Possibly a bonkers idea - cavity wall insulation

[cojmh]

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.

Thanks for the detail.

The insulation board that we have been using is celotex thermaclass cavity wall insulation (details here) which are interlocking and a very snug fit. They don't allow any air movement from inside to out (or vice versa). The only holes in the insulation are where the wall ties go through.

In terms of rebuilding the wall as a solid wall then adding EWI would mean my total cost would double as I would in effect be doing both options 2 and 3 .... which unfortunately I cannot really justify cost wise.

Thanks for the advice on the thermal break block and XPS .... I will get that done too.

[NoraBatty]

Seems my initial post didnt load and has been lost.

Do not get too caught up in building regs for insulation.
These only apply to new builds, or extensions etc.
For existing buildings, as much as practicable is fine.
It is also worth noting that the gains from insulation depth are not linear, but logerythmic.
You will feel the most difference in the first 1 inch of insulation. Logic does say 2 inches would gice double the warmth, but in reality its something like another 4 inches to get the same difference again.

I had IWI installed here.
Downstairs that meant 150mm rockwool on a swip batten system (solid stone walls)
Upstairs, we had 63mm insulated plasterboard added, with only 12mm insulation in the window reveals.
The difference, was night and day.

What action you should take here, does of course, depend on what you are trying to achieve, and what the insulation in the rest of the house is like.

Merely cutting down draughts will make a huge difference. Maybe the biggest difference, depending on if your house howls in the wind or not.

Personally, i would fill the cavity, and spend a winter deciding if it is warm enough.
If not, i would then add EWI. But i would DIY it, and contemplate the entire house.
It is easy to install on a diy basis. The hardest part, is getting a nice finish on the final render, whixh can always be subbed to a pro

[cojmh]

Hi Nora,

Thanks for the thoughts.

Unfortunately my renovation does come under building regs for new build/extension and I have already queried the values I have to get to with building control and they have told me it is a U value of 0.18 - unfortunately I cannot get away from this.

The reason I am looking to sort out the whole existing wall is that there will be a second extension added in a couple of years when funds allow that will also come under the building regs for new build/extension. At this point over 50% of the gable end wall will be covered by these rules so I just need to bite the bullet and sort it out to achieve the required U value now.

I am sure any insulation is a good thing and should be noticeable.

I have already done the easy things and unfortunately as I have already mentioned the target I need to get to is not set by me or what I am willing to accept but what I have to achieve to get the work signed off.

Interestingly I read that the ideal range for passiv haus is 0.10 to 0.15 U-value .... so the new building regs are getting much tighter with respect to energy efficiency.

[Mart]

This is so stupid I'm very embarrassed to suggest it ...... but what the hell ..... but given the EWI and rebuilding wall costs opening up a larger(?) financial space for brainstorming - could you turn that 700mm outdoor area into an unheated indoor area by building a 'simple' green house side extension of some sort?

Our house has a lot of what I call buffer areas. There's the conservatory, pantry/downstairs toilet, side extension/storage room, and even the mostly glass porch. The parts of the house that touch these areas are of course touching colder areas, but not as cold as the 'true' outside.

At worst, you get to have a laugh, and dismiss the idea, at least knowing you've been thorough.

Doh, the perfect example of an 'ohnosecond' as I pressed submit - if render is not going to get permission, then I suspect a two storey* glass box is out of the question.

*Gable wall could be IWI on the inside of the loft, with the glass box ending at two storey, not reaching the roof apex.

Edit - Oops, forgot to say IWI for the loft.

[cojmh]

I don't believe in stupid suggestions when thinking out of the box!!

I think (but am not certain) that I would need to get planning permission and from my own experience with the planning this time around and reading other applications from people in my area - they are anti anything that makes the house look different. So I am not sure how far I would get.

The gable end wall is where I have been moving (and will continue to move in the future) all of the services. So for example all foul water drainage will be on this side, plus AC unit and boiler flue etc.

making it into an enclosed space to act as a heat buffer is not as simple as it might sound.

But thanks for the thought

[Joeboy]

I like option 3 done to the max and wonder if there are any financial sourcing shuffling can be done to reduce the final chimney sum?

I take it an aerogel solution doesn't get it done either?

[cojmh]

To be fair, I was expecting the main objection to my option 3 (rebuild the wall) to be how can I consider taking down the outer wall on a cavity wall that is 2 stories high plus roof. Would this be a structurally sound thing to do to a house.

I guess it might not be as bonkers as I initially thought as to be honest that was the bit that worried me the most!

[resybaby]

Another, maybe daft suggestion at first glance, but can you not use that multi multi layer foil stuff used in roof constructions, but on a vertical stud frame? Am most certainly no expert on building regs nor insulation, but if good enough for roofs, maybe it may be possible to use 'the other way up' as such - even in duplicate layers?

I'll get my coat

[Joeboy]

To be fair, I was expecting the main objection to my option 3 (rebuild the wall) to be how can I consider taking down the outer wall on a cavity wall that is 2 stories high plus roof. Would this be a structurally sound thing to do to a house.

I guess it might not be as bonkers as I initially thought as to be honest that was the bit that worried me the most!

Personally I feel.thats a question for the architect. Why is it your going to these extremes. What's the endgame?

I ask as it must be quite something? I'm sorry to hear that you are being forced by the local council to these extremes.

[cojmh]

I like option 3 done to the max and wonder if there are any financial sourcing shuffling can be done to reduce the final chimney sum?

I take it an aerogel solution doesn't get it done either?

I looked at aerogel as it was suggest to me by Smegal.

When we did the sums on it it didn't work out to be good enough on the U-value score to be thin enough for IWI and the costs were eye watering per m2.

I think there are a few areas that I can look at to reduce the cost, such as the skips .... going for a grab lorry instead. Materials are about as cheap as I can get but I might be able to push the builders merchant for a discount given that I have already spent a ton of money with them.

Labour is as cheap as I think I am going to get it .... which only really leaves the scaffolding which I am not sure how to get cheaper .... but will keep digging.

Thanks for the thoughts