Zero-VOC Sheathing & Subfloor

This post is an overview of low and zero-VOC sheathing, decking and subfloor options

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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 the 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 it was not possible to measure them accurately.” APA Source

While 0.1 ppm is high for someone sensitive, the fact that the panels rapidly offgas makes them a safe option for most people with chemical sensitivities.

I would suggest testing plywood that is a few months old before ruling it out.

For a detailed review of the glues and chemicals that offgas from plywood and OSB sheathing see the post on formaldehyde and pressed wood products.

A Note on Purebond Formaldehyde-Free Plywood

Purebond plywood is not rated as a structural plywood for sheathing, subfloor, or decking. It also doesn’t handle moisture as well as typical plywood.

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. Please don’t do this!

Purebond is made with “soy-based glue” which likely means polyurethane. I do not prefer this over regular plywood.

Plyboo is a bamboo-based plywood made from soy glue as well and is also not structural.

Exterior Sheathing Options

There are other options than plywood and OSB for sheathing. The first thing you need to do is check with your building codes and also consult with an architect to make sure you have the structural support needed.

The three main types of sheathing are: plywood, OSB, and Huber’s OSB.

Plywood is the best option for most sensitive folks. OSB take longer to offgas. I have found that Zip Sheathing (a type of OSB) needs a solid 4 months (at least) to offgas. I like the Zip-R which is OSB-backed in polyiso foam. The foam is to the interior side, which seems to block the offgassing.

It’s highly unusual to use anything other than plywood or OSB.

Solid wood can be used but it’s very pricey and not done often. Non-wood options are far riskier.

Non-wood options are sometimes used but are usually unadvisable, often far more expensive, and often don’t meet codes: those include Georgia-Pacific DensGlass gypsum, MgO board, and even rigid foam can technically be used as exterior sheathing in some areas (though almost never advised by a good architect). Cement board is sometimes used, especially in tropical areas. USG Structural Panel is a thick, strong concrete board (unlike crumbly tile backer boards) that is sometimes used on foundation walls when the framing is metal. It has a slight odor but it is mild, I expect that it has polymers added since it has 10% undisclosed ingredients and a faint odor.

Consult with your architect or engineer before choosing an unusual product.

Subflooring

Subflooring is usually plywood, OSB, or Advantech (which is a form of OSB). Plywood is generally the best choice for the chemically sensitive.

You can also use solid wood.

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.

You have to spec specific brands that have structural ratings. This is not something that has been in use for long, and it would be unusual to find a good architect who would approve this.

Huber did recently acquire a MgO board company that has a structural rating.

For those who can’t tolerate wood, it is possible to pour a concrete upper floor, though this is not usually done in North American homes.

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 house wrap. Consider double-sided house wrap tape so that you get more of an air barrier.

For subfloors, 1-by subfloor was laid diagonally to the floor joints. The subfloor could be planks or tongue and groove. Here is a pic of what a solid wood plank subfloor looks like.

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.

Roof Sheathing (Decking) 

Plywood, OSB, and Huber products are also the most common roof decking.

Purlins or skip sheathing can be used as an alternative 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) says Paula Baker-Laport, Architect.

Huber Advantech products are applied with a polyurethane adhesive. You would want to check for the tolerability of polyurethane adhesives (as well as the Advantech product themselves).

Corinne Segura is a Building Biologist Practitioner with 8 years of experience helping others create healthy homes.

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