OSB & Offgasing

BigGeorge

Am looking to build a passive house in the coming month & am hearing conflicting opinions as regards offgassing from OSB (mainly) & other construction products due to VOC/ formaldehyde content. OSB seems to get the most attention as there is typically a lot of it used in this type of construction

In the case of OSB, is it worth specifying a low offgasing grade or even the exterior grade for interior use in walls & floors?

justflow1983

OSB 3 is a low VOC variant that should be OK for you. Its formaldehyde free and suitable for both interior and exterior uses.

gears

Have you looked in to using Panelvent as a more natural alternative.

justflow1983

Panelvent is a great material, and is better at vapour diffusion, but I fail to see how its in any way more "natural." It is made from pulped wood chips rather than solid wood chips, and bound in a resin just like OSB.

http://www.panelagency.com/panelvent.html
http://www.woodsidestory.com/products/osb/product_data_osb_3.html

BigGeorge

Sounds like the OSB3 is the job - are these any restrictions , e.g different agreement cert, on its use versus regular full fat OSB?

gears

justflow1983 wrote: »

Panelvent is a great material, and is better at vapour diffusion, but I fail to see how its in any way more "natural." It is made from pulped wood chips rather than solid wood chips, and bound in a resin just like OSB.

http://www.panelagency.com/panelvent.html
http://www.woodsidestory.com/products/osb/product_data_osb_3.html

Looking at those sites it looks like I may have been blinded the credentials of the company who sells them here. Apart from a very low formaldehyde content there's not a whole lot in it.

justflow1983

No big deal gears. Sometimes the sales pitches on those things are a little overzealous.

Panelvent is a material that, in climates with periods of low humidity in summer or winter, will allow much more moisture to migrate out of the building fabric than OSB will. In that sense its a better product. On the other hand, its questionable whether it has any real advantage in the irish climate. I've seen some stuff recently suggesting that here in Ireland, you're better off using OSB 3 and a vapour open external insulation like a wood softboard. Using OSB 3 would also require that you use a VCL that allows back diffussion such as the Intello or Siga membranes.

justflow1983

BigGeorge wrote: »

Sounds like the OSB3 is the job - are these any restrictions , e.g different agreement cert, on its use versus regular full fat OSB?

Not that I know of, but I would suggest asking the supplier here in Ireland. PM sent with link.

soldsold

justflow1983 wrote: »

Using OSB 3 would also require that you use a VCL that allows back diffussion such as the Intello or Siga membranes.

Does this mean the wall structure would have to include softboard, osb AND an intelligent VCL?

This is new to me if thats the case, or am I taking this up wrong?

S

justflow1983

Timber frame build-up I generally use is as follows, from Inside to Outside:

Plasterboard > Battens > VCL> Studs w/ insulation > Sheathing > Softboard> Moisture Barrier (tyvek/solitex) > Battens > Cladding

In this case the intelligent VCL allows moisture to migrate back into the building. Since the OSB is somewhat vapour closed to the exterior its something I would want included for redundancy. Either way you need some sort of VCL there.

Alternatively I've seen:

Plasterboard > Battens > Sheathing > VCL > Studs w/ insulation > Softboard > moisture barrier > Battens > Cladding

In this case you're more vapour open and don't need the intelligent VCL but I'd be concerned about convection/wind heat losses in the insulation, lowering your effective U-Value. This is due to wind infiltration pushing cold air into the assembly.

I'm not an expert, so maybe I'm being redundant with my layers. However, the first way is the way that seems most appropriate to me.

soldsold

A couple of points probably worth clarifying/ discussing a bit more:

a) 15mm OSB is considered to be a vapour barrier, so if 15mm OSB is used internally I would have thought there is no need for the expense to buy and install a separate VCL - assuming softboard is used externally to allow OSB to be used internally

b) Softboard is used as a wind tightness layer, so your point about needing to prevent wind from blowing away the heat should be already covered by using softboard? I have read on another post that 20mm softboard is considered to be windtight?

Your structure looks very good, but is it an overkill?

For example, Ive just had my extension roofed and the structure will end up as (from inside to out):

Plasterboard
Insulated service cavity
OSB3 15mm, joints taped
225 rafter pumped with cellulose (turbofill)
60mm softboard (would have used 20mm if I had known how much work it takes to screw counter battens through 60mm softboard)
Vertical counter batten
Slating batten
Slates

I dont think this roof has any condensation risk but the rafters, softboard and battens/ slates are all that are on for now so Im interested in the vcl issue for this reason.

Steve

BigGeorge

Most of the wall buildupds I've seen from PH suppliers & the architect have the second wall build up, with render applied directly to the softboards , using a system like Sto or Weber. This appears to be driven by the 'need' to increase breathability are you move outwards.

As regards the roof buildup using a warm roof, I've seen it with and without the softboard on the outside. A PH build i saw on sunday had no softboard on the roof, but appeared to had just insulation ( lot of it) behing the moisture control layer and enclosed by the VL; this was on a very exposed site & am really unsure how this fare in the medium term. Are there any suggestions on a tried and test roof buildup?

justflow1983

soldsold wrote: »

A couple of points probably worth clarifying/ discussing a bit more:

a) 15mm OSB is considered to be a vapour barrier, so if 15mm OSB is used internally I would have thought there is no need for the expense to buy and install a separate VCL - assuming softboard is used externally to allow OSB to be used internally

b) Softboard is used as a wind tightness layer, so your point about needing to prevent wind from blowing away the heat should be already covered by using softboard? I have read on another post that 20mm softboard is considered to be windtight?

Your structure looks very good, but is it an overkill?

For example, Ive just had my extension roofed and the structure will end up as (from inside to out):

Plasterboard
Insulated service cavity
OSB3 15mm, joints taped
225 rafter pumped with cellulose (turbofill)
60mm softboard (would have used 20mm if I had known how much work it takes to screw counter battens through 60mm softboard)
Vertical counter batten
Slating batten
Slates

I dont think this roof has any condensation risk but the rafters, softboard and battens/ slates are all that are on for now so Im interested in the vcl issue for this reason.

Steve

1) I have a report from a supplier on my desk saying that OSB3 has the same vapour diffusion properties as Panelvent in the Irish climate. If that's true then it wouldn't be a very good VCL. On the other hand they're trying to sell me OSB 3 so it looks like its something I need to investigate further.

Ciaran O'Connor's woodspec book for Ireland indicates both OSB and a VCL on the inside face of a stud. In the US the sheathing is always on the outside face of the stud so they'd always have a VCL on the inside face.

2) Is the softboard you're using a wood fibreboard? The samples I've got aren't very porous, so you maybe right on it being windtight. Another thing I want to check, but something I've been pretty concerned about. Recent research is showing high losses in real world U-value due to infiltration losses, to the point where there are recommendations to put a sealant on the face of the studs and then mounting external sheathing.

Its more than likely that the softboard is windtight, in which case I'll be happy to have the extra option.


BigGeorge.... I always put battens between insulation and render/cladding. The sto system allows that fine if you use their render-backer. I just really like the rainscreen principle, its probably just a personal quirk.

A good starting point on buildups is woodspec.ie. There aren't very many tried and true buildups at the moment because the long-term experience with TF is in the states where they do more vapour-closed assemblies. A lot of this vapour open, german style construction is newer and still being figured out, although the science behind it is incredibly sound so I wouldn't worry too much. FWIW I'm a fan of ventilated roofs, where theres an air gap between roof surface, it avoids ice damming in the cold-as-heck place where I grew up (northern US on the Canadian border).