I'm concerned about toxic offgassing from OSB subflooring and roofing. Are safer products available?


I'm concerned about toxic offgassing from OSB subflooring and roofing. Are safer products available?

Asked by Chris Kivett, Summerfield, NC

I'm concerned about using OSB subflooring and roofing due to offgassing. Should I use plywood instead? Are there different grades that don't have as many chemicals?

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Ian MacLeod's picture

First of all, there is not a huge difference in offgassing between OSB (oriented strand board) and exterior-grade plywood. You are right to be concerned, however, about offgassing from wood panels, as the EPA has classified formaldehyde as a probable human carcinogen.

Having said that, there are important differences in the types of formaldehyde resins and their use in wood panel products. The first important distinction is between urea-formaldehyde (UF) and phenol-formaldehyde (PF).

  • UF is more toxic and should be avoided at all costs in anything you put in your home.
  • PF is more water resistant and stable than UF, which means formaldehyde will offgas at lower levels and very slowly, but for a longer period of time. PF products are considered to be relatively hazard-free.

Most exterior plywoods and OSBs use PF. Unfortunately, many interior plywoods, MDFs (medium-density fiberboards), and particleboards contain urea-formaldehyde. These materials inside your home can have a greater impact on your indoor air quality than exterior structural panels or OSB.

For interior use, Columbia Forest Products makes a line of formaldehyde-free hardwood plywood panels that use a soy-based adhesive. For MDF interior panels, Medex and Medite II are formaldehyde-free and are your best options. I would request formaldehyde-free wood panels wherever possible, but they may not always be available. If formaldehyde-free panels are not available for your project, use products that contain less-hazardous PF.

There are other important differences between plywood and OSB that are worth discussing.

  • One is that OSB is much more resource-efficient than plywood. Plywood requires large-diameter trees for its slices of veneer, whereas OSB uses smaller trees and utilizes practically the entire tree. The trees used to make OSB often come from tree farms, reducing pressure on older-growth forests.
  • OSB tends to be cheaper than plywood as well.
  • One major drawback of OSB in the past was that it tended to swell when wet, especially around the edges. This is especially a problem with subfloors, where the tongue and groove have to match between adjacent panels, and where any bumps in the subfloor can translate directly into the finish floor. OSB manufacturers have been working hard to reduce this problem.
  • Some newer types of OSB, so-called "New-generation" OSB panels, use isocyanate resins that reduce the swelling risk.Isocyanate-resin panels do not contain formaldehyde and are considered non-volatile when cured; they offgas less than PF panels. They also use edge sealers to prevent water infiltration at the side of the panel.Louisiana-Pacific TopNotch Sub-FlooringandWeyerhaeuser Structurwoodare examples of this kind of product.

Both plywood and OSB, if they get wet over a prolonged period of time, will buckle and delaminate, so you should use only Exposure-1 ply or OSB and materials should be kept as dry as possible to avoid any problems.

Structurally, plywood and OSB panels are essentially the same, although some structural engineers prefer Structural-1 plywood for shear walls. Check with your contractor and make sure local codes are met.

Another environmental consideration is whether you can get Forest Stewardship Council (FSC) certified panels. Certification ensures that the trees come from sustainably managed forests or from other renewable sources.

In California, we can sometimes get both FSC plywood and FSC OSB, but not all the time or in all the sizes we want. Check with your local lumberyards on price and availability. The supply often depends on a local lumberyard's relationships with regional mills, so it is always best to ask.

For more information:

Read Amy Green's Q&A "Does priming and painting OSB help prevent offgassing?"