New workshop, checking on insulation etc.

[Cabinetman]

I have a super space inside an old barn in which to make it. The barn is fairly typical 2x4’s ( full size as it’s old) with horizontal planking on the outside.
I have acquired a whole lot of Polyiso - Kingspan type 1 1/2” with paper on both faces, R 6 per inch.
The space is 12’5” ish wide so I have no need to cut the insulation to go between the 2x4’s . I plan to line the inside of the existing walls with Tyvek type moisture barrier stuff onto the 2x4’s then having read about bridging and continuous insulation I want to put up 3 layers of the polyiso against the Tyvek then 5 or 6 horizontal Furring strips 1”x3” with long long screws through it all into the 2x4 then plasterboard screwed to the furring strips. I’m not intending hanging heavy things onto the walls.
So have I got that about right?
The floor.
Existing floor is an old not very level but dry concrete floor, I am intending to make up 9, 2x4 frames (the space is 12’ x 24’ ) these will be screwed to each other on top of packing pieces on top of vinyl cushion flooring pads which just gives a little and acts as an extra waterproof layer on top of the plastic dpm. I’ve done this twice before so fairly happy about this bit. Two layers of insulation between the floor timbers all air gaps are to be filled with expanding foam. Then plywood screwed on top.
The top bit.
Th workshop is in an addition with a pent roof on the side of the main barn, so this gives me space above the workshop, the roof joists will span 12’ and will be 2x6’s, one layer of continuous insulation with Furring strips and plasterboard under the joists with an additional two layers of insulation between the joists. There is plenty ventilation above this new ceiling through into the barn proper, do I need a dpm anywhere up here?
All thoughts are very welcome.
Ian

Ps the pool equipment is going.

 

[Mike G]

Tyvek (Housewrap, presumably) is a breathable membrane which allows vapour out whilst not allowing water (liquid as opposed to gaseous) to pass through. In other words, it's the very opposite of a vapour barrier. Unless your walls are leaking, I can't see what purpose it will serve. I'd leave it out. Far more important if you are planning to heat the space is to have a vapour barrier on the inside (the heated side) of the insulation. In a workshop this can often be just OSB or ply, which is good enough without being perfect. Same deal with the roof.......a vapour barrier on the inside of the (underside) of the insulation should stop any moisture getting into the structure and condensing. It looks to me like there are sarking boards under whatever is on the roof, so, again, as long as the roof is in decent order I'd leave things alone up there. Your joists are too thin if you are planning on storing anything on top of them. They'll need to be 8" for that span.

 

[Cabinetman]

Looks like I need to do some research, did I use the wrong name for the plastic sheet needed?
well 8” it is then! Must admit I was guessing with 6”, I’m only putting a dust extractor up there and the weight will be spread over several joists, but who knows I might put all sorts up there in years to come. Thanks Mike.

 

[duke]

Ian, if I'm reading this right, why are you not insulating the stud wall cavity? If you do you could eliminate a layer or two of kingspan.
As Mike has said the tyvek or house wrap is for the exterior surface and 6 mil vapour barrier on the warm interior side.
Also make sure to add bridging or blocking at the half way point to your ceiling joists above your shop. You never know what else you will store up there.

 

[Cabinetman]

@Mike G , I think I’ve been confused by all the houses here in Pennsylvania that I’ve seen being built, they are sheathed on the outside of the studs with OSB and then Tyvek which is a WRB Water Resistive Barrier, finally they add the Siding.
Not sure if they do it the same further North where the workshop is, but the Climate there is hot and sometimes humid in the summer and bloomin cold in the winter. Someone joked? that the sheeting should be in the middle of the insulation as then it copes with the varied Climate.
If I have it right ( it’s complicated) A WRB is needed to prevent any ingress of water though the outer siding getting into the fabric of the building and a vapour barrier is used to prevent wet air from the inside of the building getting to the studs and insulation but the two should not be used at the same time? as any moisture would then be trapped.
My thinking is that as there will be virtually no moisture produced inside the workshop and the outer skin of the building is two layers thick and there isn’t any evidence of or seen to leak in a huge storm recently. I’m probably not going to bother using any, plus with the continuous insulation going onto the studs there will be plenty of ventilation there. Yes the roof is in excellent condition.

@duke, thanks, the reason I’m not putting insulation between the studs is that the building is a few inches wider than it’s easy to build and so I have the room to be able to use the continuous insulation method which means the studs can’t act as bridges for heat loss. And the idea really appeals to me lol.
Good advice on the blocking in the ceiling, I hadn’t considered that.
Ian

 

[Mike G]

@Mike G , I think I’ve been confused by all the houses here in Pennsylvania that I’ve seen being built, they are sheathed on the outside of the studs with OSB and then Tyvek which is a WRB Water Resistive Barrier, finally they add the Siding........

I think that this is going to be a massive story in the USA over the next decade or two. This construction method is fundamentally flawed, with what is effectively a vapour barrier on the outside of the insulation. Problems are already being discovered, and there are many instances of frame repairs being required during the renovation of houses from the 80s and 90s. It's going to be a huge scandal. The idea of draping a vapour-open membrane over a vapour-closed board is just ridiculous. It's only about protecting the OSB during the construction of the building. The only reason that the scandal isn't already happening at large scale is that the internal vapour barriers have reduced the amount of water vapour getting into the wall structure........but wherever alterations are made, or someone screws something to the wall, or changes a socket location, or puts a service in or through the wall, there's a danger that the vapour barrier will be damaged, vapour can then ingress, and will condense on the inside of the OSB. It will then run down the back of the sheathing board and pool on any horizontal surface such as noggins and especially the sole plate. There it will cause mould to form, and eventually rot to occur.

My view of American building techniques is........erm..............not positive! Don't get me started on cast-concrete plinths, timber framed houses in hurricane-prone areas, or the wall/ 1st floor junction in 2 storey houses.

 

[Cabinetman]

Very interesting, I spent a few hours yesterday trying to get my head around this and came to the conclusion that most people don’t know what they’re doing, and it’s not just single build people, I came across several Architects asking what to do in particular situations and questioning the regulations. They were usually from different States where the rules are different. The last point you raised Mike about the first and second floor joint point did come up in my reading, lots questioning the rules around insulation and plastic sheets.
I can’t see that what I’m doing will cause any damage to the structure, in effect I am constructing an insulated box inside an existing barn and I’m not interfering with the ventilation or altering the framework.

 

[Mike G]

Very interesting, I spent a few hours yesterday trying to get my head around this and came to the conclusion that most people don’t know what they’re doing, and it’s not just single build people, I came across several Architects asking what to do in particular situations and questioning the regulations. They were usually from different States where the rules are different. The last point you raised Mike about the first and second floor joint point did come up in my reading, lots questioning the rules around insulation and plastic sheets.
I can’t see that what I’m doing will cause any damage to the structure, in effect I am constructing an insulated box inside an existing barn and I’m not interfering with the ventilation or altering the framework.

Yes, your approach is spot on. Just leave out the Tyvek on the outside as it's unnecessary, and make sure you line the inside of the walls with something which resists the passage of vapour.

 

[Cabinetman]

Great thanks!

 

[GaryR]

Vinyl siding is not water proof. The Tyvek vapour barrier is there to prevent bulk water that makes it past the vinyl siding from soaking the OSB sheathing. My understanding is that omitting the vapour barrier over the OSB is a code violation.

 

[Mike G]

...... omitting the vapour barrier over the OSB is a code violation.

Tyvek is a breather membrane, not a vapour barrier, but yes, that's the problem.

In a properly thought-through wall, the OSB would be on the INSIDE of the frame, and the Tyvek breather membrane would be directly over the outside of the frame. There would be battens over that, with the cladding (you guys call it siding) thereby spaced 25mm off the frame. You use all the right materials, but in the wrong places.

 

[GaryR]

Yes, I think that design is a good one. And yes, here the sheathing typically goes outside the frame. More recently the rainscreen wall has become more common, which is frame>OSB or plywood>vapour retarder>3/8 to 1 inch battens>cladding, similar to the design you suggest. With appropriate flashing and screens to keep bugs out of the gap. One problem with using vinyl cladding over the battens, though, is that it is hard to get the vinyl to lie flat. It tends to dip into the spaces between the battens.