External Wall Insulation over Thermalite Blocks - will it help?

_DCG_

Hello.. wondering if anyone can help please... 
We live in semi detached which has side and back extensions that were added in 1996. The side extension is made up of a garage and second reception room on the ground floor with a bedroom and bathroom above. This extension has always felt much colder than the main house - in particular the second reception room is very very cold and needs a lot of heating. However that second reception room does have a full flat roof which we know is not well insulated (likely to be 100mm if that given date built). 
Transpires that the extensions are single layer walls and were made out of 9 inch Thermalite blocks which are supposed to be self insulating, however doesn't feel like they are very effective to me!
We now have two options and I am trying to work out which one will make the most difference and essentially give us the most bang for our buck:
1) Add external wall insulation to the full side wall - Benefitting rooms will be a bedroom, a bathroom, the garage (which we do plan to partition to create a utility) and the second reception room. Cost £2k.
2) Insulate the flat roof which covers all of the second reception room, part of the kitchen and part of the main living room. Cost £6k. 
Obviously the wall insulation is far cheaper and it feels like more rooms should benefit from it being done. My concern here is whether adding EWI to walls built out of Thermalite blocks will not actually make much difference, in which case that £2k would be better off saved and put towards the roof.
Any thoughts or advice would be gratefully received, _DCG_

matelodave

You could do a few sums - work out the heat loss through the wall without insulation and then do it with insulation and see what the difference is. That should tell you how much heat you need to counteract the heat loss  through the wall and how much less if you applied insulation). You could then decide whether it was cost effective to lash out £2k or more to save perhaps a kwh or so. You could do the same for your roof to decide if £6k was cost effective.

There are plenty of heat loss calculators on the website, all you need is the u-value of the blocks and the u-value of your proposed insulation

TBH, even if you save 10kwh* a day for say six months = 182 days x 10 x 3p/kwh(for gas) = £55 a year, payback is 36 years. You probably lose a lot more through the roof  but its still gonna take a long time to pay back and £6k will buy you quite a lot of gas. However if you heat with leccy then the payback time will obviously be less.

* you need to do your own sums to work it out, this is just a quick & dirty guess

I did the calculation to see if it was worthwhile insulating under my floorboards - cost around £2k, savings around £100 = payback 20 years. Plus all the disruption of tearing up and replacing all the flooring. I guess it might have made the place feel a bit warmer but it was more money than I was prepared to spend - we just turn the heating up a bit

tacpot12

matelodave said:

...
I did the calculation to see if it was worthwhile insulating under my floorboards - cost around £2k, savings around £100 = payback 20 years. Plus all the disruption of tearing up and replacing all the flooring. I guess it might have made the place feel a bit warmer but it was more money than I was prepared to spend - we just turn the heating up a bit

This is the problem in the UK; energy is just too cheap! If energy was more expensive, it would be worth investing in better insulation. 

Flippancy aside, a good level of EWI should make a difference, but because hot air rises, you will get more benefit from increasing the insulation in the roof.  Why no do both? You are going to have significant disruption, so you might as well do the best job you can possibly do now, then it is done.  

Petriix

£6k seems extortionate to insulate a flat roof. I built a whole new flat roof including timber and 130mm of solid insulation for about £1k. I suggest that you get a couple more quotes or consider DIY. 

silverwhistle

I'd agree: my first thought was that you'd swapped the prices by mistake. Definitely get some more quotes.

_DCG_

Hi all, thanks so much for the replies, really appreciate you taking the time to help.
matelodave - Yes those calculations are exactly what we need to do. I have so far struggled to find the u-value of these 9 inch Thermalite blocks, will have another look. I don't know what EWI material the company is proposing, it just says 100mm. 
tacpot12 - Would love to do both, unfortunately we can only afford one and since the maximum value of the voucher is £5000, to do both would be £13k  and much as we want to reduce our bills and better insulate the house it would take forever to see the payback
Petriix - We've also had a few other people comment that £6k (£11k without the voucher!!) is very high. This £11k quote is to fit a Bauderflex warm roof system and its £11,220 so even more! The roof area is 9.3m width x 4m depth so roughly 37 metres squared. We're just inside the M25 so have to factor in London / south east prices. Is this still very high? Unfortunately we're most definitely not DIYers and would likely cause far more harm than good so we do need to get it done by someone who knows what they are doing.

DORA_THE_EXPLORER_2

Be careful with EWI it can cause damp in the property... Google it. I have terrible damp with CWI and the CIGA guarantee is not worth the paper it's written on.. 

_DCG_

DORA_THE_EXPLORER_2 said:

Be careful with EWI it can cause damp in the property... Google it. I have terrible damp with CWI and the CIGA guarantee is not worth the paper it's written on.. 

Oh goodness, thanks Dora. So you had cavity wall insulation done and now you have damp problems as a result?