Preparing ply for internal workshop walls

[shipbadger]

Hi all,

The contractors are due to finish erecting my new workshop today. Yes, I know there are those who will say I should have done it, but I already have too many other projects on the go. Anyway, to keep costs down they have delivered 30 sheets of Far Eastern 9mm ply for me to clad the interior. There is a breathable membrane behind the outer cladding and the uprights are 3x2 inch. I propose to add 2inches (50mm) of Kingspan leaving an air gap between it and the membrane. What I am not sure about though is if I need to treat the inside of the ply with any sort of coating. The side facing into the workshop will be painted. Can the ply be left in it's raw state or should it be given a coat of primer or primer and undercoat or something I'm not yet aware of?

Tony Comber

 

[Gary]

As I understand things to be, the exterior cladding keeps the weather off the building. The kingspan will insulate it, the membrane will stop and moisture from condensation. So therefore no need to prime the inside of the ply.

However I could be wrong. Hope this gets confirmed before I make the same error.

 

[Triggaaar]

the membrane will stop and moisture from condensation. So therefore no need to prime the inside of the ply.

The membrane is there to stop the weather (as well as the cladding). It's breathable so that it doesn't cause moist air to stick around and condense, but it doesn't actually remove the moist air. You need something on the inside to stop moist air going out through the insulation (then any that does will hopefully continue through the breather membrane).

If the kingspan is silver coated on the inside, and the joints don't also match joints in the ply, that might be enough. If you put the ply vertically, maybe you can tape the kingspan where the ply joints are.

Depending on what heating and ventilation you have, you could still get condensation on the inside (from warm moist air that cools).

 

[jasonB]

The ply should be fine as is But you need to stop warm moist air traveling through the wall from hot inside to cold outside as there is a high risk of the moisture condensing out within the insulation.

You can either add a polythene membrain fixed to the inside face of the studs or tape over the studs and any joints in the kingspan with foil tape, you will need 4" wid etape to cover the stud and join onto the foil face of teh kingspan

Jason

 

[Dibs-h]

The ply should be fine as is But you need to stop warm moist air traveling through the wall from hot inside to cold outside as there is a high risk of the moisture condensing out within the insulation.

You can either add a polythene membrain fixed to the inside face of the studs or tape over the studs and any joints in the kingspan with foil tape, you will need 4" wid etape to cover the stud and join onto the foil face of teh kingspan

Jason

Jason

From all the info I've come across on the web - it seems (consistently) that when using Ply for the inner face, due to the impermeability of ply wrt water vapour, a separate polythene (or otherwise) vapour barrier isn't required.

Much appreciate the info - if it's otherwise.

Cheers

Dibs

 

[Gary]

The membrane is there to stop the weather (as well as the cladding).

How does the membrane stop the weather, it's between the insulation and ply?

 

[Dibs-h]

How does the membrane stop the weather, it's between the insulation and ply?

The construction is (from the outside)

- cladding
- membrane
- studs (with insulation in them)
- Ply

Now as breathable membrane usually can stand several metres of hydrostatic pressure - that means waterproof to me on the whole. So if it's waterproof, I can't see it not being weatherproof.

HIH

Dibs

 

[Triggaaar]

There is a breathable membrane behind the outer cladding and the uprights are 3x2 inch. I propose to add 2inches (50mm) of Kingspan leaving an air gap between it and the membrane.

How does the membrane stop the weather, it's between the insulation and ply?

You'd put a vapour barrier between the insulation and the ply (could use a damp proof membrane if you had it spare), but we're talking about breather membranes on the outside of the insulation, just like you have under roof tiles.

 

[jasonB]

Gary as Dibs-h says teh breather membrain is between the outer cladding and insulation not on the inner warm side of the structure. For our purposes it is waterproof and will stop any rain that may get driven behind the cladding going any further. Its a bit like a Gortex outer layer, it stops the rain making you wet but lets the moist sweaty air out.

A Vapour barrier goes between the inner surface finish be it plasterboard, plywood etc and the insulation layer.

Dibs, can't say I have heard of one not being needed with plywood but as vapour resitivity of any material is related to thickness it would depend on how thick your plywood is. And if its anythiong like some of the cheap WBP thats about at the moment there are so may voids in it that you could poke your finger through let alone water vapour :shock: Ply certainly have a better resitivity than say plasterboard, almost 10 times better but nowhere hear that of polythene.

Its a long time since I have plotted temperature graidients and worked out U values for various methods of construction but I would say in most cases you could possible get away without a vapour barrier in our workshops but a few quid spent now will be better than having to strip off cladding etc at a later date if you find you have interstitual condensation problems.

J

 

[Dibs-h]

Dibs, can't say I have heard of one not being needed with plywood but as vapour resitivity of any material is related to thickness it would depend on how thick your plywood is. And if its anythiong like some of the cheap WBP thats about at the moment there are so may voids in it that you could poke your finger through let alone water vapour :shock: Ply certainly have a better resitivity than say plasterboard, almost 10 times better but nowhere hear that of polythene.

Its a long time since I have plotted temperature graidients and worked out U values for various methods of construction but I would say in most cases you could possible get away without a vapour barrier in our workshops but a few quid spent now will be better than having to strip off cladding etc at a later date if you find you have interstitual condensation problems.

J

Cheers Jason

Sod's Law - just been Googling on the subject and can't find a single bleedin link now. Although did find something on Wiki that shows that Exterior grade ply has a permeability of 40 units and Polythene 1.7 units.

As you say for the sake of a 20 quid roll in what could easily be a 10-20 grand build, not worth the headache should any interstitial condensation occur.

Many Thanks

Dibs

 

[Triggaaar]

We're not asking the ply to do the main job, the silver foil on the Kingspan is a good vapour barrier, you just need to tape the joints. If you weren't taping the joints the plywood might be enough in those areas, but unless you're sure, best to play safe.

 

[shipbadger]

Thanks for all the replies, if I've got this right the answer is:

Don't need to do anything to the ply.

Need to ensure joints in the ply don't correspond to joints in the insulation.

Best to tape joints in the insulation, especially if behind an unavoidable joint in the ply.

Best of all is to add a layer between the ply and the insulation.

So one remaining question - people have mentioned damp proof membrane, vapour barrier and polythene as the layer under the ply. Does it matter which and is there a prefered material?

Tony Comber

 

[Triggaaar]

So one remaining question - people have mentioned damp proof membrane, vapour barrier and polythene as the layer under the ply. Does it matter which and is there a prefered material?

They're basically all the same thing, just different grades. DPM and vapour barriers are made from polythene or similar. I think vapour barrier is about 500 gauge, DPM is about double that. If buying, go for the vapour barrier as it should be cheaper, and it's easier to move about (think DPM is a bit more unweildy). Note that you can buy polythene sheet that is too thin (less than 500 gauge) so do check.

Need to ensure joints in the ply don't correspond to joints in the insulation.

That's only if you weren't either taping up the joints or using a vapour barrier. If you are, then your insulation will have joints at every joist, and your ply will join every 3 or 4 joists. For example, I've just put some OSB over some kingspan (equivellant) against a brick wall, and since I had no joists to worry about, I just ran the insulation horzontally and the OSB vertically, so none of my joist line up.