air tightness in new build

Joscia

I've done several timber frame extensions over the last decade or so of and always used 500 gauge plastic as a vapour barrier. I also taped all joints and openings. As there is a lot more about air tightness these days, does anyone know if 500 gauge plastic is good enough used with proper tapes, ie, (Moy. Sieka) I did a course a couple of years ago and understood that it is fine. As there is a huge difference in price I'd like to know if there's a big difference if done correctly.

BryanF

I've used it on timber frame previously.

Phirstclass

The 500 gauge plastic will act as a very effective Vapour barrier if taped & sealed at all junctions & penetrations, offering a high level fixed vapour resistance with protection from air & moisture exfiltration due to convection into the building envelope.

However where the more expensive, but much more sophisticated 'intelligent Vapour Control Layers' come into their own is during seasonal changes when moisture within the build up of your wall from e.g. wind driven rain needs to diffuse inward it will be allowed to do so due to the variable resistance of the membrane. It becomes much more vapour open.

Timber frame stick buildings in Ireland & UK tend to have a much higher moisture content than those built in other European countries and this moisture needs to be able to migrate over time. By installing a fixed vapour resistant barrier such as polythene this moisture will be trapped from diffusing back into the interior and this may lead to a build up of interstitial condensation and mould over time if it cannot dry to the outside.

Joscia

Joscia wrote: »

I've done several timber frame extensions over the last decade or so of and always used 500 gauge plastic as a vapour barrier. I also taped all joints and openings. As there is a lot more about air tightness these days, does anyone know if 500 gauge plastic is good enough used with proper tapes, ie, (Moy. Sieka) I did a course a couple of years ago and understood that it is fine. As there is a huge difference in price I'd like to know if there's a big difference if done correctly.

Cheers phirstclass. Would a breather membrane on the outside not allow the moisture to escape? I fit a breather paper on outside skin with 2x1 battens with exterior riblath on top. This is plastered with 2 coats of sand and cement. With 25mm airgap between.

Phirstclass

Yea, breather membrane with air gap in front will definitely promote outward drying if conditions allow. You want your construction to be increasingly vapour open from inside to outside, so avoiding osb/ply as the external sheeting is a good idea.

In summer particularly though as the wall absorbs radiant heat any water vapour within the wall will want to move from areas of high pressure to low pressure. This can often be to the inside so using an intelligent system can become up to 50 times more vapour open than a vapour barrier allowing good dry back into the building thus reducing risk of moisture build up.