Traditionally, houses are framed in wood. This wood skeleton determines the shape of the house and its structural integrity. Framing a house is usually a pretty straightforward affair, but by applying some new thinking and advanced framing techniques (AFT; also called California framing), Meadowlark Builders has refined a system that reduces the amount of lumber used in the frame and increases the amount of insulation. Our modified AFT creates a sturdy, energy-efficient envelope for less money.
With conventional framing techniques, the exterior walls of a home are composed of about 25 percent lumber. Add a few windows and only about half the exterior wall can be insulated. With AFT, we use about 25 to 30 percent less lumber in the frame, and thus we can place 30 percent more insulation in the walls of a home.
AFT is a method of framing for load paths—lumber is only used where it’s needed. For example, this diagram shows how framing members line up from rafter to the basement rim joist.
These diagrams show how we reduce the wood used in the exterior envelope. Notice the 2-stud, or California, corner. An interior wall buck allows us to insulate an area that is usually a solid block of wood.
Notice also that headers are sized for the load, and even those load-bearing headers use engineered lumber that is stronger and allows insulation to be placed in the wall.
Although these homes are more efficient than their conventionally framed counterparts, we can go one better. By adding two layers of 1-inch rigid polyisocyanurate insulation board (polyiso) on the exterior, we create a thermal barrier between the wood members and the exterior. This forms an excellent air infiltration barrier and also adds an R-10 insulation factor to the total wall assembly.This construction technique rivals structural insulated panels (SIPs) in efficiency, but it costs less. Below are some pictures of this technique in action (this installation uses polystyrene, a product we no longer use due to it’s high carbon footprint):
On the interior of the walls, the rigid polyiso gives us a few more options for insulation. We can safely use cellulose since the dew point is outside between the foam layers, and with staggered seams on the foam board, the air infiltration should be minimal. We can also insulate with open-cell spray foam that won’t shrink or settle and that adds even more air-sealing properties.
Notice how we carefully caulk the frame with the cellulose technique, and that our rim joist between the first and second floor is still insulated with the open-cell foam. This potentially leaky area needs insulation that will stay in place and seal it.
Another benefit of this technique is that we can hold the siding material off of the house with sleepers, also called a Rain Screen. The siding material will last longer and need to be painted less often. This vent will also keep the home cooler in the summer.
We also use this modified technique for roofs. The following picture shows a roof that has the thermal break, an R-15 insulation rating, and a vented cold deck. With spray foam on the underside between the rafters, this roof will be durable and energy efficient. Best of all, ice dams can’t form on this roof in winter, and the roof won’t add heat to the house in the summer.
We like our modified AFT because it’s cost-effective and creates a very tight and energy-efficient house. Other building systems have good points as well, but AFT has earned its place in our bag of tricks.
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