Timber Frame Consultants, LLC

Closure Systems for Timber Frame Homes

There are lots of post and beam closure systems on the market today. Each project is different, and there is no one product or system that is clearly the best in every case. The factors and options are laid out in this article

If you have further questions about which product is right for you, please call ITFB at 413-774-4279.

Why does the kind of closure system matter?

Insulation: All of the closure systems include insulation, but they use different kinds. Some of them have higher R values than others. R value is the measurement of how well insulation resists heat flow, so a higher R value means better insulation. For more information go to our Source article on insulation, and visit the Department of Energy's Insulation Fact Sheet.

Cost: The question here is labor vs. materials. The panels discussed below cost more than the traditional stud-wall closure. However, panel installation is fast, keeping labor costs low. If, on the other hand, labor will be inexpensive it could be cheaper to frame and insulate the walls.

Interior: There are lots of ways to connect drywall to timbers, they all work, but some create more problems than others. The original method, which is still used, simply butts the dry wall against the timber. The drawback is that timbers twist and check some as they dry, which can buckle the drywall or leave a gap. Habitat Post and Beam slides the drywall into a dado in the timber, while panel systems tend to slide the drywall behind the timber, concealing the joint completely. See the drawing.

The Options

2x6 stud wall

It is exactly how it sounds. It is also called an infill wall, because the insulation fills in the bays between the studs. It's a simple system; the materials can be found at the lumber store and do-it-yourselfers will have no trouble closing in their own frame. Habitat and Yankee Barn sell their own prefab stud walls.

According to the Structural Insulated Panel Association, the insulation lost to studs is significant. Their data indicates that a 2x6 wall performs as well as a SIP with 4" of foam insulation, but a 2x4 wall performs about R4 worse than the SIP. Predictably, there is no Stud Wall Association.

SIPs

Structural Insulated Panels, also called stress skin panels, are as the name claims, structural. Two layers of oriented strand board (OSB) are separated by a layer of insulation. Kevin Smith of Winter Panel touts the benefits of the continuous R value of panels. They sell sheets of closed cell urethane foam at 4.5" and 6.5" with R values of 25 and 38, respectively. They cost between $4.48 and $5.33 per square foot. He claims that a stud wall is only about an R15.

SIPS detractors, like Habitat and Yankee Barn, argue that OSB is not a strong enough nailer to hold cabinets, and extra framing is necessary. They also point out that some people have had issues with rodents chewing tunnels through the SIPs, causing trouble and cutting down on insulation. There is evidence that expanded polystyrene insulation is more vulnerable to invasion than extruded polystyrene or urethane insulation.

According to Smith, a four person crew can install panels, though second level walls and roof panels will probably require a crane. He also said that lots of people do attach cabinets to OSB without a problem.

Curtain walls

This is a non-structural panel outfitted with interior drywall instead of a second skin of OSB. Though Winter Panel sells curtain walls, Kevin Smith doesn't recommend them. The changing shape of timbers leads to the old buckling problem.

Agriboard

This company makes a structural panel which they say is environmentally friendly, has an R value of 25, does not use toxic adhesives, and goes up quickly. Read their brochure  for more information.

Toxins

Some people have residual fear about SIPs based on the unhealthy urea formaldehyde insulation that they were made with in the 1970's. That type of insulation was phased out in the 1980's. If there is a formaldehyde gas issue now, it comes from the glues used in OSB and other particle boards. The EPA recommends using exterior grade OSB exclusively because it uses phenol formaldehyde, which off-gasses much less than the interior grade.