We are hoping to add a small extension to the Edwardian chalet bungalow we are renovating (http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16366&page=1#Item_7)

This will be on the gable end wall , single storey, about 8 m long and 2 m wide and will be for utility/boot room/hot water tank type stuff with a shallow pitched roof - the finished outside height being c. 3.8 m.

Somewhere I saw a suggestion that an externally insulated block wall would be a better bet than standard issue cavity wall so started to look for more detail. I now know that there are many variations on a theme but apparently no standards so I am not really any the wiser. In fact overwhelmed bu options and materials and jargon I am not familiar with...

We need a render finish outside and can't really go very much more than 300 mm wide because of a side access.
The suggestion I saw said 100mm blockwork with rendered over insulation outside. But it seems that this is not going to get past BC, at least without a SE going some calcs. I have been looking for some more formal description of the build up, how it works with foundations etc but without success.

I guess I could easily be accused of guilding a rather old lily with this house which is currently about as air tight as a collander, but I really want to do the best I can where I can afford to plus the extension will end up being an area where I don't have to add IWI to the 9" solid walls, which is a good thing!

I mentioned the external insulation desire to the architect and he seemed concerned that there could be problems with render cracking / expansion joints etc.
The old solid walls I have assumed are at about U value of 2. The insulation we are fitting upstairs I hope will achieve better than 0.18.
Is this sort of level achievable for the new wall within the parameters mentioned?

Any thoughts to help simplify this and point in the right direction would be very gratefully welcomed.

Posted By: Dursingle storey

When you say the 100mm block version will need calcs by a SE, do you mean that BC have suggested other block thickness WOULD be acceptable without a SE, eg. 140mm? I'd have thought they would want the lintel sizes, and perhaps confirmation of racking??

Single storey and small should be OK with 100 solid concrete blocks and 200mm polystyrene and mitigation of all thermal bridging

Can you stretch to a 350 mm cavity wall in 100mm block thickness on 650 mm wide foundation and use 100mm PIR insulation this should give you a very decent u value about .18 I reckon. And the 50 mm cavity gives extra assurance against penetration of rain particularly if you happen to be in an exposed area. You can always increase u value by internal insulation with thermal plasterboard. Then render external wall. I looked at external insulation and came to the conclusion from what I saw that external insulation with render over is easily damaged and also if not done really well the joints in the boards show through when wall gets wet.

The detailing of this form of construction is very well documented including the concerns you express. Look at the details within the tech sheets of the major insulation manufacturers. Kingspan in particular have very good detailing.

With a wall 8M long you are on the boundary of needing an expansion gap so would seek advice on this.
My BCO was an ex brickie and had worked in Germany where external insulation on solid block was common place. He believed this was not as good as a cavity wall and also he would never fill a cavity wall with insulation regardless of how good it was supposed to be particularly in an exposed area.

Thank you both. Will study further.

Thanks - I am leaning towards the cavity wall as per revor's suggestion, Done properly it looks pretty good.
As you say Kingspan have particularly good detailing once you find the right document.
Not really sure between the insulation + 50 mm cavity and the full fill insulation +10 mm "cavity". Will study further.