Energy Efficient Framing

GreenBuildingAdvisor.com

Our contractor might disagree with the use of the phrase "Energy Efficient Framing" since it was his crew that put all the energy into framing the house, but with a little extra energy put into planning the framing it's possible to save a lot of lumber and get some extra insulation in the walls ultimately leading to a more efficient building envelope. Mom and Dad really wanted a super-insulated home so we came up with a system that would allow for a lot of insulation in the roof and walls, as well as introducing thermal breaks between the interior and exterior sides of the walls whenever possible.

Key components of Energy Efficient Framing are:

Stacked Framing - Aligning all the framing members vertically, from roof to wall to floor will allow for direct transfer of loads down to the foundation. This can eliminate the need for double top plates on walls and other members used to transfer loads horizontally.


Double-Stud Wall 24" O.C.- Using 2x6's at 24" on center instead of 2x4's at 16" on center is often recommended because even though it uses slightly more lumber it allows for two extra inches of insulation in the wall. We wanted even more insulation so we decided to go with a double-wall system comprised of two parallel walls framed with 2x4's at 24" on center and spaced 1" apart. This allowed for a total of 8" of insulation in the wall cavity and also created a thermal break between the inside and outside surfaces of the wall.

2-Stud Corners - Conventionally frames walls usually will use at least three studs in the exterior corners of the wall to support sheathing on the outside and drywall on the inside. All this wood means there isn't much room for insulation. Energy Efficient Framing typically uses two studs to support the sheathing and metal clips to support drywall allowing for more insulation in the corner. With the double-wall system we used we end up using four studs in the corners, but still have plenty of room to get a lot of insulation in there.

Eliminate Unnecessary Headers - Headers are only needed to transfer loads over openings in load bearing walls, but often they get put in over any opening regardless of whether or not they are actually needed. The north and south walls of this house are the load-bearing ones, so those are the only walls we put headers into.

Engineered Lumber for Long Spans -For floor and roof framing longer spans require deeper members and to get these out of solid lumber is not only expensive but it's a lot of tree. While we used conventional lumber for all the wall framing we used engineered wood joists for the floor and roof framing. The second floor and roof is made up of two 14' long spans, but we able to use 32' joists for the roof framing so a single member made both spans and formed the roof overhang.

Design Module - The overall foot print and form of the house is based on a 2' module to work with conventional sheet material sizes, thus minimizing material waste. The main level footprint is 28'x40' and the second floor level is 28'x24'.

THIS LINK provides additional information on Energy Efficient or Advanced Framing Techniques.

South elevation taking shape

View across the kitchen

View across the bedroom

The main purpose of Energy Efficient Framing is to improve the efficiency of the home, not the process of building it. While we did make an effort to eliminate unnecessary framing elements and minimize material waste the bigger goal is to maximize insulation in the roof and walls.

We Have a Slab

For the sake of simplicity, cost, and because they really like the look, Mom and Dad decided to go with polished concrete for the finished floor throughout the main level of the house. This meant that a little additional planning, and a lot of extra care needed to be taken to ensure that when the slab got poured during the first month of construction it would remain in good condition throughout the rest of the construction process. Construction is a dirty job and having the finished floor be the primary work surface throughout construction can be risky.

Piping stubs can be seen coming through the layer
of insulation below the floor slab. A footing runs
down the center of the house below a load bearing wall.

Plumbing (under the slab) and radiant heating (in the slab) were the two critical items to get done right, because once the slab goes in there is no going back to change anything. In a typical home with a basement the plumbing is done long after the basement floor slab goes in, but in our case the entire first floor plumbing layout had to be done with only the foundation walls for reference. The same goes for the radiant floor layout which had to be done without any reference to the interior partitions that were to come along later.

Once the floor slab was poured the contractor had to cut control joints in the concrete. Concrete will expand and contract with changes in temperature and humidity, especially with a heated slab, and when this happens the concrete can crack. These control joints will allow us to define where the cracking occurs, like in a perfectly straight line between to rooms, rather than at random through the middle of a room. There is always a chance that cracking will occur where you don't want it, but that's just the risk you take when going with a finished concrete floor.

For the floor finish we looked into several options and ultimately decided upon a process of polishing and staining the concrete after it had cured. This would allow the general contractor to proceed with framing the house, and hold off on really finishing the floor until the house was fully enclosed.The subcontractor who would do this was Jon Meade, a Portland-based "concete artisan" who specializes in countertops on floors.

View of the foundation prior to pouring the slab

The almost completed floor slab, you can see where control joints are being cut.

Foundation: No Basement = Less Concrete

It was always our intention for this house to have the smallest impact on the site as possible, and that includes how the house sits on the ground. After spending the past 35 years living in a house with a basement that leaked Mom and Dad were determined to avoid this problem in their new home.

As part of the desire to minimize the overall energy usage of the house we developed a passive-solar design that situated the house with a southern orientation, maximized south-facing glazing and minimized northern, and incorporated a thermal mass for storing thermal energy gained from the sun over the course of the day. The most effective way to do this (and to avoid the potential problems of a wet basement) would be to build a slab-on-grade. Since we intended to use the floor slab as a thermal mass for storing heat gained from the sun it also made sense to incorporate radiant in-floor heat and use the slab as the primary heat source of the home.

We were also interested in minimizing the overall amount of concrete used in the home, another reason for avoiding a full basement. Even though the Dragon Cement plant is less than 10 miles away from the site we still had concerns about the high embodied energy in concrete. This article addresses the carbon-load and energy intensity of producing and transporting concrete. Although the amount of concrete we saved by not building a full basement is insignificant in global terms it was still important to us to minimize the carbon footprint of this home.

For the sake of simplicity we initially considered several monolithic slab techniques, but since the slab was to be the primary heat source for the home it made sense to isolate the slab from the foundation wall. The problem with a typical monolithic slab is getting enough insulation around the outside of the slab to keep the heat you pump into it in a radiant application from escaping to the cold outside during the winter. The detail below shows a wall construction with an R-value of somewhere between R-21 and R-30 depending on the insulating materials and thicknesses used, but around the foundation there isn't even R-10. Since the framed wall needs to sit on top of the foundation it's nearly impossible to get adequate insulation in place if your monolithic slab is also your heat source.

BuildingScience.com

Additionally, because the slab is continuous with the wall in this detail the wall becomes part of the thermal mass being heated by the radiant floor system. While we want to have a substantial thermal mass it's possible to have too much mass to heat efficiently, and in this type of detail there is a high surface area relative to the mass which would lead to greater potential for heat loss to the earth below (not to mention the fact that this particular detail doesn't illustrate any insulation below the slab).

The detail we ended up going with is a modification of the one below, a more traditional foundation wall with separate footing, foundation wall, and most importantly a thermal break between the floor slab and the foundation wall. While both of these details illustrated have the same amount of insulation between the slab and the outside air, the placement of that insulation makes all the difference.

BuildingScience.com

The detail below is the one we developed, which also includes a layer of 2" rigid insulation extended down the inside face of the foundation wall to help prevent frost from penetrating to below the floor slab. At one point we considered insulation on the outside of the foundation wall but ultimately abandoned this out of concern for the durability of a finish over the insulation.

The soil on site was very sandy which made for quick excavation for the foundation wall. The photo below shows the footing form-work with a step down towards the front of the house where grade slopes away. 

Below shows crushed stone backfill around the footings. You can also see the perimeter drain in the lower left corner of the photo below where the footing steps. Even without a basement it is important to drain water away from the building foundation due to concrete's ability to wick moisture.

Mom and Dad enjoying their new foundation:

The completed foundation walls with partial backfill and the beginning of insulating to the interior side of the wall.

Without having a basement in the house there was definitely some concern about storage space which it why it was great to have completed the detached workshop building before construction began on the house. The workshop provided a perfect staging area for the contractor and ultimately the storage space that Mom and Dad needed. Not having a basement also meant that all the mechanical systems of the house would be on the same level as the living area, where space within the 1120 square foot footprint was at a premium. We managed to squeeze the boiler, water heater, radiant manifold, electric panel and solar inverter, washer, dryer and freezer into a 78 square foot utility room.