200mm pumped cavity - with rads?

johnpawl · 21-11-2010 12:40AM #1

I am considering constructing a house with exterior walls consisting of - 100 mm inner leaf, 200mm pumped cavity, 100 mm outer leaf. I will have the house air tight. It will be heated with rads. I'm wondering is my method of insulation more suited to UFH due to the concrete helping maintain a regular temp etc and would I be better off with insulated slabs for a quick heat up time?
I'm more inclined personally to pump the cavity but I'm not sure so I'd appreciate a few point of view as I need to make up my mind fairly quickly!!

holdfast · 21-11-2010 10:11AM

At that size of cavity I would be more worried about laterally stability of the wall, i.e the two walls working together to resist wind loading and structural loading.I think myself that block building will be replaced by timber frame more and more due to the increased insulation. Get an engineer to look

L driver · 21-11-2010 11:39AM

Johnpawl,
Think I agree with above. Think we should be moving away from cavity build. Your wall build up will be at least 16" wide which means a strip of at least 4' probably, then you have thermal bridges and air tightness details.
Do a bit more research, whatever way you go start with an insulated raft, wish I did. Cavity build is kinda like the walkman!

sydthebeat · 21-11-2010 05:50PM

id kinda agree as well with the previous two posters.

using 2 x 100 block leaves with 200mm pumped cavity = 200mm concrete + 200 mm polystyrene

you could just as easily build a 215 block on flat leaf and externally insulated with 200mm polystyrene EWI. = 215mm concrete + 200mm polystyrene

the difference being:
pros
the much greater reduced thermal bridges in the latter
much greater increased thermal mass in the latter
less engineer involvement with the latter
much better suited to UFH in letter
much easier to make airtight in latter

cons
possibly more expensive in latter

beyondpassive · 22-11-2010 05:59PM

The handy thing about cavity walls is they are robust outside an inside. The inner leaf will contribute to the thermal stability of the house. The big problem up to now with the cavity is the massive cold bridge at perimeter. Thermal bridge break can be achieved by using foam glass blocks or Aerated Autoclaved concrete block. However there is an insulated raft now available which can be used with a 200mm to 300mm cavity giving a positive cold bridge, ideal for passive house type builds and especially useful for slow response radiant heat sources like underfloor, where the slab can easily maintain high comfort temperatures.

200mm pumped cavity - with rads?

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