Spray foam insulation is the advertised form of insulation material today. With the fantastic air sealing qualities along with significant R-values, this type of insulation is quickly becoming the insulation that everyone wants to use to insulate their home. But spray foam insulation does not work as well on homes with laminated lumber and with more and more homes being constructed with these more structurally sound products, this is a major problem for the spray foam industry.
Laminated Structural Members
Laminated beams are extremely strong and have all but replaced steel beams in basements and crawlspaces. For walls over 12 feet tall, these structural members meet building code requirements for wind shear and structural load support also, making them increasingly more popular for tall wall construction.
The laminated wood is plywood that cut into strips the range in width from 3 1/2 inches all the way to 30 inched wide. The strips are then glued together and pressed in a large machine that holds constant pressure on the entire beam until the glue drys, creating a super strong framing member. The exterior of the beam has a smooth surface that can be slippery and harder to handle, but creates a tough outer layer that adds to the member’s rigidity.
Open Cell Spray Foam
Open cell spray foam expands very rapidly and has to fill the cavity completely in order to insulate and air seal the cavity completely. The excess is cut off with a knife flat to the face of the framing. A vapor retardant paint must then be applied to the surface in order for open cell spray foam to meet vapor barrier requirements as it will absorb moisture over time.
The Issue With Laminated Lumber and Open Cell Foam
Because of the smooth surface of the laminated lumber, the open cell foam only adheres to the sheathing on the back of the wall cavity. When the foam cures completely, this leaves gaps between the spray foam and the wall framing of up 1/2 inch on either side. On very tall walls, the foam can sometimes be pulled right off the wall when cutting because it simply has not adhered properly to the surface of the wood. The most important aspect of this is that the air sealing quality of this product is gone and the R-value (R-3.5 per inch) is no better than fiberglass at 3 times the cost.
Solution
Applying an elastomeric coating (paint that is flexible enough to expand and contract with the framing) can be used to give the foam a surface to adhere to. This coating should only be used on the framing members and not the wall sheathing as the wall needs to breath to allow moisture to escape.
The Issue With Laminated Lumber and Closed Cell Spray Foam
The smooth surface also has an affect on closed cell spray foam, although not as great. The smooth surface prevents adhesion to the laminated members, which reveals very small cracks of 1/8 inch or less along the members. Simply taking a heavy grit sander to the sides of the laminated beams will give the closed cell foam enough grit to be able to hold onto the laminated framing. Because closed cell spray foam does not need a vapor barrier after 3 inches is sprayed into the cavity and does not absorb moisture, it can be a much better choice for foam to use, plus the R-value is R-7 per inch. Even though open cell spray foam is a less expensive product, the extra preparation and vapor retardant requirements make it a more expensive option.
Regardless of your choice of spray foam, working with laminated lumber requires some form of preparation to assure adhesion. Without the adhesion, there is no point in using a high end insulation product like spray foam.
