Zero-VOC Sheathing & Subfloor

Here is an overview of low-VOC and zero-VOC sheathing and subflooring options.

Structural Plywood

Structural plywood (Softwood Plywood - SWPW) can be used for exterior sheathing, roof decking and subfloors. It is made with phenol-formaldehyde glue (PF) in the US and Canada.

The Rate of Formaldehyde Offgassing in PF Plywood

"Formaldehyde levels in this test chamber were well below 0.1 parts per million (ppm) in air from all tests of fresh panels, and emissions rapidly approached zero as the panels aged. In fact, the levels were so low and so close to the "background" levels in the test chamber that is was not possible to measure them accurately." Source APA

The rate in outdoor air is about 0.0002 - 0.006 parts per million (ppm) in rural and suburban outdoor air. Source

While 0.1 ppm is high for someone sensitive, the fact that the panels rapidly offgass should make them tolerable for someone with MCS. I would suggest testing plywood that is a few months old.

A Note on Purebond Formaldehyde-Free Plywood

Purebond plywood is not rated structurally or moisture-wise for sheathing, subfloor or decking. I have spoken to the company and they don't recommend it used for this purpose though I do see people building with this product. I am not sure the consequences of building your subfloor or decking with a non-structural plywood.

Non-Wood Options:

Exterior Sheathing

A non-wood option is Georgia-Pacific DensGlass, which is around 3 times the cost of OSB. It is very low VOC; they are going for GreenGuard certification. It is also a lot more mould resistant than OSB or plywood. Make sure with all materials it fits the codes where you live (in terms of high winds and earthquakes). Make sure it is also compatible with your exterior finish.

MgO board is now starting to be used as exterior sheathing. Another zero-VOC option. It is heavy, structural and more expensive than plywood.

Rigid foam can also be used as exterior sheathing without any ply or OSB. The rate of XPS offgassing is very low, but it does contain a flame retardant. The other option is Polyiso. The Johns Manville brand does not contain a flame retardant. Here is some info on how to brace when using rigid foam as sheathing. This is not structural and should be checked against local codes.

Another structural option is Rewall which is made of recycled beverage cartons and cups shredded and compressed. It claims zero-VOCs. It is paper-faced.

Subflooring

It is possible to use structural cementitious sheeting board (MgO) as a zero-VOC option however when I did this in my tiny house I needed additional framing support underneath. It worked out well for me. Here is an example of the support you need underneath. This is probably only suitable in a tiny house.

For those who can't tolerate wood it is possible to pour a concrete upper floor.

Roof Sheathing (Decking)

Purlins or skip sheathing can be used as an alternate form of roofing which eliminates the need for solid sheathing. This will only work with certain roof types (metal vented attic, in some cases cedar).

Pre-Plywood Options

In the days before plywood, solid wood was usually used:

For exterior sheathing, 1-by lumber laid diagonally was used. This is not an airtight option so humidity and energy issues should be considered. Make sure to use housewrap. Consider double sided housewrap tape so that you get more of an air barrier.

For subfloors, 1-by subfloor were laid diagonally to the floor joints. The subfloor could be planks or tongue and groove. Here is a little more info and a pic of planks. This is a zero-VOC option but it will cost you quite a bit more. If you use this method make sure you use a subfloor adhesive otherwise you will have a very creaky floor. Liquid Nails is the lowest VOC option I have seen at less than 20g/l, but I find Almighty Adhesive to be more tolerable.

For decking, 1x decking butted up to each other can be used. This is how roofs were built before plywood or OSB. This will allow for many types of roofing types over it.

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3 comments:

  1. What about MgO Board? George Swanson, builder from Austin, TX, uses MgO for exterior applications as well as interior. I think he also uses it for roof sheathing... Thoughts?

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  2. We used it as flooring on the 2nd floor of the house we're building. Also going to use as inside walling and outdoor decking with tiles over the top. We used Modakboard MgO board from Australia. Great company! Sent samples wrapped in foil for me and everything.

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