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Mold performance of some construction products with and without borates. (Solid Wood Products).



Many construction products contain borates to improve physical and processing characteristics or to impart flame retardancy. In other construction products, they are used for biodeterioration control giving performance against decay fungi and wood-destroying insects. In this study, solid wood, wood composites, and gypsum wallboard treated with various borate loadings were tested, and work on cellulose insulation was reviewed. The objective was to ascertain whether current commercial levels of borates used in these construction materials would also render them resistant to mold growth. It was found that the presence of borates significantly decreases the amount of mold growth. These results could be important due to the rising concems regarding illnesses associated with 'sick building syndrome' caused by in-house toxic mold growth. While borate-containing materials should not be considered as a substitute for good design and maintenance that prevents moisture ingress, they could be considered as a part of an integrated strategy for the control of mold infestation.

Molds are fungi that grow superficially on materials, but generally do not decay or weaken the strength of such materials. Moisture content is probably the most important factor in determining the rate and extent of mold infestation. Construction materials going into a new building are generally dry and would not suffer from such fungal colonization. If the materials remain dry, this will remain the case; however, should high humidity, condensation, penetrating or rising dampness or other forms of moisture ingress occur overlong periods, the materials can become susceptible to mold growth, as well as other fungi.

In the event of poor design or maintenance, mold growth can be found on many domestic and commercial construction materials in service. These include lumber, wood composites, gypsum wallboard, and insulation products. Previously it could have been argued that it was not important to control mold fungi that did not adversely impact the structure. However, growth of a number of these organisms has now been associated with negative health effects.

Recently, poor moisture management in homes, hotels, and portable school buildings has been linked specifically to the growth of toxic mold fungi. In a survey of homes in 24 U.s. cities, nearly 50 percent of them had moisture problems (14). The majority of the molds found in homes are Cladosporium, Penicillium, and Alternaria, which are now known to cause chronic sinus infections, respiratory infections, and asthma. A potentially lethal mold, Stachybotrys atra, although not as common, was also found (17). Stachybotrys atra produces airborne toxins that can cause inflammation and injury in the gastrointestinal and pulmonary tissues in children and adults. An outbreak of pulmonary hemosiderosis (bleeding lung disease) killed 10 children in Ohio and another 60 infants nationwide before it was traced to toxic mold growth resulting from inadequate home ventilation and humidity controls; at least 28 more children have died since the initial outbreak (14).

This study reviews the presence of borates in some construction materials and evaluates their ability to control mold growth. It would be of value if, in addition to the primary purposes of incorporating borates into construction materials, some performance against mold in homes was achieved as well.


The earliest use of borates against wood-infesting fungi was in the development of preventatives against mold and sapstain in sawn lumber. The extensive American studies in the 1 920s and 193 Os were brought together by Scheffer and Lindgren (24) and at that time they suggested that borates were the most effective means of protecting lumber. Further specific efficacy against staining fungi (e.g., Ceratocystis spp., Aureobasidium spp.) has also been shown in many other studies (1, 7, 8, 10, 12, 15, 19, 20, 22, 26). Borate retentions required to give protection of lumber against some of these organisms are shown in Table 1 (16).

Today, lumber to be used in construction can be treated to control the growth of decay fungi (mold fungi are not the primary targets) and wood-destroying insects. In lumber treated with borates, the basidiomycete decay fungi are fully controlled (9) and this is probably true in other cellulosic materials as well.

Lumber receiving commercial treatment with borates in the United States is treated with disodium octaborate tetrahydrate (DOT-[Na.sub.2][B.sub.8][O.sub.13] 4[H.sub.2]O). It receives a borate retention of 0.17 pcf [B.sub.2][O.sub.3] ([approximately equal to] 0.9% DOT) in accordance with AWPA C31 (4) and ICBO evaluation report 4890 to control decay, beetles, and native termites, or 0.28 pcf [B.sub.2][O.sub.3] ([approximately equal to] 1.5% DOT) to protect against Formosan subterranean termites as well. Wood composites are treated with zinc borate (ZB-2ZnO 3[B.sub.2][O.sub.3] 3.5[H.sub.2]O) and generally meet a retention of 0.75 percent ZB to control decay and termites (including the Formosan termite) in accordance with AWPA recommendations.


Gypsum wallboard or "drywall" is a major component of construction. It is not usually a structural component and is often thought to be an inert material, not specifically prone to biological attack. However, gypsum products have been proven to be susceptible to mold, decay, and termite damage (11). Gypsum has also been shown to supply calcium nutrients to specifically support fungal growth in other commodities, which can lead to structural problems. Gypsum products are an ideal substrate to support mold growth under the right conditions, with both a high paper and starch content, in addition to their high moisture-holding capacity and surface area.

Lightweight boards are now being produced in North America by incorporating foam. This improves ease of handling, lowers shipping costs, and saves energy. Decreasing the gypsum content in boards by adding foam, however, leads to overdrying at the surfaces and a decrease in the strength of the bond between the gypsum and the paper, which results in peeling. Problems with sagging can also be seen in ceiling applications.

Borates are used by some manufacturers to overcome these problems (18). Borates successfully raise the calcination temperature of the gypsum, protecting the board from overdrying. This is accomplished by the water-soluble borate infiltrating the surfaces and edges of the board, creating a dense layer on the surface and a harder outer edge. This dense layer improves the adhesion of the paper backing. Gypsum in the presence of borates also forms larger and thicker crystals, increasing the rigor of the boards, which decreases the likelihood of sagging. In addition, borates accelerate curing, prevent wrinkling on the surface (18), and increase the fire retardancy of boards (21). All this can be accomplished without increasing the overall weight of the board. Currently, manufacturers adding borates target a retention between 0.1 and 0.3 percent boric acid overall, which gives a higher surface retention, due to water movement during drying.


Thermal and sound insulation materials are very important in the construction of homes. Insulation can be made from various materials including synthetic foams, mineral or glass wool, and natural fibers such as cellulose (13). Cellulose insulation is made of recycled newsprint milled into a fibrous form that has a high thermal insulation value and low cost. However, cellulose insulation is prone to both flaming and smoldering combustion, so fire retardants are added (25).

Borates are well-known fire retardants that prevent flaming combustion, suppress afterglow, improve char formation, and form protective glazes at the burning surface. Borates are also reported to protect cellulose insulation against fungal and bacterial growth (25). Cellulose insulation is a good carbon source, has high moisture-holding capacity, and a very large surface area to support mold growth.

Grinda and Kerner-Gang (13) tested insulation materials' resistance to mold fungi and wood-damaging basidiomycetes. Their study concluded that mineral insulating boards and granular volcanic rock cannot be utilized by mold fungi but can be overgrown by them, while the foams they tested contained components that could be utilized by the fungi. This showed that untreated insulation materials (even those considered inert) could be affected and overgrown with mold. Viitanen (27) studied the influence of insulation materials on wood biodeterioration. He concluded that insulation materials influence the growth of mold and decay fungi in contact with wood. At high humidity, he found that borate flame retardants prevented growth of brown-rot fungi in both the cellulose insulation and surrounding lumber. It was also shown that mineral wool is destroyed by decay fungi, and at the same time actively supports surrounding lumber decay (5,23,27).

Typical commercial flame retardant retentions range from 15 to 25 percent borate by weight (a combination of boric acid and/or borax) in order to meet national fire standards (6,25). Phosphates or sulfates are sometimes used as partial replacements for some of the borate, but probably should not be considered in warm humid environments due to corrosion and fiber deterioration problems (28). Ammonium sulfate and ammonium phosphate, the most common replacements for borates, are acidic, can corrode, may release ammonia gas under certain conditions of pH and temperature, and promote fungal growth (6).


Four different types of wood-based construction products were examined: southern yellow pine solid lumber (approximately half and half sapwood and heartwood), aspen oriented strandboard (OSB) obtained from Michigan Technological University, treated aspen oriented strandboard siding obtained from Louisiana-Pacific Corp. (from their treated Smart[R] range of products), and Douglas-fir plywood purchased from a local retail outlet. All OSB was bonded with pMDI resin and the Douglas-fir was bonded with phenol-formaldehyde. Bearing the latter in mind, all samples were evaporatively aged for 1 month to ensure there was no remaining free formaldehyde, which would have prevented fungal growth. Two 3 by 4-inch samples were cut from every board tested.

The southern yellow pine solid lumber was pressure treated using a standard vacuum/pressure full-cell Bethel process (1/2-hour vacuum [[approximately equal to] 26 in Hg] prior to and during solution introduction and then 1 hour of pressure [150 psi] submerged in the treatment solution). It was treated to a target retention of 0.28 pcf [B.sub.2][O.sub.3] ([approximately equal to] 1.5% BAE).

The non-commercial OSB was prepared by Michigan Tech University. It was made from dried bolts of 2-1/2- by 3/4- by 1/40-inch disc-cut flakes prepared from fresh aspen (Populus tremuloides). The borate was added into the blender after spray additions of the pMDI adhesive (MF-184, ICI -7% content) and wax (0.75% solids, Borden EW-403HS wax emulsion). The flakes were arranged in a non-oriented pattern. After being mixed with resin and preservative, the boards were pressed to a thickness of 3/4 inch using an 18- by 18-inch laboratory hot platen press. The press temperature was 400[degrees]F, and the press time was 330 seconds.

Borate retentions were determined by acid reflux extraction followed by Inductively Coupled Plasma Atomic Emission Spectroscopy analysis (Table 2).

The gypsum boards were manufactured by taking 800 g of oretch stucco and adding 575 mL of distilled water that contained 0.16 g of sodium chloride, 4 g of starch, and the required borate loading. Diluted alpha foamer mixed with water was added to the stucco mixture and stirred. The contents were poured into a 0.5-inch board press that was lined with a 14- by 14-inch gypsum paper envelope where it was allowed to set. The boards were then placed into an oven at 138[degrees]C for 45 minutes, trimmed, and placed into a 40[degrees]C oven overnight. Three- by 4-inch panels of boards containing 0.0, 0.1, 0.3, and 1.0 percent boric acid retentions were tested.

A modified version of ASTM: D 3273-94 (2) was used to test the various products' resistance to mold growth. The incubation chamber consisted of a plastic container (23.5 by 12.25 by 16.25 in.), tilted on its side (to give a 45-degree roof angle to prevent condensation from dripping on the samples), filled with 2 to 3 inches of water. The water was heated to 32.5 [+ or -] 1[degrees]C, using a thermostatically controlled heating coil. A plastic bowl (11.5 by 13.5 by 5.25 in.) was filled to within 3 inches from the top with a mixture of damp potting and unsterilized soil to act as an inoculum source, covered with plastic mesh, and then floated in the plastic container. The boards were placed on the plastic mesh above the inoculum source. The lumber and wood composite tests ran for a duration of 4 weeks and the blocks were turned over after 2 weeks. The gypsum board samples were examined after 6 weeks. All samples were scored visually and rated according to ASTM D 5590-94(3) (Table 3).

The decision was made to use unsterilized soil in order to provide a more natural mixed culture condition and give the samples the opportunity to select the most suitable organism for growth on that substrate and the organism least susceptible to the treatments. It also gave some opportunity for biological succession.


Overgrowth of mold in terms of visible discoloration was scored and results are shown in Table 4. It was noted that of the untreated materials, the southern yellow pine sapwood (Fig. 1), the Douglas-fir plywood, and the aspen OSB were most susceptible to growth, in that order. It was also noted that the papered and primed single surfaces of the commercial OSB were resistant to mold growth. Of the treated materials, greater protection was achieved with greater borate retention, and zinc borate was found to give better performance than disodium octaborate tetrahydrate.

The amount of mold observed on the gypsum samples decreased with the increase in borate retention (Table 5). Gypsum boards treated with 0.3 percent boric acid showed sparse mold growth, which was significantly lower than the untreated boards. The best result was seen on the 1.0 percent boric acid containing boards, which exhibited little to no growth after the 6-week period (Fig.2).


With such limited indicative studies, it is of course difficult to make categorical recommendations. However, in all of the studies conducted, the results consistently indicate that the addition of borates to construction materials decreases the growth of mold fungi. It was found that the higher the borate concentration, the lower the ability of the mold fungi to colonize a given material. It can also be seen that the type of construction material (wood type, gypsum, and insulation), borate type, and borate loading are all important in determining the product's performance against mold growth.

Results from the lumber and wood composites revealed that the current commercial loading of borates in the tested products gives a significant reduction in mold growth. The ability of zinc borate to control mold appears better than disodium octaborate tetrahydrate, and this is consistent with data produced by Laks et al. (15).

It was also of interest to note that treated southern yellow pine boards showed more mold growth on the heartwood compared to the sapwood. This is probably due to the ready treatment and higher borate retentions achieved in the sapwood.

Results obtained from the gypsum study again showed that the higher the borate retention, the lower the ability of mold fungi to colonize the material. The current commercially favored retention of 0.3 percent boric acid gives reasonable control of mold fungi.

No mold testing of cellulose insulation was carried Out in this study. However, from the review of the literature, very high borate retentions (15% to 25%) are used for flame retardancy, and therefore, significant performance against mold growth could be expected. The results obtained with the much lower retentions of borates in solid wood and wood composites, which are likely to perform similarly, also corroborate such a conclusion, as does the research of Siddiqui (25) and Viitanen (27). It is also clear from the literature that cellulose treated with ammonium phosphate or ammonium sulfate alternatives would actually exacerbate the potential mold situation (6). This probably occurs due to both the additional nutrients that these flame retardants supply, and also because of their greater hygroscopicity. A difficulty then arises in suggesting the performance of mixed flame retardant types, where for example phosphates and borates are used in conjunction. Clearly these are likely to perform less effectively t han the stand-alone borates, but do merit some specific testing of their own.

In future studies, work should try to more accurately determine the toxic threshold of borates in these products against mold fungi, and investigate other ways of improving mold resistance. Further work, if considered, should also include specific testing with Stachybotrys atra or other known mold pathogens.

Since some products are already manufactured with borates, consumers of these materials can be advised that they are receiving greater benefits in terms of biodeterioration control and mold growth control than they realize. Those involved with tackling the problems of mold infestation in houses should be made aware of the specific borate-containing products to be used as a possible part of an integrated control strategy. However, it should be recognized that the use of borate-containing materials alone can-not be considered a complete preventative against mold.

The authors are, respectively, Technical Associate, US Borax, Inc., 26877 Tourney Rd., Valencia, CA 91355; and Vice President, Nisus Corp., 215 Dunavant Dr., Rockford, TN 37853. (*)Forest Products Society Member.


(1.) Amburgey, T.L. 1990. The need for cobiocides when treating with borates. In: Proc. of the First Inter. Conf. on Diffusible Preservatives. Forest Prod. Res. Soc., Madison, WI. pp. 51-52.

(2.) American Society for Testing and Materials. 1997. Standard test method for resistance to growth of mold on the surface of interior coatings in an environmental chamber. ASTM D 3273-94. Annual Book of ASTM Standards. Vol.6.01. Paint-Tests for Chemical, Physical, and Optical Properties; Appearance. ASTM, West Conshohocken, PA. pp. 338-340.

(3.) _____. 1997. Standard test method for determining the resistance of paint films and related coating to fungal defacement by accelerated four-week agar plate assay. ASTM D 5590-94. Annual Book of ASTM Standards. Vol. 6.01. Paint-Tests for Chemical, Physical, and Optical Properties; Appearance. ASTM, West Conshohocken, PA. pp. 622-625.

(4.) American Wood-Preservers' Association. 1999. Standard C3 1-99. Lumber used out of contact with the ground and continuously protected from liquid water-treatment by pressure processes. Book of Standards. AWPA, Granbury, TX. pp. 114-115.

(5.) Bech-Andersen, J. 1985. Alkaline bui]ding materials and controlled moisture conditions as causes for dry rot Serpula lacrymans growing only in houses, in: Inter. Res. Group on Wood Preservation, Doc. No. IRG/WP/3458. IRG Secretariat, Stockholm, Sweden.

(6.) Bower, J.G. 1978. Borates and Cellulose Insulation -- Partners in Quality Control. RSI Roofing, Siding, Insulation-Regulating Insulation (Special Rept.). Harcourt Brace Jovanovich Inc., New York.

(7.) Byme, T. 1990. Recent research in boron treatment of Canadian wood species: Stain and mold preventives, in: Proc. of the First Inter. Conf. on Diffusible Preservatives. Forest Products Res, Soc., Madison, WI. pp. 87-90.

(8.) Da Costa, E.W.B. 1953. The use of chemicals in the control of blue stain in Pinus radiata. Forest Prod. News Letter 194:3-7. CSIRO Collingwood, Victoria, Australia.

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(10.) Fenton, R. 1962. The use of boron solutions as a means of reducing stain and rot in Corsican pine posts and poles during seasoning. Interim Res. Release. October. Forest Res. Inst., Rotorua, New Zealand.

(11.) Fogel, J.L. and J.D. Lloyd. 2000. Biological performance of gypsum products containing borates. In: Inter. Res, Group on Wood Preservation, Doc. No. IRG/WPOO-30237. IRG Secretariat, Stockholm, Sweden.

(12.) Forsyth, P.G. and T.L Amburgey. 1992. Prevention of non-microbial sapstains in southern hardwoods. Forest Prod. J. 42(3):35.

(13.) Grinda, M. and W. Kerner-Gang. 1982. Evaluation of the resistance of insulating materials to mold fungi and wood-destroying basidiomycetes. Material und Organismen 17:135-156. (German with partial English translation)

(14.) Jacobs, D.E., W. Friedman, P. Ashley, and M. McNairy. 1999. The healthy homes initiative: A preliminary plan (Full Rept.). U.S. Dept. of Housing and Urban Development, Office of Lead Hazard Control, Washington, DC. pp. 1-82.

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(16.) Lloyd, J.D. 1996. International status of borate preservative systems. in: Proc. of the Second Inter. Conf. on Wood Protection with Diffusible Preservatives and Pesticides. Forest Prod. Soc., Madison, WI, pp. 45-54.

(17.) Mann, A. 1999. Mold: A health alert. USA Weekend, Dec. 3-5. pp. 8-9.

(18.) McBroom, R.B. and A. Vizel. 1999. Effects of boric acid on gypsum board. Global Gypsum, June, pp. 18-21.

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(20.) Miller, D.J., J.J. Morrell, and M. Mitchoff. 1989. Controlling Sapstain: Trials for product group II on selected western softwoods. Res. Bull. 66:1-10. Oregon State Univ., Corvallis, OR.

(21.) Mitsui Toatsu Chemicals Inc. 1992. Fire-resistant gypsum board is covered with paper containing boric acid and/or borate salt. Japanese Patent 92022869.

(22.) Orman, H.R. 1954. The relative efficacy of certain chemical dip treatments in preventing sapstain in Pinus radiata. Australian Timber J. 20(11):831-905.

(23.) Paajanen, L. and A.-C. Ritschkoff. 1991. Effect of mineral wools on growth and decay capacities of Serpula lacrymans and some other brown rot fungi. In: Inter. Res. Group on Wood Preservation. Doc. No. IRG/WP/1481. IRG Secretariat, Stockholm, Sweden.

(24.) Scheffer, T.C. and R.M. Lindgren. 1940. Stains of sapwood and sapwood products and their control. Tech. Bull. No. 714. USDA, Washington, D.C. pp. 123.

(25.) Siddiqui, S.A. 1989. A Handbook on Cellulose Insulation. 1st ed. Robert E. Kreiger Publishing Co., Inc., Malabar, FL. pp. 23-33, 57-61.

(26.) Urbanik, E. 1965. Effect of hydrophobic agents on the preservation of spruce wood against molds. Prace Instytutu Technologii Drewna 12(4):3-14.

(27.) Viitanen, H. 1991. Preservative effect of cellulose insulation material against some mold fungi and brown rot fungi Coniophora puteana in pine sapwood. In: Inter. Res. Group on Wood Preservation, Doc. No. IRG/WP/1484. IRG Secretariat, Stockholm, Sweden.

(28.) Winandy, J.E. 2000. Serviceability modeling - predicting and extending the useful service life of FRT-plywood roof sheathing. In: Inter. Res. Group on Wood Preservation, Doc. No. IRG/WP00-20210. IRG Secretariat, Stockholm, Sweden.

Toxic Threshold for Some Mold/Sapstain Fungi (16)

Fungal species Lowest conc. of BAE (a)

 (kg/[m.sup.3] of wood)
Alternaria alternata 2.0
Aureobasidium pullulans 2.0
Phialophora spp. (8 species) 0.1 to 2.0
Phoma spp. (2 species) 2.0
Rhinocladiella spp. 0.1
Sclerophoma spp. 0.1
Torula spp. 2.0
Trichocladium asperum 2.0

(a) Boric acid equivalent.

Mean borate retentions

Wood type BAE (a) ZB/DOT

Southern yellow pine 0.0
Southern yellow pine 1.5 [+ or -] 0.06 1.27 [+ or -] 0.05 DOT
OSB 0.0 0.0
OSB 0.56 [+ or -] 0.01 0.66 [+ or -] 0.01 ZB
OSB 1.76 [+ or -] 0.04 2.07 [+ or -] 0.05 ZB
Commercial OSB siding 0.44 [+ or -] 0.0 0.52 [+ or -] 0.0 ZB
Commercial OSB siding 1.08 [+ or -] 0.0 1.27 [+ or -] 0.0 ZB
Commercial Douglas-fir 0.0 0.0

(a) Boric acid equivalent.

Evaluation of results rating system (3)

Observed growth on specimens Rating

None 0
Traces of growth (%) 1
Light growth (10% to 30%) 2
Moderate growth (30% to 60%) 3
Heavy growth (60% to complete 4

Average severity of mold growth on boards

 Growth on sample
Wood type BAE(a) Side A (sapwood) Side B (heartwood)

Southern yellow pine 0.0 4.0 4.0
Southern yellow pine 1.5 1.5 2.5
OSB 0.0 3.0 4.0
OSB 0.56 2.0 3.0
OSB 1.76 1.0 2.0
Commercial OSB 0.44 2.0 0.0 (paper covered)
Commercial OSB 1.08 1.0 0.0 (paper covered)
Douglas-fir plywood 0.0 4.0 4.0

(a) Boric acid equivalent.

Average mold growth on gypsum boards containing borates

BA in gypsum board Growth on sample

 0.0 4.0
 0.1 2.0
 0.3 1.0
 1.0 1.0
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Author:Fogel, Jennifer L.; Lloyd, Jeff D.
Publication:Forest Products Journal
Geographic Code:1USA
Date:Feb 1, 2002
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