In green building circles it is generally acknowledged that prefab or modular homes are more environmentally friendly than traditional stick built on site homes. Walls are constructed inside a warehouse so the materials aren’t exposed to the elements, which helps keep the materials dry; there is less material waste because excess materials from one job can be used on the next one; There is less waste on site because walls were constructed elsewhere and homes go up faster because they are already partially built.
Straw bale houses are also becoming increasing popular because of their properties of low embodied energy (straw is the waste product from wheat, the “chaff”), combined with excellent insulating properties with R-values ranging anywhere from R-20 to R-50, depending on the thickness of the wall. They are strong, durable, and the majority of the material is biodegradable at end of life.
So now imagine a building made of prefab straw bale houses. These houses combine the insulating and durability advantages of straw bale, with those of prefab, producing less waste and being built in a faster period of time. Here in Canada, staw bale SIPs (structure insulated panels) are being produced by the team at NatureBuilt Walls. The walls were the brain child of Chris Magwood, who runs the Endeavour Centre in Peterborough, and Ian Weir, who’d taken the green building course at Fleming College. They are now joined by Neeraj Jain, and Ryan McLaughlin, who bring additional specialized strengths to the company.
I had a lot of questions about the walls, so I contacted Neeraj and we talked about these SIP walls and all of their properties. (Note that the interview has been condensed and I am paraphrasing Neeraj’s answers):
1. What are the SIPs made of?
The SIPs are made of straw bales, FSC wood for the frame and are covered with an inch of concrete on either side.
2. What is the expected lifespan of the SIPs?
There are straw bale houses in England and the US that are still standing today that are well over 100 years old, so I’d say our homes will also last over 100 years.
3. What is R-value of a Nature Built SIP wall?
We are in the middle of testing the exact R-value of our walls through a research project being carried out at Queen’s University, but we estimate that our R value will be between 35-40.
4. Can they be used for roofing as well?
No, they are too heavy. One 8′x8′ panel weighs about a ton.
5. Given that it’s an organic product, how do you protect against mould, rot and pests?
The straw bales are packed very densely, it’s like having a wall of solid wood, so there are no pockets or cracks for animals to infiltrate. Further, the straw itself has no nutrition value, so pests would not recognize it as a food source.
With respect to water infiltration, like any wood-based product, it’s how you well you seal the straw to protect it from water. Our walls are completely covered with an inch of cement on either side, so it would be hard for water to find a pathway. But, even if water did infiltrate, because the walls are breathable, there is a way for them to dry out so mould and rot won’t start.
6. Is the straw used for the SIPs waste straw or is it grown for this purpose?
No, the straw used is the leftover stalks from wheat production. Straw is primarily used as bedding materials for animals, so it is generally considered a waste material. Also, by putting it inside walls, it is another way to capture and store carbon.
7. Which type of windows and doors (brand and/or material) are best to use with your walls? Or does it matter?
It doesn’t matter which type of windows and doors are used. After quite a bit of experimenting we now design our panels around the openings, instead of cutting openings within the panels themselves. So, for instance, we will make two panels that will stop four feet apart, and the opening will be where the door will go. To fill the area above the door frame, we can do a variety of things such as add straw bales on site, or use different insulations such as Roxul mineral wool. There are a lot of options. We also build long narrow panels that can be used along the bottom of the house, then windows can be built in resting on top of the panel.
8. Does the wall provide any type of thermal mass for the building?
Absolutely. As I mentioned, these walls are incredibly heavy. They are also about 16″ thick when cement is applied to both sides, so there is a lot of thermal mass there to help regulate a building’s internal temperature.
9. Can you custom spec the walls or do they come in just one size? What is the largest wall that can be specified? Can you stack them for two stories?
Right now our only consistent requirements are height and depth. Walls are usually 8′ or 9.5′ high and the depth is always 16″. Lengths can vary depending on the design, however, because the walls are transported to the site on a flatbed truck, we like to limit the wall length to about 10′. Our residential designs can be up to three stories high. To build higher structures, the skeleton needs to be built in steel.
10. How thick is the final wall when it’s completed?
The wall is 16″ thick which consists of an inch of concrete on either side of an interior filling of 14″ thick straw bales.
11. How do you install wiring and plumbing behind the wall or is it best done through interior walls?
We don’t install plumbing on exterior walls, but we can make conduits for electrical wiring on the exterior walls. While it’s easier when we have the electrical plans when we’re building the walls, we can also add the conduits for the wires after the walls have been constructed.
12. Do you plaster directly over the walls, or do you apply studs and drywall?
The wall is as it is. It can be finished further, if desired, but it’s not necessary. The trick with these walls, especially on the exterior is covering the seams. One of my favourite examples of how this was done well, was our project in Red Rock, Ontario, near Thunder Bay. The architect who designed the building incorporated some architectural trim that was simple, cost effective and also covered the seams.
On the inside of the house it’s not usually much of an issue because the seams tend to line up where the rooms divide. Even if the seams are exposed, as in one of our projects, drywall can be applied directly onto the cement using drywall mud. No studs are necessary.
13. What kind of paint can be used on a Nature Built wall, which I assume is a breathable wall.
A silicate based mineral paint needs to be used on these walls particularly on the outside. Our advisor, Chris Magwood, figures it’s not too important to keep the walls breathable on the interior of the house, but it’s essential to stop mould growth and rot, to keep the building breathable on the outside in order for it to dry out. We can actually apply the paint in the factory so it’s already done by the time it gets to the site.
14. Can you please define a “breathable” wall for our readers?
A breathable wall is one that has no vapour barrier. Let’s say that water is traveling down a wall, even without the actual water infiltrating the wall, the water vapour can. If there is no way for the water to get out, it will eventually produce mould. A breathable wall such as ours, allows for water vapour to evaporate and keep the interior walls dry.