Lots of good advice here Richard, perhaps get your builder to join us if he is interested?

Why not use wet plaster on the walls rather than plasterboard? No special products needed.

If requesting air tightness consult from an air tester make sure you ask for advice on building in PH like values. Most of an air testers expertise is about getting aweful leaking builds up to the generous regs retrospectively. Does that air tester have experience with top quality builds? How many < 1m3/m2/h @50Pa builds has he tested?

Posted By: GreenfishDoes that air tester have experience with top quality builds? How many < 1m3/m2/h @50Pa builds has he tested?

Posted By: tonyNone I should think, certainly not domestically.

Hmm, there's a well-known counterexample and others less well-known.

Yea, but they are net on our side they are on the side of the big boys who build them

Can they manage that now then?

none!

Guidance from BRE and others (and Building Regulations Approved Document F) has always been that if targetting an air tightness of below 5m³/m²/hr @ 50 Pa or below, then mechanical ventilation should be considered in the design.

The guidance in particular I recall from Brian Anderson at BRE a few years back was that If achieving an air tightness below 3m³/m²/hr @ 50 Pa. then mechanical ventilation should be used to ensure that build up of 'pollutants' is controlled.

From the EST ventilation guidance I linked to on the last page (GPG 268):

"Research has shown that if relative humidity levels exceed 70 per cent for prolonged periods, there is a high probability that the condensation occurring on cold surfaces will lead to mould growth. A ventilation rate of between 0.5 and 1.5 air changes per hour (ach) for the whole dwelling will usually be sufficient to control condensation."

My suggestion would be to "build it tight and ventilate right - and recover the lost heat from ventilation (but make sure whichever MVHR system you go with has a high heat exchange efficiency; low power use; Insulated ducting; Is SAP Appendix Q rated; is installed by someone who knows what they are doing, rather than a cowboy to ensure that the ducting isn't reminiscent of spaghetti junction - and is actually commissioned; choose a system that is not noisy in operation for the occupants; is easily maintainable; and ensure that occupants are aware of the necessity of cleaning/replacing the filters at the relevant times...)". Have I missed anything?

Hardly catchy, but reflects the aim.