Wood Frame Covering and Insulation

[Skavenger]

Hi,

I am building a workshop in my garden here. I have just started to build the timber frame and was originally going to cover the outside of the frame with 11mm OSB then breathable membrame, battens and some form of cladding.

I am now having second thoughts, which is making positioning my common studs a knightmare!!!

I need to keep the costs down and this is the reason for now not being sure. Whilst going ahead with my plan I then thought about the inside of the workshop. If I cover that with OSB as well that will double the cost of the OSB. So I need some options of what methods to cover the timber frame both externally and internally.

I have seen a few methods on where the breathable membrame is secured onto the timber frame then battens are fix and the cladding without OSB on the outside of the frame. On the inside the guy then insulated between the studs put on a vapour barrier and then OSB on the inside.

If I OSB on the outside it would look ok from the inside as you would just see wood but it would not be insulated and if you insulate then it needs to be covered.

The workshop will be used for woodworking, metalworking, welding etc.

 

[John D v2.0]

Looks is one thing but more importantly in you're adding insulation where will your insulation be relative to your osb? You want your vapour barrier on the warm side and the osb is not exactly breathable. Should be breathable membranes only outside the insulation unless you have a ventilated drained void on the inside of the impermeable layer.

 

[^woody^]

Pay more and insulate it, or pay less and don't.

It's not really a technical question.

 

[Skavenger]

Looks is one thing but more importantly in you're adding insulation where will your insulation be relative to your osb? You want your vapour barrier on the warm side and the osb is not exactly breathable. Should be breathable membranes only outside the insulation unless you have a ventilated drained void on the inside of the impermeable layer.

Sorry but I am an enthusiastic ametuer - and I can't quite follow your answer, I am a little confused regarding the different vapour / waterproof barriers and where they are used?

In this build the guy has put OSB on the outside of the timber frame, then a "modern breathable membrane". Then he will put battens and then cladding I guess. This was my first choice and if I continue down this route I would then need to think about insulation between the studs in the wall and a covering? Poss vapour barrier plasterboard? or ply?

In this build the guy seems to have stapled the membrane direct to the stud walls, then battens and later cladding. On the inside he had used rock wool between the studs, then a vapour barrier (looks like polythene sheeting) and then OSB.

 

[John D v2.0]

I'm also an enthusiastic amateur! But basically u need the vapour to be able to escape before it cold and condensed. The OSB will block it, so u need it in a warm location. If you have no insulation then that's not a problem as long as it's as cold inside as out.

 

[Skavenger]

In this build the guy has put OSB on the inside of the stud walls, then filled the voids on the outside with Kingspan, then breathable membrane then battens and then cladding.

I'm also an enthusiastic amateur! But basically u need the vapour to be able to escape before it cold and condensed. The OSB will block it, so u need it in a warm location. If you have no insulation then that's not a problem as long as it's as cold inside as out.

Hence the reason for putting the OSB on the outside then?

 

[chappers]

In this build the guy seems to have stapled the membrane direct to the stud walls, then battens and later cladding. On the inside he had used rock wool between the studs, then a vapour barrier (looks like polythene sheeting) and then OSB.

This is your best bet if you only want one lot of OSB and want to insulate it.

 

[John D v2.0]

In this build the guy has put OSB on the inside of the stud walls, then filled the voids on the outside with Kingspan, then breathable membrane then battens and then cladding.

Sounds sensible to me. Although if you can use glass wool it will be cheaper overall even if you need to double the thickness of your studs.

Hence the reason for putting the OSB on the outside then?

Assume you mean because he didn't insulate. And I've no idea why he did it that way or where the insulation went. He mentions sheep's wool but he doesn't mention wall insulation. Pass!

 

[Skavenger]

OK so I have sourced OSB boards - turns out that Wickes is cheaper than most (even TimberCLick). 11mm OSB 3 or the walls and 18mm OSB 3 for the roof.
Only need about 3 sheets of each?

For insulation I am weighing up Rock Wool v Celotex. Looks like I will board out the inside first and then insulate from the outside.
If I fit Rock Wool it's gonna be a pain to keep it in place while I staple on the breathable membrane, whereas Celotex would be easier.

If I fit 50mm Celotex there will be a 20mm gap as the timbers are 70mm wide do I need any form of membrane between the OSB and the celotex as I would with the Rock Wool? What out if I fit Rock Wool - do I need a gap?

 

[John D v2.0]

There shouldn't be a gap between your air barrier and your insulation. With celotex if you tape/foam/whatever then the insulation is basically your air barrier. With wool you need something else ie your OSB. The last thing you want is draughts through and on the warm side of your insulation.
With you r wool you just need to fix it at the top and get the width snug then it should hang OK, no doubt you can get special insulation fixings for the purpose.

 

[^woody^]

I really can't see the point of even bothering with insulation and going through this palaver of board or no board, for a shed then is mostly glazed and so small. Cost savings oposed to cost of doing it will be nil.

 

[Skavenger]

There shouldn't be a gap between your air barrier and your insulation. With celotex if you tape/foam/whatever then the insulation is basically your air barrier. With wool you need something else ie your OSB. The last thing you want is draughts through and on the warm side of your insulation.
With you r wool you just need to fix it at the top and get the width snug then it should hang OK, no doubt you can get special insulation fixings for the purpose.

Sorry John I am a little confused. where you say air barrier do you mean vapour barrier?

If I OSB the inside and then fit 50mm celotex flush up against the inside of the OSB, hold the celotex in place with expanding foam or tape there will be a 20mm gap between the celotex and the breathable membrane any moisture here should escape out of the membrane. There will also be a gap between the membrane and the cladding.

 

[John D v2.0]

Sounds fine, you almost certainly don't need the gap before the membrane as it's breathable but it won't be an issue of you do. The one outside the membrane is important.