Vapour Control Layer behind WBP Plywood

[self builder 123]

Hi

We are currently building a tiimber framed workshop.

The outer leaf is concrete block. Inner frame 150mm timber with mineral wool insulation with 9mm sterling board covered with breather paper.

Preformed trusses clad with sarking board, roofing felt then metal box profile sheeting.

Inside we are going to line the frame and the underside of the ceiling rafters with WBP plywood.

Would you recommend installing a vapour control layer directly behind the WBP, as I understand that WBP is moisture resistant thus preventing any moisture from passing through ?

All things considered would it hurt to include it ?

 

[^woody^]

WBP is not water resistant in this context.

It will depend on what is going in front of he ply as to where the vapour check should be, but probably the best place is in front of the ply on the room side

If you are not using plasterboard, then how do you propose to get the necessary fire resistance for the frame and roof?

 

[wavetrain]

woody if the plywood is going to be the inside room surface then the dpm should be on the outside surface of the ply.

The point about fire resistance is interesting as I am considerring building an outbuilding mainly for storage and am contemplating different ideas.

Plasterboard is a no no in a workshop context IMO as it would so easly be damaged.

You could always go for block cavity block and forget the wood entirely

PS can someone reply to my thread please?//www.diynot.com/forums/viewtopic.php?t=169977

 

[^woody^]

woody if the plywood is going to be the inside room surface then the dpm should be on the outside surface of the ply.

Yes that's right ...., if the ply is intended to be the inner surface.

But it really shouldn't be - in which case to avoid the possibility of condensation between the inner surface and the vapour check, then the vapour check should be immediately behind whatever is to be the surface/innermost component

 

[wavetrain]

I do not think there is enough thermal resistance in 18mm of ply to worry about considering the amount of other insulation.

Woody I do not think the temperature for the outside surface of the ply in this setup would ever be below the dewpoint.

 

[^woody^]

Its not about dew point.

Vapour will enter the wall and then hit the vapour check and go no further.

So it will then either remain within the surface component and be able to evaporate back into the room, or will remain behind the surface component and between it and the vapour check - in which case it will not evaporate readily back into the room and could therefore allow elevated moisture and mould growth

The principles to work on are to have the waterproofing layer as near to the outside as possible, and the vapour check layer as near to the inside as possible.

 

[self builder 123]

Hi

Many thanks for your replies.

The building has been approved by our planning department.

The construction plan for the walls would be :

100mm outer concrete block leaf : 50mm cavity : Breather Paper : 9mm Sterling Board : 150mm timber with glass wool insulation : VCL : 12mm WBP Plywood.

I take it that this would be okay ?

 

[^woody^]

Approved by planning or building control?

I don't know how building control could approve a structure with no internal fire resistance to the whole structure.

If the ply burns, the frame goes and then so does the roof

And if this is a workshop, then the chances of a fire are increased somewhat. And if it is a commercial workshop, then other precautions come into play too.

 

[self builder 123]

We have both planning permission and a building warrant.

Are you surprised that plywood has been allowed on both the walls and the ceiling ?

What would you have expected to see ?

Is it standalone - would that make the difference ?

Cheers

 

[^woody^]

I would not have thought that a bare ply lining would meet the requirements of part b (1) linings and (3) structure of the b/regs.

Whether the Scottish regs are different I don't know, but presumably if approved then that's it.

Without being alarmist, it just seems that an internal face of dry plywood would not last too long in a fire situation, and neither would the frame or roof once the ply burns.

I would also have expected the boards to need to be tongue and groove, and not just butt jointed to enable fire resistance at the vulnerable joints, and also consideration of the fixings too

 

[self builder 123]

Woody

I take it plasterboard on both the walls and the ceiling would meet the fire regulations ?

Is this correct ?

 

[wavetrain]

Hi

Many thanks for your replies.

The building has been approved by our planning department.

The construction plan for the walls would be :

100mm outer concrete block leaf : 50mm cavity : Breather Paper : 9mm Sterling Board : 150mm timber with glass wool insulation : VCL : 12mm WBP Plywood.

I take it that this would be okay ?

VCL, such as 1000 gauge polyethylene should be fine.

BTW what's the calcualted U value for the above system?

 

[self builder 123]

Wavetrain

As this is a simple workshop our BCO did not specify the need for any insulation at all, but we thought that it best to insulate.

Not sure what the U value is, but I am hoping that the 150mm glass wool in the walls, with 300mm in the ceiling should really help keep the place warm !

 

[Alastairreid]

I take it plasterboard on both the walls and the ceiling would meet the fire regulations ?

Is this correct ?

12.5mm plasterboard with 3mm skimcoat gives 30min fire resistance.

 

[wavetrain]

Wavetrain

As this is a simple workshop our BCO did not specify the need for any insulation at all, but we thought that it best to insulate.

Not sure what the U value is, but I am hoping that the 150mm glass wool in the walls, with 300mm in the ceiling should really help keep the place warm !

I'm not being critical BTW. I know of workshop built 35 years ago inside an old garage with a 2" wooden inner frame with 2" polystryne and clad with 1/2" chipboard including a polythene sheet as DPM. (note the imperial measurments ) It makes a big difference in the summer in that it stays cool inside if the door is kept closed, and the fast thermal response means it heats up quickly when the heater is switched on.

The U value of the walls of your workshop might be 0.2 - 0.25 so you will be delighted by its thermal performance when finshed.