Possibly a bonkers idea - cavity wall insulation

[cojmh]

I am interested to see what people think ....

Basically I need to insulate a gable wall end of a two story building with the traditional double pitched roof. It is roughly 35m2 of wall area to insulate.

There are two doors on the ground floor level but otherwise there are no windows or other apertures just a few small things like cameras/outdoor lights that can easily be removed and put back afterwards.

I have 3 options - although one might be a bit too bonkers ...

Option 1 - Internal wall insulation - but this has been ruled out really due to taking up far too much space on the interior. It will take up nearly 10% of the room width in places.

Option 2 - External wall insulation - Nothing complicated about this but it is not cheap and I have concerns about the life span of the installation. It will also take up around 150mm of a 700mm path down the side of the house. Not the end of the world but a consideration.

Option 3 - For nearly the same price as the external wall insulation - my builder has said we could remove the outer course of brickwork - move it out by 30mm and build a new outer course the full height of the house with everything new (new insulation, new wall ties, new bricks, brick weeps etc.)


Option 3 is appealing from the perspective that the end result is closest to what I want. The life span of the installation is the same as the house - there is minimal change in terms of the house appearance and it shouldn't impact much on us living in the house.

BUT ..... I am somewhat concerned about how risky this move would be. I understand that the main structure of the house is on the inner course of block work .... but there has to be an element of strength added by the outer course of brickwork. Is it possible/wise to do this?

I am currently in the process of finding out if any changes are needed to the footings too (so this is an unknown that is being explored).

Thoughts are welcome but we will have to move forward with option 2 or 3 at some point.

[Countrypaul]

When we renovated our current house we ended up removing the outer leaf of the cavity walls so that the finished walls were alll the same brick. The dormer bungalow went from hipped to gable end with a higher roof line. The orignal had been modified previously so there were at least 2 sorts of brick (I seem to recall 3 but not 100%) - the current finish is just one type of brick above the DPC. The builders seemed to think it was not unusual, but we only had a single story to removed the outer leaf from.

How big is the existing cavity? Have you considered injected polyurethane CWI? You don't mention it, but it would be done in probably half a day and might give you what you need. We opted for that but it was a major renovation so injected from inside result no tell tale plugged holes on the outside (made life difficult for the EPC surveyor!)

[spread-tee]

Blimey, either your builder is doing a really good deal for you or you have over priced the EWI a lot IMHO

https://ewistore.co.uk/contact/?mh_matc ... AwEALw_wcB

Check out this site for prices, I think you could get the stuff for a few hundred quid and most of it is pretty easy for a decent DIYer, maybe just get a spread to put on the topcoat. That should keep your costs down to a minimum.

Your option 3 is do-able but it seems like loads more work than the EWI to be honest.

Desp

[cojmh]

How big is the existing cavity? Have you considered injected polyurethane CWI? You don't mention it, but it would be done in probably half a day and might give you what you need. We opted for that but it was a major renovation so injected from inside result no tell tale plugged holes on the outside (made life difficult for the EPC surveyor!)

Thanks for your response - in answer to your question:

The cavity ranges from 50mm to 85mm ..... the reason for the range is that a previous owner effectively built an extension around and over an existing garage. So the cavity between the original garage wall and the new outer wall is 50mm -ish .... in a couple of places we have measured 70mm. The rest of the extension was totally new and that measures 85mm in the one place I have been able to measure it.

However it looks like they have used thermabead (polystyrene beads) around the garage and standard loft insulation in the rest of the cavity. So neither provide a lot of thermal resistance.

I have asked what I need and apparently to achieve building regs (a U value of 0.18) I need a minimum of 90mm PIR and 10mm air gap otherwise I will need additional insulation either inside or outside. Or I could go with 150mm of rockwool in the cavity.

So however I slice it ... the cavity is just too small to achieve what I need as it stands without significant internal or external insulation.

Thanks for your thoughts

[Mart]

Know nothing about the comparative costs, but would your overall house 'rating' improve with the EWI and the increased thermal mass that adds (as the outer wall, becomes internal(ish)?

[cojmh]

Blimey, either your builder is doing a really good deal for you or you have over priced the EWI a lot IMHO

https://ewistore.co.uk/contact/?mh_matc ... AwEALw_wcB

Check out this site for prices, I think you could get the stuff for a few hundred quid and most of it is pretty easy for a decent DIYer, maybe just get a spread to put on the topcoat. That should keep your costs down to a minimum.

Your option 3 is do-able but it seems like loads more work than the EWI to be honest.

Desp

I have looked at the ewi store before and some components are cheap but there are a few additional considerations (which I didn't cover earlier).

1. I need a certified install for building control and also my house insurance has specified that everyone working on the house has to be competent and insured.
2. The insulation has to be polystyrene as the existing cavity wall insulation is thermabead so I cannot use PIR on the outside otherwise it will create a moisture trap.
3. To avoid needing planning permission for a rendered finish (which I would be very unlikely to get) I need to go with a brick slip install which is not so easy to do DIY and adds a lot to the cost.
4. I am not the best with heights so I am not sure I could do an install at the top of the wall .... just being realistic.
5. I did look into courses to do the work myself and become a competent person. It would be about £750 + VAT for a tailor made course to do everything I would need on this job.


To try and put this into context I have had two quotes for the EWI (brick slip) and they have been £15K and £7.8K and I am waiting on one more quote. (this includes labour, scaffolding, materials but I don't know each element cost) and I was told it would take a week to do.

I have worked with my builder to come to a costing and excluding any extra work on the footings, the total is about £8.7K and the costs come out as:

Materials - £4,000 - ish (minus odds and sods)
Labour - £3,250
Skips - £736
Scaffolding - £750 - ish (preliminary quote)

Now I know my builder personally and I he has contacts for scaffolding etc. so I am sure I am getting a good rate with him .... but not sure on EWI if that is a competitive quote.

I know the builder cost is higher by about £900 but as I previously said given the cost over the lifetime of the building this will be less.... I am not fully convinced how long the brick slip/render coating will last.

Thanks for your thoughts and would welcome anymore

[cojmh]

Know nothing about the comparative costs, but would your overall house 'rating' improve with the EWI and the increased thermal mass that adds (as the outer wall, becomes internal(ish)?

Thanks for the thought.

I am not sure is the honest answer .... I have been looking at this purely as an exercise to achieve the required 0.18 U value - which I have no choice but to achieve - and also the physical aspects (how will things be affected by the insulation (room sizes, walkway widths, the look and life of the finished wall/EWI.

As part of the renovation - this part of the house is having the concrete floors removed then 150mm of insulation added into the floor before under floor heating is added. So I am both insulating against losses through the floor and adding thermal mass (the 75mm thick screed holding the heating pipes). So I think (but could be totally wrong) that the thermal mass will not really add much with EWI.

Happy to hear otherwise if you know how to work it out?

[resybaby]

What sort of internal insulation did you consider? Must have been something pretty major if it would take up 10% of the availible floor space in some areas.

I would have thought simple poly/plasterboard backed internal sheeting would be the starting point myself for a cavity wall. Iirc its about £40 a sheet. Easy to work around with sockets/windows/doors etc to.

[cojmh]

What sort of internal insulation did you consider? Must have been something pretty major if it would take up 10% of the availible floor space in some areas.

I would have thought simple poly/plasterboard backed internal sheeting would be the starting point myself for a cavity wall. Iirc its about £40 a sheet. Easy to work around with sockets/windows/doors etc to.

I have been working with the architect to figure out the internal wall insulation and they have said I need this:

I have done the calculations also with Celotex, and it does not seem to be possible to achieve the required U-value with 100mm of insulation.
The calculations to achieve 0.18 W/m2K required a 50mm air gap, 10cm board, 100mm studs with insulation in between and 37mm insulated plasterboard, totalling a thickness of 198mm over the existing walls.

My limit was 100mm for considering internal wall insulation so we missed it completely.

I went and did the sums on the celotex website too and it confirmed what the architect said (so he is not making it up).

I presume this is also what my building control officer will look at too when they assess the installation for sign off.

This is all just to conform to the up to date building regs.

I also don't fully get my head around why 90mm PIR with 10mm air gap in the cavity will give the same as 50mm cavity and the above 200mm of internal installation but this is not my field so I am just going with the experts.

So unfortunately the insulation backed plasterboard by itself is nowhere close to what I am required to achieve.

Thanks for your thoughts

[jonc_uk]

i would only put EWI on a solid wall. Otherwise you still have air movement within the wall negating the insulation effect.

A rebuilt outer leaf with PIR would be ok as long as the PIR was very snug, foamed between each sheet etc. otherwise you still have the same problem - air movement around or up the inside of the sheets - same poor insulation problem. You would have to watch the builders like a hawk and be prepared to make them redo parts - not fun. All practical if you are building your own wall though.

My ideal would be strip the outer leaf, rebuild it snug as a solid block wall with no cavity and then EWI. You then have increased thermal mass, good air tightness and good insulation. So much of the practical effect of insulation is lost because of air movement.

For added effect, use a thermal break block at the bottom of the new leaf (Marmox Thermablock) and go down to foundation level on the outside with a layer of XPS.