Zero heating wall construction systems – which is best

[Anonymous]

We are self-building an 11.5 x 9.5 two storey house in East Anglia. We want to build a zero heating house (woodburners for winter backup). We are currently working on a 540mm finished external wall thickness. Exterior rendered. Thermal mass if possible. Reasonably tight budget. Concern about interstitial condensation. Ground floor is suspended timber.

We have looked at a number of wall systems, as follows, with potential problems for each one:

1. Hemcrete – no problems except cost. Zurich will guarantee but only as an infill material so a complete structural timber frame is also needed. Double that for price of 500mm (installed) Hemcrete and add cost of erection and external render.

2. Double skin of blockwork with 300mm cavity filled with Rockwool. Dense concrete gives better thermal mass so possibly dense inner skin and aircrete outer. Ties have to be found – 450m. Zurich approve as long as ties are okay. BedZED had own ties made. Problem with large amount of embedded energy. Also difficulties around window and door reveals (no problem there with Hemcrete) and very dependent on good craftsmanship to avoid cold bridging and leakage. Construction may be slow. Rain ingress into cavity during construction. Seems a lot of bother. Why bother with external skin of blockwork at all?

3. Timber frame with 300mm studs – possibly Masonite or similar. Major problem is that we do not like the hollow sound of dry-lined walls. Also the lack of thermal masss. Would cellulose filling eliminate this? Or maybe Rockwool for weight.? How could we get to c. 0.1 u-value with rendered surface and no cavity (no point in extra skin of expensive purely decorative blockwork).

4. Clay perforated blocks with woodwool external insulation – seem like a good idea but expensive and may not achieve the u-values for a zero heating house.

5. Hybrid masonry / TF systems. We discovered these last night on this forum. How well tested is it? Which goes up first? Which is structural. The idea of building to the FF and tying it all together before going up sounds good as about one-third of our FF is in the roof so we have large gable ends.

6. Or – could we put up a thick masonry wall and insulate it externally? Window reveals would be set a long way in? How would the roof/wall connection work?

In every case – what about interstitial condensation?

Your help would be much appreciated. Otherwise going mad with too much information.

Thank you!
Lizzie ???

[David Olivier]

Bit concerned that your ground floor is suspended timber

They tend to leak air.

HTH

David

[SimmondsMills]

Lizzie
the block / timber frame hybrid you saw on the forum is not very widely tested, but various permutations have been used in the uk over the years. we have not built this yet – meeting the builder again this week! you could also get the engineer to design it so the blockwork is totally loadbearing and the timber external studs are far smaller: say 50 x 50 even, knot free. this aproach would seem fairly robust – keep timber studs/noggings/rails etc to a minimum – design in minimal timberwork!

a more 'tried and tested' method may be a loadbearing blockwork and insulated render system – such as permarock's 250mm EPS (polystyrene) Platinum system (low k value for eps better than 'normal' polystyrene). blockwork is 'parged' with a levelling screed then system glued on – not sure what the glue is – think may be bitumen below dpc, not sure above, worth asking them. explore this with jeremy richings at permarock (or other similar company like Sto renders). he is on 01509 262 924

I beams: line with a heavier panel? see discussions elsewhere on forum…

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