External wall build up & condensation

BigGeorge

Am speaking to a number of contractors on a PH build. One of the has proposed the following external wall build with a U value of about 0.1 but am concerned about condensation in general , especially between the interior OSB & plasterboard due to lack of breathability.

Any opinion on this would be much appreciated

Outside -> inside:

- Cementation board
- Ventilated cavity & battens 45mm
- OSB 12mm
- 300mm I Beam with 300mm insulation of Rockwool
- Membrane & 12mm OSB again
- Battons with 50mm of insulation in service cavity
- OSB 10mm
- Plasterboard & plaster

sinnerboy

I would tweak as follows

- Cementation board
- Ventilated cavity & battens 45 50mm
- add breather membrane to protect
- OSB 12 9mm - to increase vapour permeability here
- 300mm I Beam with 300mm insulation of Rockwool cellulose for many reasons
- Membrane & 15mm OSB tape sealed at joints and at all boards edges - this is your vapour check and air tight layer
- Battons with 50mm of semi rigid rockwool insulation in service cavity
- OSB 10mm - don't inhibit vapour here - if you are looking for racking strenth use
- Fermacell Plasterboard & plaster

sinnerboy

That will give you a U Value 0.12 .

If you want to get down to 0.1 you need an extra 100mm of insulation

beyondpassive

This is the type of spec I suggest for clients for Passive. I'd be more concerned with the vapor diffusion through the outer OSB.

Aquapanel or equal renderboard
50mm cavity
22mm T+G Gutex or steico racking board, brougth around jambs and reveals no breather required or 9.5 panelvent.
220 Stud with 75 rail or else twin wall stud ( a 140 and a 75mm studs sperated by osb plates as web connectors, the poor mans I beam)
12mm OSB liner as vapor control and airtightness. Cellulose filled to 60Kg/msq
50mm battens rockwool filled services cavity.
9.5 OSB as fixing grounds and 12.5 plasterboard.

beyondpassive

here's one form a couple of years back.

This is for info only, there has been a number of changes in the intervening period.

Firstly, the comprband strips arent always enough to prick up the huge tolerences in Timber frame. it may be worth installing a sole plate for measurement and to receive the the timber frame. Second, the rail shown in the detail should be on the inside. The big issue with timber frame as it is with masonery is the intermediate floor detail and airtighness. You really need to be tough on the frame companies to ensure they put enough attention into building junctions to standards equal to either the Canadian Housebuilding manual or the Passivhaus Bauteilkatalogue (construction detail catalogue).

BigGeorge

Can you explain the reasoning for replacing the interior OSB with the wood fibre & the '9.5mm OSB as fixing grounds'.

I suppose I'm trying to understand if the change above are personal preferences or from observations of what has been installed in recent times and is giving difficulty already. Is there a software calc I can run to determine any issue such as Builddesk

thank folks

beyondpassive

BigGeorge wrote: »

Can you explain the reasoning for replacing the interior OSB with the wood fibre & the '9.5mm OSB as fixing grounds'.

No! thats the exterior side OSB thats been replaced.Its following the rule that materials should be more diffusion open as you go out fro the VPL (thats vapour controll layer by the way.

The 9.5 OSB on the inside of the services cavity is for nailing stuff into and it also adds a little to thermal mass. Hangig shelves, cupboards, flat screen TV's

BigGeorge

Is there any issue with condensation / moisture by putting OSB up against the plasterboard on the interior ( lack of breathability etc..)?

sinnerboy

sinnerboy wrote: »

- OSB 10mm - don't inhibit vapour here - if you are looking for racking strenth use - Fermacell Plasterboard & plaster

I think so