Using insulation rolls in timber cavity workshop walls

[Pord]

I'm at the stage of internally lining my 6m x 4m workshop. Construction is (inside to out) OSB3, timber stud framing with 95mm cavity, breather membrane, ventilation gap, T&G weatherboard.

In an effort to keep costs down I'd like find a way to use the plentiful and cheap rolls of roof insulation on Gumtree, rather than batts which are generally not available second-hand. Anyone had success with this approach? Presumably slumping is the main issue? Because of the dwangs each 'cavity' is approx 550mm wide and 1100mm high.

 

[MikeG.]

You'll be fine, so long as they are stuffed in tightly.

 

[Peter Sefton]

I think slumping would be your main issue, but this could be overcome by fixing at the top with a batten and letting it hang.

Cheers Peter

 

[Phil Pascoe]

If they're stuffed in tightly they won't have room to slump.

 

[Inspector]

I know you guys build a little differently than we do in Canada but in your description of the walls you don't mention vapour barrier behind the inside sheets of OSB. It is usually 4mil polyethylene, stapled to the studs after the insulation is on. and sealed with a sticky black snotty sealant along the edges and around poly pans the electric boxes are in. The idea being to keep the water vapour in the building from migrating into the wall cavity and condensing in the insulation. So no vapour barriers on your walls and ceilings or just didn't mention it?

Pete

 

[Pord]

Pete, you're clearly not au fait with the MikeG School of Workshop Building! The OSB3 is the vapour barrier. All joints will be sealed for further vapour resistance.

 

[Inspector]

Is the OSB3 a painted or sealed product? Here it isn’t but there are versions for construction that have a treatment that is supposed to keep it from swelling from rain while building. It isn’t a vapour barrier though and the treatment doesn’t last long.

Pete

 

[MikeG.]

Is the OSB3 a painted or sealed product? Here it isn’t but there are versions for construction that have a treatment that is supposed to keep it from swelling from rain while building. It isn’t a vapour barrier though and the treatment doesn’t last long.

Pete

Pete, OSB is so full of glue that it works perfectly as a vapour barrier (regardless of what it is coated with), with a very high resistance to the passage of vapour. In a workshop which doesn't have high levels of heating or moisture output (cooking, showers and so on) it is more than adequate for the task. Lining the inside of a workshop (or other outbuilding) with OSB therefore obviates the necessity for a vapour barrier. In "breathing walls" there is a rule-of-thumb principle of having the vapour resistance of the inside of the wall 5 times higher than the outer face, and OSB on the inside achieves this beautifully, without the need for a plastic VCL. Of course, to show compliance with the Building Regulations such a wall would require a dew-point calculation, but workshops typically don't need to be in compliance with the regs so this is superfluous.

As an aside, I predict a world-wide scandal regarding timber framing where OSB is used on the OUTSIDE of the frame and insulation. I think we will see failures where vapour gets into the wall from the warm side (inside) and condenses on the inside of the outer face of the wall (ie inside the OSB), runs down and causes eventual mould growth and even rot of the lower parts of the frame. I always specify a breathable sheathing board (Panelvent is one one such) on the outside of such construction, or a breather membrane without board if I have put the sheathing board on the inside face of the frame.

 

[Inspector]

Appreciate the reply. My shop was built above the garage at the same time as the house. It has the poly vapour barrier over the 2x6 fiberglass batt insulation as left by the builder. I chose to use OSB rather than drywall because it was cheaper and I can attach to the walls and ceiling anywhere I want. Looks like from what you said my exterior OSB will not have any issues from moisture from inside. Sorry about sidetracking the thread. Always interested in how constitution differs in other countries.

 

[MikeG.]

Appreciate the reply. My shop was built above the garage at the same time as the house. It has the poly vapour barrier over the 2x6 fiberglass batt insulation as left by the builder. I chose to use OSB rather than drywall because it was cheaper and I can attach to the walls and ceiling anywhere I want. Looks like from what you said my exterior OSB will not have any issues from moisture from inside. Sorry about sidetracking the thread. Always interested in how constitution differs in other countries.

Well my view on this matter isn't mainstream here yet. I think more and more fellow architects are coming around to my way of thinking, but the building industry is way behind us.

 

[Pord]

Thanks all for the replies. Will crack on as planned with the roof insulation in the cavity walls.

Must post some pics of the build so far...