Wood Versus Concrete

205 Crichton Street - 15 Dufferin Road, All Green Projects, Build Projects, Design Projects, Plan Projects

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I have been a long time proponent of wood construction, especially in the Ottawa-Gatineau region.  Our history is so deeply rooted in the forestry industry that building with any other material could be considered a form of industrial-cultural sacrilege.

My new passive house project at 15 Dufferin Road – 205 Crichton Street will be built in two phases.  The first phase involves the construction of a new 2,000 square foot home on an infill lot within a heritage conservation district.  In the second phase a 1,200 square foot second floor addition will be added to an existing single story home built in 1945.  The project will be certified to the Passive House and LEED Platinum standards.

The most notable thing about the otherwise unremarkable piece of architecture found on the site is the fact that it was built with masonry when most other homes were (and still are) built with wood.  (The home’s walls are built using 10″ blocks below grade and 7.75″ block with 4″ brick above grade.)  This fact motivated me to consider alternatives to the new pre-manufactured wood wall assembly I had intended to use.

In the last several years I have designed high R-value walls based on a wood structure for other certified passive houses now built at 279 Crichton Street3336 County Road 3 and 5840 Red Castle Ridge.  Each building has abnormally thick walls with very high levels of thermal resistance and very low levels of permeability (which are necessities of an energy efficient home).  However, each building’s wall assembly is somewhat different than the others.  The variation between them is in:
the type and quantity of insulation(s) used;
how the space for the volume of insulation is created; and
what materials are used to make the assembly impermeable.

In the past I have only seriously considered concrete as an appropriate material for below grade applications.  All of the buildings mentioned above sit on foundations made (in three different ways) out of concrete and foam.  There are several factors that have made wood the standard for above grade construction and concrete the standard for below grade construction. Here is my outline of why residential designers and builders in Canada like to create buildings using wood instead of concrete:

Factor Wood Concrete
Cost +
Ecological Impact of Extraction +
Embodied Energy +
Thermal Conductivity +
Simplicity of Construction +
Adaptability +
Traditional Building Material in Ottawa-Gatineau Region +
Durability when dry + +
Durability when wet +
Sound Attenuation +
Flammability +

Although one might attribute different weighting to the factors I list here, more often than not, wood is my preferred building material (above grade in residential construction).  Notwithstanding my opinion of concrete versus wood, I chose to build my current passive house project using steel reinforced concrete insulated within expanded polyurethane forms (EPS).  Why? Well, I do believe in the adage: Don’t knock it until you’ve tried it.  Until now, I have never been involved in a project that uses concrete (poured on site, in blocks or in panels) from the footings to the roof.  (I have done quite a few basements using a variety of insulated concrete forms, also known as ICF.)  Projects that I plan, design and build for myself provide me with the opportunity to try things that I have not done before.  As a result, I am in a position to make recommendations to clients based on personal experience.

The most significant difficulty in designing and in constructing better built buildings in a northern climate are the challenges associated with air sealing the building envelope, especially where different materials intersect. The actual construction is made increasingly difficult as temperatures fall below freezing, since products used to seal joints become more difficult to apply effectively. These two points influenced my choice of building materials because my project was to be a winter-build.

I believe that the primary advantages to building high performance walls with insulated concrete forms in the winter are that:
the insulation (integral to the forms) extends the construction season – allows concrete to be poured in cold temperatures;
the wall is simple, containing only two materials – there are fewer places in the assembly that have the greatest potential to fail because of poor connections between materials; and,
the wall is monolithic and impermeable – it does not require tape, caulking, adhesive or additional membranes to become airtight.

(I will comment on the disadvantages of building with ICF in the winter once we have had the experience.)

This is the typical ICF wall that I have chosen to build at 15 Dufferin Road:

0.5″ gypsum (at 0.45 per inch = R0.23)
2.625″ expanded polystyrene (at 4.493 per inch = R11.794)
6″ concrete (at 0.5 per inch = R3)
with 10-M vertical steel bars at 16″ on centre
with 10-M horizontal steel bar at 36″on centre
6″ expanded polystyrene (at 4.493 per inch = R26.958)
2.625″ expanded polystyrene (at 4.493 per inch = R11.794)
2.5″ expanded polystyrene (at 4.0 per inch = R10)
2.5″ expanded polystyrene (at 4.0 per inch = R10)
0.625″ strapping
1.0″ wood siding
(The nominal R-value is: 73.776)

 

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