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Fire-Rated No-Formaldehyde Substrates Can Discolor Exotic Veneers

The increased use of no-added-urea-formaldehyde (NAUF) wood products is great, but can occasionally cause unsightly "bleaching" of gorgeous veneers. What to do?

Exotic veneers like this deliciously buttery English sycamore can be devastatingly discolored by some types of non-urea-formaldehyde boards. Woodworking experts have found some solutions to the problem: it's all in how you put it together.

Medium-density fiberboard (MDF) and particleboard are the materials providing the backbone for much of what we see in finished interiors. They are usually composed of sawmill waste and some still use urea-formaldehyde (UF) binders. They are cheap, mill cleanly (MDF more so than particleboard), are made from a waste product, and are fairly durable if installed properly. We almost never see MDF or particleboard, as they are either painted or covered by glued-on veneers that can be designed to look like a myriad of different materials. But reach under a countertop or behind a bookshelf to find an unfinished edge, and you can feel the telltale roughness of particleboard or the glassy smoothness of MDF.

Hidden but deadly

UF is a toxic substance and it was recently classified as a carcinogen (see "Formaldehyde Causes Cancer; Styrene a Likely Carcinogen," EBN June 2011). LEED credit IEQc4.4 requires the use of no-added-urea-formaldehyde (NAUF) products, and most major panel manufacturers produce NAUF MDF and particleboard products (see GreenSpec, Masterformat Section 06 40 23: Interior Architectural Woodwork Substrate).

GreenSpec editors have recently heard from some design teams encountering performance issues resulting in de-lamination and discoloration of veneers applied to Class A fire-resistant NAUF substrates. In some instances, subcontractors have refused to use that combination of materials because of manufacturer disclaimers and the risk of discolored veneers. One architect reported having to give up on the IEQc4.4 credit because of this issue.

It's not the materials; it's how you combine them

NAUF substrate manufacturers recognize the discoloration problem, but identify the complexity of mixing different types of materials and adhesives as a prohibitive factor in comprehensively addressing the problem, as this excerpt from substrate manufacturer Sierra Pine's statement on the issue demonstrates:

We do not know why these rare cases present themselves where particular customers' processes or materials create adverse reactions. There are too many factors, procedures, materials, and end uses adopted by our numerous customers (including, but not limited to environmental conditions, surface coatings, veneer glues, pressing time and temperatures and/or chemical systems used to manufacture FR [flame retardant] products) for us to control, anticipate, or provide guidance on every potential adverse event.

Panel Source, another NAUF substrate manufacturer, has a document outlining veneering guidelines. Highlights from the veneering guidelines include acclimatizing MDF panels to room temperature for 48–72 hours, protecting them from high humidity, and adhering to manufacturer specifications for press time, pressure, temperature, and curing time. It appears that in some instances closely following guidelines may not be enough. In a May 2011 article published by the Woodwork Institute, Dick McClure lays out some strategies for understanding and averting the problem.

First he identifies the instances in which problems are most likely to occur:


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Affected veneers are typically of the exotic varieties (figured Makore, English Sycamore, Wenge) and tend to be much thinner than other veneers. Second, none of the affected veneers had any type of backing or barrier sheet between the substrate and adhesive and the veneer. Third, all of the substrates were specified as 'fire-retardant panel.' Oddly though, we have not seen discoloration occurring with any of the thicker domestic veneers that may or may not have a backer sheet on them.

Not everything needs to be fire-rated

McClure also identifies some potential solutions to these problems. First, he recommends asking questions to make sure that fire retardant panels are actually needed. They are sometimes specified unnecessarily, and if asked, the "code inspector may tell you that only exterior walls and/or interior walls with door openings are required to be fire-compliant." The second solution is to put a barrier between the substrate and the veneer. He writes, "a cross-grain veneer glued on prior to the finish veneer appears to be a workable and successful technique to prevent any penetration to the surface veneer."

Another option is to "request the manufacturer of the substrate to provide a paper or phenolic face and balance sheet to their fire-retardant substrate during the factory lay-up process." McClure acknowledges that these solutions add time and cost to projects, and the Woodwork Institute is continuing research into the problem.

Brian Feagans, AIA, at Ratcliff Architecture, worked on a project that encountered veneer discoloration and told GreenSpec about his firm's experience. The problem became apparent four months after completing the first phase of the project, but luckily before the second phase began. In this instance, the problem manifested itself in "roller marks" over areas where the adhesive was applied thinly. When thickly applied, the adhesive coating created enough of a barrier between the fire-treated NAUF MDF, and a thin exotic veneer to avoid the discoloration. However, when applied too thinly, discoloration occurred. Thanks to a suggestion from the casework fabricator, the team addressed the problem in phase two construction by applying a sacrificial barrier in the areas where the fire-rated material was necessary, and using non-flame-retardant NAUF MDF where it wasn't.

Slap on more adhesive?

McClure told GreenSpec that while in some instances adhesive thickness may prevent the problem, it isn't wise to rely on thick application of adhesives to prevent it:

Applying adhesives is hard to do consistently; they may be rolled on or sprayed on, and the bottom line is that something is penetrating the almost looks like the veneers are being bleached. It is either the borate or the silica in the fire-proofing that is causing the problem...the surefire suggestion is to use a backer or barrier [with these problematic combinations of materials].

McClure went on to talk about the gravity of the problem and the Woodwork Institute's efforts to raise awareness, saying most problems seem to show up six to twelve months after installation and that communication before installation is crucial. "The only way to fix it is to remove, replace, and refinish [both substrate and veneer]; otherwise it is never going to be right again."

Communication between architects, contractors, and inspectors can help identify which areas truly need a fire-resistant coating. Then it is important to make sure the manufacturer knows that not all the substrate needs to be fire-rated material. In areas where the fire rating is required, make sure that a backer or barrier is specified. McClure told GreenSpec that in one project "I saw 22,000 square feet of walls that had failed. In a meeting with the owner, architect, general contractor, and fabricator, we came to the conclusion that many of the walls didn't need to be fire-resistant. Communication and coordination could have avoided a significant amount of the problem in this instance."

Looking on the bright side

Now that we know more about this problem, communication and coordination can avoid it altogether.

There is an unexpected and positive outcome that may result from this issue. If designers, contractors, and inspectors are using more care in identifying areas that need flame-retardants, we may see a reduction in the rote specification of flame-retardant material. This would be a good thing from a green building perspective, given the history of toxicity concerns around some flame-retardant chemicals.

Published October 12, 2011

(2011, October 12). Fire-Rated No-Formaldehyde Substrates Can Discolor Exotic Veneers. Retrieved from

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