Hi ali. I do not agree with waterproof renders. They do not allow for movement in the same way soft permeable mixes do, meaning that they tend to crack more. Once they crack, the water gets in, gets trapped behind the waterproof render and if you are really unlucky - freezes, blowing your render of the wall.

Also see discussion here http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=61&page=1#Item_19

I would go for partial fill cavity unless you can use rendered external insulation.

so rendered insulation on 200mm blockwork tony ? would this be okay to go up three storeys ? i'll check reg's.
thanks, i'm sure i read before you werent keen on keeping cavity for rendered blockwork. with increasing insulation standards that style of construction appears to be becoming uneconomical. any thoughts on spray urethane as a cavity insulation? product blurb looks positive and seems to make sense.

i just want to bring this live again with the question being that if the masonry is to abut existing walls and the new wall is to be insulated externally but the original wall is insulated internally, at best fully filled, then theres still the thermal bridge where the masonry abuts the old.
if you're struggling with that - imagine a capital T - bottom right corner is external, theres insulation on top of the horizontal line and to the right of the vertical line, but heat can track through the horizontal line.
even more so if the original cavity is only partially filled.

Would that class as a designed in internal winter cooling system? generic to internally insulated properties.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=931&page=1#Item_12

i know i'll insulate that wall externally, especially as its north facing anyway there wont be much left of that original wall anyway after the extension. front has to stay un-changed due to too many fiddly areas around bay windows, roof overhang, etc and its a semi. it's gonna look beefcake enough as it is when i've finished.
now i just need to convince the client to hack off the render to the front of the house so it can be re-rendered with mesh folded round the corner, where the new external insulation is, to prevent the render cracking where old would meet new.
has anyone found that to be a problem with external insulation systems, cracking render i mean, not convincing clients ? any other solutions to completely re-rendering of the front i'd be keen to hear. the only alternative i can think of is some kind of mastic joint.