Choosing a building envelope
When it comes to choosing a building envelope there are many variables to consider: performance, durability, health, comfort, and of course, cost.
© Mike Reynolds
What is the best wall system?
I don't know - what is the best way to get from here to there? That depends on what are you looking for. You have to choose between the quickest way, the safest way, the most scenic way or which route has the most gas stations or least amount of hills. So what is the best wall system? That also depends on what you are looking for.
What is the most durable wall?
What is the most energy efficient wall?
What is the most soundproof wall?
What is the most affordable wall?
That is just a short list of things to consider, and there isn't one right answer. In order to find the right answer, you need to ask the right questions.
Your wall system is an environmental separator between outside and inside. Its intent is to slow the movement of heat and air between outside and in, and manage moisture infiltration from both sides.
There have been many changes over the previous few decades in building codes, building materials, and how we condition our homes. These changes mean homes are better insulated, more airtight and, most importantly, air conditioned. There is also much more understanding of how buildings work and why they fail.
If a home will be air conditioned in summer months, it is agreed on by building scientists that the worst thing you can do is to install an interior polyethylene vapour barrier. Since we build outside with wet materials, buildings start out wet. Over their service life they will continually get wet from the inside and the outside. So forget about trying to 'keep them dry' with a vapour barrier, that's impossible. A wall should be designed so in can dry properly, in both directions.
To build in a cold climate and respect the laws of physics, building scientists agree that a wall needs four major components to be truly successful, and in the following order of importance:
A water control layer:
To protect the exterior surfaces of your home from damage caused by precipitation and groundwater.
An air control layer:
To slow the movement of air through wall assemblies, preventing heat loss and moisture damage in walls.
A thermal control layer:
To slow the movement of heat from inside to out in cold months. This is your insulation. It also helps keep heat out in summer.
A vapour control layer:
Commonly known as a 'vapour barrier' but a better term to use would be a vapour retarder. A moisture control barrier should slow diffusion of moisture through walls without preventing them from properly drying.
Some materials within a wall assembly may serve more than one function, air barriers in particular. Many materials within a wall assembly can slow air flow without trapping vapour.
Dense-packed cellulose wall:
Dense-packed cellulose wall © Gergana Gatina, Ecohome
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 centers, 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, 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, and well-experienced ones.
The addition of a exterior EPS foam sheathing can reduce any risk of moisture damage, as it brings up the temperature of the exterior condensing surface where the cellulose is. This is an insurance policy that is highly recommended when building like this in more northern regions.
Conventional wood framing:
This is the most common method of home building, and one that people are most used to seeing.
Building codes have greater requirements for thermal performance as of 2012, it is now required in most areas that you break thermal bridges with some form of continuous insulation.
With that, performance has improved, but there is still the matter of an enormous quantity of wood that makes up the frame, so the stated R value of batt insulation does not reflect the performance of a full wall assembly.
Add up all the studs, and without a thermal break 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 batts, you can easily have small gaps. It takes as little as 1/8th of an inch space between batts and studs to start an air convection and increase the rate of heat loss.
Wood is a renewable resource, so it is very much a 'green' material choice. To improve the performance you can increase the size of studs (going from 2x6 to 2x8) or add several inches of insulation to the exterior.
Mineral wool batts have higher R value than fiberglass, and they are much easier to cut and install properly.
Prefab wall © Celine Lecompte
In years past Pre-fab homes were generally just 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.
Building with ICF blocks is great for reducing sound transmission and stopping the infiltration of air, insects and rodents.
The thermal performance of ICFs (R Value) is reasonable but not a high end performer unless blocks with thicker foam are used or additional insulation is added. The increased performance realized is though reduced air leakage, not the R value or thermal mass.
This is a great system that meets the additional challenges of building below grade.
In the interest of saving money and reducing greenhouse gas emissions, we recommend limiting its use above grade, as dollar for dollar you can get much better performance for your money with other wall systems.
That said, an ICF house built on rock could easily be standing for centuries or a millenium, so there is an argument to be made about its full lifecycle, which is great, so long as it is in a desirable location a few generations from now. That can be hard to predict.
Installation requires great care, the biggest risk comes from a ‘blowout’ where a wall section fails. This is not the ideal 'do it yourself' technique, we recommend getting pros.
SIPS- Structural Insulated Panels
© Céline Lecomte
Foam SIPS usually consist of a panel made of polystyrene or urethane sandwiched between two sheets of OSB (Oriented Strand Board).
While SIPS with a 5.5 inch foam core were considered a high performer less than a decade ago, increased thermal requirements of building codes and a greater demand for energy efficient homes has brought that wall system back to the category of average.
An excellent long term money saving investment would be to look at thicker SIPS, moving to a 2x8 or 2x10 frame instead of 2x6, offering you a chance at either 2 or 4 additional inches of foam. The relatively small increase in cost will offer a much greater energy performance.
They can be 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. The panel itself is airtight, if the joints are well sealed then the house can be as well.
We would recommend the inclusion of a flexible membrade over the joints to avoid problems with any movement and shifting.
See about getting windows and door openings cut at the factory, otherwise you will likely 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 afterwards.
There is a new player in the SIPS market in Eastern Ontario that is manufacturing prefabricated straw bale panels. All the benefits of straw bale construction are still there, but not the same risks and problems. So far we have found only one manufacturer that is doing this, so products are currently available only in Ontario, Western Quebec and some northern U.S. states.
The R.E.M.O.T.E wall
(Residential Exterior Membrane Outside-insulation TEchnique)
The R.E.M.O.T.E wall © Alain Hamel, Ecohome
Designed for extreme northern climates, the REMOTE wall was developed by the CCHRC (Cold Climate Housing Research Center) In Fairbanks, Alaska. Walls are framed and sheathed, with the inclusion of a stick-and-peel exterior membrane to control air and moisture movement, and it does so exceptionally well.
With the inclusion of cavity insulation, the vapour barrier in a REMOTE wall is nicely sandwiched about 1/3 of the way through the wall, exactly where it should be. This type of construction is highly praised by building scientists as being among the most durable.
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.
Log homes: Log home builders speak of the ‘thermal mass’ of wood keeping you warm, but this myth has been dispelled by software energy modelling. They are also highly prone to air-infiltration du to 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 less than R10. 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.