Choosing a Building Envelope
When it comes to choosing a building envelope there are many variables to consider: R value, thermal bridging, air infiltration, embodied energy, fire resistance, toxicity, insects and rodents, and of course cost. The following should help you make sense of it all.
© Mike Reynolds
To be clear up front, we have a favourite: the double-stud framed wall, very efficient and environmentally friendly for a number of reasons.
Double stud framed wall:
This is a wall of offset 2x4s (interior then exterior) with a thick cavity filled with cellulose insulation. It is recommended to have studs in each wall at 24 inch centres, so when offset you have a stud every 12 inches. The thickness is not fixed, and is completely up to you.
This wall system uses less wood than conventional framing and when filled with cellulose is non-toxic, has low air-infiltration, is resistant to insects and rodents, can be very well insulated (12 inches or more) with no thermal bridging.
Keep in mind how important it is that the cellulose be well packed to avoid settling and leaving you with empty pockets at the top of the walls. For this reason we recommend having it installed by pros.
Conventional wood framing:
This is the most common, but not a particularly efficient way to build a home. Add up all the studs, and you will have a significant portion of your wall that is nothing but solid wood, with an R value of approximately 6 (R1 per inch).
On top of that, unless great care is taken while installing insulation, you can easily have little gaps, which can actually start an air convection and increase the rate of heat loss. Though the batts themselves are R20, when you factor in all the studs your overall average is more like R13.
If you are going to go this route, be sure to see our sheets on advanced carpentry, there are tricks for reducing the amount of wood needed to frame walls, leaving more space for insulation. Another way to save on material cost and waste is to have your wood pre-cut to length in the factory.
To beef up your R value, you can also add some foam. 2 inches of rigid insulation on the outside stops thermal bridging, reduces air infiltration, and gives you another R 13 or so. It’s an easy install, being held on with strapping which you will need regardless for your finished siding.
Roxul has recently released a product called ‘comfort board’ which is a rock wool panel that may be an effective replacement for foam. Foam is a great insulator, but it is a petroleum product. Rock wool on the other hand is natural, and almost entirely recycled post industrial waste.
Another material saver would be skipping the sheathing layer all together, using metal cross bracing on the interior (a V shaped metal band that slips nicely into a skill saw cut that you then nail to each stud) and attaching your exterior foam directly to the studs.
This will save you a lot of material, and a lot of money. The downside of this technique is that sheathing acts as a noise barrier, so the absence of it might be noticeable if you are building in a high traffic area.
© Céline Lecomte
In years past Pre-fab homes were generally 2x6 framing, and weren't known for high performance, but rather affordability and ease.
In production however they've always had the ability to reduce waste due to the controlled setting of a factory and the abiity to have all cuts are calibrated.
The walls usually come insulated, so they must be kept well protected from rain and snow, or you risk ruining your insulation. The advantages are speed of installation, and a factory precision that is hard to match onsite.
Building codes and a general trend towards smarter building practices has seen some serious leaps forward in the quality offered by pre-fab manufacturers, including high R-value walls and breaking thermal bridging.
If you are considering this route, there are pre-fab walls that meet LEED standards, and even turn-key prefab homes that have successfully achieved LEED Platinum certification. Pre-fab walls can seriously speed up the building process, and reduce waste as well. Simply find a quality manufacturer, and if you send them your plans, they send you completed wall sections including all your window and door openings.
ICF - Insulated Concrete Forms
© Creative commons
This is a system of polystyrene blocks that are stacked and filled with concrete to form the entire wall system of your home. ICF homes are fairly well insulated, reduce sound, stop infiltration of air, insects and rodents, and have the benefit of thermal mass to help retain heat.
But if you remember the stats on green house gases from the production of concrete (one ton GHG per ton of concrete), your carbon footprint with a home like this will be about as high as it gets.
Installation requires great care, the biggest risk comes from a ‘blowout’ where a wall section fails. So if you’re doing it yourself, be ready with ply wood and bracing to plug any holes.
SIPS- Structural Insulated Panels
© Céline Lecomte
SIPS consist of a panel made of polystyrene or urethane sandwiched between two sheets of OSB (oriented strand board). They make for a pretty quick install, are very well insulated, and quite airtight if installed correctly.
It is important to seal the joints between panels very well, and pay careful attention to sealing the joint where the wall meets the roofline. Keep in mind this is still a petroleum based product, but significantly outperforms conventional 2x6 framing. Other things to watch for are condensation at joints, so flexible sealers are recommended to avoid problems with any movement and shifting.
See about getting windows and door openings cut at the factory, otherwise you will be using a chain saw on site and will treat your neighbourhood and local environment to a blizzard of little foam balls that you will be finding for years.
There are many other methods of wall construction, what we have covered here are the more common and cost effective techniques. There are a variety of natural fibres that are used for home construction and insulation.
Hempcrete: a mix of concrete and hemp fibres. It offers great sound proofing, not the greatest R value. It's labour intensive during installation, but an absolutely beautiful textured finish. Hemp wall panels are just finding their way onto the market, and will likely offer significantly reduced labour costs.
Straw bale walls: see our page on insulation
Log homes: Log home builders speak of the ‘thermal mass’ of wood keeping you warm, but there is little to support that. They are also highly prone to air-infiltration from cracks, also known as ‘checking’ and use enormous amounts of wood. Wood has an R-value of somewhere just over 1 per inch, so for an 8 inch wall you are looking at around R10 at best. So as pretty as they may be, efficient they are not.
Food for thought:
Whatever wall system you choose, when it comes to building floors, the lowest environmental impact is with engineered wood (floor trusses, I-beams, jointed wood). This allows for the use of smaller trees and factory scraps that would often have just been landfill.
Engineered trusses and joists will cost you more, but they install quickly and accurately with no shimming, compared to solid floor joists which can vary as much as ¼ inch in height. They also facilitate longer spans, allowing for a bit more versatility in floor plan designs.
Like many green building practices, the extra cost upfront for engineered joists will bring you a net savings in the end with time and labour. Engineered floor joists are like roof trusses, a web of 2x4’s. Consequently running plumbing and wiring through those existing openings is a lot quicker than drilling hundreds of holes, so even just financially speaking it makes sense.