Do you mean draughty, vapour open, or air permeable?

Masonry can be vapour open but it is dangerous in terms of rot and interstitial condensation to have vapour open and timber frame and the better insulated you are the colder the outside of your frame will get.

Hi Folks , i lived in Finland.They have been doing prefab for years. The current trend is pushing against Passiv houses. Unhealthy buildings. People are becoming sick and now have to live in Quarantine type housing developments. I am aware it depends on ventilation, air change rates etc. Its just a development to bear in mind .

This outfit do prefab at a decent price. Exemplary quality and price. Put PV on the roof and away you go

see www.sunhouse.fi

Posted By: Ed DaviesLot of FUD about this evening, isn't there?

You mean about Grexit I assume......

Ed Davies asked: "Wasn't there a company doing straw bale panels like this in the west country? Maybe in Bath?

You may be thinking of Modcell but I think it may be worth looking at Ecofab http://www.eco-fab.co.uk/

I think there are others if the OP searches for pre-cut timber homes but I don't know enough about any of them to make a suggestion.

Go on Ed - another chance to do your Trademark one 'l' thing.

SIP + CPS kits do not breathe because they rely on OSB as a sheathing to the panel, which does not breathe. OSb or sterling board traps moisture and does not allow for moisture to pass accross it due to the glue that holds all the strands of timber together. So the panels need ventilation both sides. Where moisture hits it without venting it will create an environment for spores.

So …Where not detailed properly it can be a disaster.

I’ve built up wall panels on site from simple timber members with for instance more breathable MagPly on the exterior, ran the condensation risk calcs and they are much better, comply and remove the risk.

Further .. the step away from OSb and toward moisture permeable construction methods - allows for the use of warmer and less toxic insulation such as Sheepswool or cellulose fibre.
And the methodology ten allows for adding further layers of breathable insulation externally such as Insulation foil to move easily up to super insulation levels in line with Germany, but without the passive house side effects of living in a hometically sealed box .. if that’s not your desire.

It’s not for everyone.
Have tackled whole houses , extensions and upgrades to fabric this way over the last 25 years and found no issues .
Examples at
www.retoolarchitecture.com

OSB3, ply etc do breathe. Nothing in the 'sandwich' needs to be super-permeable, though premium-price things like Panelvent are sold on that belief.

The construction (for southern England at least) that works extremely well when modelled in WUFI, which I design and specify whenever suitable, uses 11mm OSB3 gapfilling glued and screwed incl all edges, on the outside of stud framing. Warmcel blown insulation between, plasterboard & skim inside, which may be punctured by trades. EPS or woodfibre glued like EWI outside the sheathing, choice of claddings like any EWI. No VCL. The OSB, along with the blown Warmcel, serve as robust airtight layer, but are more than enough vapour permeable, as is the EPS - another 'non breatheable' myth. This is for roof slopes as well as walls, forming a seamless 'tea cosy' of both insulation and airtightness at the eaves.

Posted By: Andrew McAvoypassive house side effects of living in a hometically sealed box

Another myth

Off the top of my head, for truly vapour open construction in kit form you have a number of options.

Straw SIPs:

Modcell - https://www.modcell.com/
Ecococon - https://ecococon.eu/gb/


Closed Panel:

Oakwrights - https://elements.oakwrights.co.uk/passive-wall-system
Arfon - https://arfontimberframes.co.uk/
MBC - https://mbctimberframe.co.uk/passive-wall/
PYC - https://pycconstruction.co.uk/what-we-offer/build-system/


Timber frame kits / systems:

Kithurst - https://kithursthomes.com (see also: https://webuildeco.com )
PH15 - https://www.phhomes.co.uk/ph15-system/
Natural Building Systems - https://naturalbuildingsystems.com/
Lowfield - https://www.lowfieldtimberframes.co.uk/low-energy/

I haven't got airtightness figures for Scottish (Caberboard) or Irish (Smartply) OSB3 - they're unwilling to specify it, not wanting to have to quality-control that alongside all the other parameters. Also, Franhofer doesn't help, not considering airtightness to be necessary amongst the tests they do on materials. To them, airtightness is all about the joints, so air-resistance of the body of the material is irrelevent. That can be nonsense, when 11mm OSB3 (or 12mm ply) sheathing is gapfilling glued and screwed (or power nailed) to studwork incl all edges supported and glued. The joints are then 100% airtight, but the body may let it down. Does it, in fact?

I met a Smartply boffin at my WUFI Training in Dublin, discussed with him the need, which he was well aware of. He said Smartply were indeed considering what to do about it, would invite me on a consultation panel. Nothing happened, except Smartply introduced their plastic coated variant. Exactly wrong ("plastic coated but breathable"? I don't think so) . What's needed is airtightness but vapour-open, not crude VCL thus airtightness.

I heard Peter Warm, the UK grand daddy of PH Certification, say ' I've never met a building that failed airtightness because of OSB3'. Good enough for me. Plus, I design/specify the OSB3 in team with blown-in Warmcel, which has significant airtightness itself. The two add together, into an adequately airtight combo that's foolproof to construct (gapfilling expanding glue) and very robust longterm - no plastic membranes or tapes to deteriorate or go gappy within months (as I've heard airtightnes guru Paul Jennings describe), and somewhat self-healing if carelessly punctured in future.

V gd Peter. I don't suppose you repeated the airtightness test 6months/5yrs down the line?
Interesting, "two strips of Compriband and the Soudal SWS system". How did you decide on that triple seal? Published advice, your previous experience, or just super-cautious?

Posted By: fostertomV gd Peter. I don't suppose you repeated the airtightness test 6months/5yrs down the line?
Interesting, "two strips of Compriband and the Soudal SWS system". How did you decide on that triple seal? Published advice, your previous experience, or just super-cautious?

We had two tests carried out 20 months apart and the results were the same. For the first test, all surfaces were finished but there was no second fix, bathrooms, kitchen or compact unit fitted. For the second test the house was complete. For sealing the windows and doors I guess it came in the super-cautious category.