So 57.5 sqm.  Having done a quick google I've average price suggestion from £150 to £230 per sqm for a builder to errect a brick and block wall (finish internally as well). So somewhere between £ 8,625 and £13,225 plus we'd also have to pay for the demolition and disposal. So perhaps ball park £15k - £20k in total.

What am I looking at for EWI, not including the rendered finish (since I am going to have to pay for that either)?

 

 

[MikeGrahamT21]

Don’t forget with new build you can claim back the VAT, may well cover that cost and a lot more on top and massively reduce complexities down the line which would end up adding even more

[Iceverge]

Keeping that piece of wall is insane in my view. 

 

Build a new wall with a wide cavity filled with EPS beads or mineral wool batts or reject a timber frame and get the vat back.