What Kind of Home Suits the Prairie Climate?

Building Details Suited to this Region

Homes built on Canada's Prairies need to be able to offer comfortable and healthy indoor space during cold winters and hot summers. Homeowners appreciate low heating and cooling costs as these costs have to be paid month after month for the life span of the home, and energy prices tend to head upward over time. Regardless of the appearance of the home and the finishes chosen, we can reduce the critical elements of an energy-efficient home to the following 7 aspects that need to be included:

      1. good quality doors and windows - excellent seals, triple glazed (DETAILS)
      2. thick walls with dense insulation that resists air infiltration and circulation (DETAILS)
      3. thermal mass such as a concrete slab for the main floor (DETAILS)
      4. air-lock entries - double-doored entry areas to discourage the in-flow of large amounts of outdoor air (DETAILS)
      5. an elevated exit point for warm air - typically an operable window located high up in the structure (DETAILS)
      6. a good quality and efficient air exchanger with appropriate sensor and controls (DETAILS)
      7. sensible orientation of windows with consideration for solar gain, natural lighting, and prevailing winds (DETAILS)

The Importance of Door and Window Quality

The easiest home to heat and cool would have no windows or doors - no holes in the wall system for cold air to seep into or for sunlight to enter and over-heat the interior. Of course, that wouldn't be a home anybody would want to live in. Oddly enough, the thing that gives character to a wall are the holes we put in it. And there are many reasons why windows are needed in every room - the ability to see outside, natural lighting, ventilation, emergency escape andwindow types so-on.

Windows and doors that leak air around their edges will undermine all the careful work of insulating and sealing foundation, wall, and roof systems. Weather-stripping and edge seals have to effectively hold out heavy cold air in the winter season, especially when conditions are windy. Opening systems with hinges tend to the best type to maintain edge seals. Doors operate on hinges, but various types of windows operate as sliding units - so-called 'single hung' windows and 'sliders' - and these types of windows are notoriously hard to seal. All types of sliding windows leak much more than a hinged unit of the same size. Casement windows (hinged along one side), awning windows (hinged along the top edge) and the opposite of awnings -hopper windows- (hinged along the bottom edge) are all acceptable window types for energy-efficient homes, and, of course non-operable windows (fixed glass units).

Beyond the operating style and edge seals, the number of layers of glass is important to the insulating value of a window. Dual glazed or double glazed windows have 2 layers of glass and thus, one air space between the layers. This air space, usually between 3/8 and 5/8's of an inch thick, is what provides the insulation for the window. Tri-pane or triple glazed windows have 3 layers of glass and 2 air spaces, and insulate considerably better than double glazed units. Some designers will specify double glazing for windows on the south wall of a passive solar home, and triple glazed for all other walls, simply to save on the window cost. For most homes built in an urban setting on the Prairies, solar heating on south-facing windows may or may not be relevant - few urban lots offer an open to any one direction. As a general practise, Advanced Design/Build uses triple glazed windows in all locations.

Certain windows benefit from special gases injected into the air space (Argon, for instance), and careful builders will also consider special glass coatings that aid in the overall performance of the window in a specific location. Air space gas treatments and glass coatings are still evolving and changing year to year, so builders rely on the window supplier to suggest the best current choice for new homes.

Bear in mind that, while a good wall system may be rated at R40 or higher, even the best window available will not likely be more than R8, and that is what the manufacturer reports as the rating at the centre of the glass. In actual use, the edges of the window (the frame) will have a much lower R value because it is more or less solid material, so it conducts cold quite well. If you place too many windows in a wall, and especially if those windows are large, the value of the tight, well insulated wall is undermined by the low R value of the windows. You can have a heavily insulated attic and a thick wall system, but too many or too large or poor quality windows and doors will drive up heating and cooling costs.


Thick Walls with Dense Insulation

Wood frame construction in Canada used vertical members of 2" by 4" softwood from its inception in the 1800's until about 1970. Residences of up to 3 stories were built with this size of lumber. That is, a wall framed with 2x4's could support 3 stories of construction. As lumber processing changed and engineers improved fasteners and bracing techniques, we evolved to a system of using 2x4's that were actually 1½" by 3½", typically spaced 16" apart or 24" apart to make up wall structures. As a result of the energy crisis of the 1970's, builders became interested in creating thicker walls in order to increase the amount of insulation that could be placed between the vertical wooden members (studs). The simplest method of doing this was to simply use 2x6 studs and have 5½" of insulation space instead of 3½". This practise is still widely used today, and many builders promote 2x6 framing as a good energy-efficiency choice for new home buyers.

While the switch to 2x6 studs did indeed allow for about 1.6 times the amount of insulation, it was a crude and wasteful method to accomplish warmer walls. The folly of the 2x6 framing method is that it is using about 1½ times the amount of lumber needed structurally. Recall that 2x4 walls allow for a 3 storey house, structurally. Using 2x6 for single and two-storey homes is a waste of lumber. Also, the concept of 'thermal bridging' is not really addressed by 2x6 studs in favour of 2x4 ones. Thermal bridging refers to the ability of lumber to conduct heat from the inside of the home to the outside surface or, conversely, to conduct cold outside temperatures into the warm interior wall surface. Lumber is the 'zero point' on the scale of insulators and conductors. Wood is not considered an insulator, nor a conductor - it occupies the dividing line between materials that insulate and materials that conduct. Clearly, building a warmer wall will require a little bit more sophisticated solution than simply using the next biggest lumber size.section of truss

thick wallBy the early '80's, serious students of energy-efficient housing had abandoned 2x6 framing and moved on to other systems that made smarter use of lumber, reduced the amount of heat transferred directly through the lumber, and allowed for wall thickness of 12" or more. A builder named John Larsen, working in northern Alberta, came up with the concept of having 2 vertical members separated by a sheet material. The original Larsen Truss used a 2x2 as the inner member and a web of plywood reaching out to a 2x2 outer member. |A typical use for Larsen trusses was to thicken an existing sheathed wall, so the 2x2 assembly was not load-bearing. Altering the size of the plywood web could add 6 or more inches of thickness but reduce the amount of thermal bridging - there was less material reaching out to the cold exterior surface of the wall. In the mid '80's Sunergy Systems Ltd, one of Alberta's leading designers of passive solar and energy-efficient homes further refined Larsen's concept. A 2x4 was used as the structural member, but intermittent webs of plywood or particle board are used to span out to a 2x2 exterior member. This innovation significantly reduced the thermal bridging, required less material, and simplified assembly of the wall trusses.

Once a thicker wall is achieved, it is important to insulate it properly. Again, we find that current practice by the majority of home builders is not going to result in significantly warmer walls. Fibreglass insulation is the default wall insulation material - typically fitted between the studs. Unfortunately, fibreglass insulation rarely performs as well as its makers would have you believe. Think about where fibreglass material is normally used - furnace filters, air filters of various types, and so-on. Fibreglass is good at letting air move through it, and this is not what good insulation does. A good insulation material will stop, or virtually stop, air flow through it and will be composed of a material that conducts heat poorly. Fibreglass insulation is basically spun sand - a material that conducts heat rather well. Beyond the physical limitations of fibreglass, one has to address the question of installation: only if each batt is carefully cut and fitted properly to completely fill each stud space will it perform anywhere near its claimed R value. A good fit might happen in stud spaces that are exactly centred on 16 or 24", but many stud spaces are not full size, and cellulose into wallthere are electrical wires and boxes that have to be fitted around. Only the most diligent and patient builder can properly fit fibreglass insulation, but the reality is that insulation is often a semi-skilled sub-trade given to the lowest bidder. The homeowner cannot inspect the work, and its quality won't be known until the first heating season is over.

The best environmental choice for insulating thicker wall systems is cellulose insulation. This material is essentially recycled paper that has been mixed with Borax as a fire retardant. Being wood fibre, it is a better insulator than fibreglass in terms of its physical properties, but the superiority of its use goes well beyond that. Cellulose is blown into the wall cavity and packed to a specific density that impedes air flow. As a loose material, it fills gaps around obstructions very well and completely fills the stud space. The net result is a high insulation value. Further, lumber walls insulated properly with cellulose burn very poorly. An accidental fire in a cellulose-filled wall will smolder, but it will not ignite and spread in the rapid manner associated with a stud wall insulated with fibreglass.


Thermal Mass and Slab Floors

A concrete slab main floor is an excellent choice for an energy-efficient home for several reasons. Firstly, it adds a large amount of 'thermal mass' to the house which has the effect of moderating temperature swings, both in a day-to-day sense, and a seasonal sense. A concrete slab holds a tremendous amount of heat, so it gives up heat if the house is cooling off or takes up heat if the house is heating up. In practical terms, this means that the slab will carry the indoor temperature during a period of time when the heating system is off (either as part of a normal cycle or during a power failure). If the slab also has in-floor heating, for which it is well suited, then the overall effect of the heating system is to provide a very stable and even heat, day to day. Seasonally, it has been found that a slab floor has a cooling effect in the warm summer months. A well-insulated home with good windows and a way to exhaust warm air in the upper levels of the building will probably not require any sort of air conditioner or summer cooling equipment.

The commonly used term, 'slab on grade' doesn't really describe what is needed for a warm slab floor in a residence. Better systems will include piles reaching well below the frost line in clay soils to provide stable support for the slab. And the slab is not sitting on grade (on the ground). A stable foundation system results from deep piles supporting a slab that is separated from the ground by foam insulation and also has an expansion allowance between the foam and the earth. Prairie clay soils will expand or contract depending on temperature (swells when it freezes) and moisture content (shrinks when it dries out). In no way can the movement of the clay soil in the first several feet under and beside the home be allowed to move the concrete slab.

The other requirement for a slab acting as a residential main floor is that the perimeter be heavily insulated with foam to minimize thermal bridging of cold outside air into the slab system. Insulating the edge of a slab fits well with truss wall systems which have their bearing member located well to the inside of the truss assembly.


Air-Lock Entries

An air-lock entry is simply the provision of small entrance room at each door to the exterior. This arrangement prevents the entry of large amounts of cold outside air (or warm outside air in the summer months) when the doors are operated, and provides a secondary barrier to whatever air leaks through the door system when closed. For instance, a room of 6 feet by 8 feet is built around the main entrance door and a pair of French doors lead from this air-lock area into the home. Although the interior French doors are not insulated or especially well sealed, the net effect is to reduce the infiltration of outside air into the main living space.

If both entrance areas are shielded with air locks, it makes a significant difference to the comfort level of the home. The use of high quality fibreglass or wood insulated doors rather than the more common steel doors is also helpful.


high windowAn Elevated Exit Point for Warm Air

Combined with a concrete main floor slab and good insulation, the provision of an operable window located high up in the structure tends to assist with summer cooling and the avoidance of the need for air conditioning equipment. This is a building detail borrowed from the Middle Eastern countries. One of their strategies for dealing with high daytime temperatures experienced in mostly earth wall or masonry buildings is to build a chimney-like structure that pulls air from the lower levels and exhausts them near the peak of the building's roof structure. This takes advantage of the natural draw of a vertical flue and has the effect of cooling the interior of the building. A more thorough discussion of this technique can be found in Hassan Fathey's book, Architecture for the Poor, (University of Chicago Press, 1973). The wind catch or, 'malkaf' in Egyptian, is actually a larger concept that addresses the extreme heat experienced in that country, but the simpler flue effect of a window or outlet near the roofline is sufficient for the typical heat of a Prairie summer in Canada.


Air Exchanger with Appropriate Controls

Air exchangers, or the more commonly named heat exchangers, control the introduction of fresh air from the exterior and the exhaust of stale indoor air. As building technology has advanced to allow the construction of very well sealed homes ('tight' homes) it has become essential to ensure that a certain amount of fresh outdoor air is introduced into the home to maintain healthy indoor air quality. This activity is generally measured in terms of 'air changes per hour' and is simple enough to comprehend - how many times per hour is the indoor air let out and replaced with fresh air. In earlier days when house construction was quite drafty and coal was inexpensive, a home may have 3 or 4 air changes per hour. A lot of energy left the house with each air change, so the heating system was running hot enough to heat all that new outside air that was leaking in. Even homes built into the 1970's were drafty enough to not require forced ventilation to maintain good indoor air quality. But with the use of vapour barriers, better doors and windows, and more heavily insulated and well-sealed walls, it has become important to include an air exchanger.

The ideal, reliable, compact and noiseless heat exchanger has not yet been built - as a house component, they are not yet especially well developed, nor priced at a level that every builder is eager to include a good quality unit big enough for the demands of a large house. However, a few brands are making headway on the quality and reliability side of the problem.

Heat exchangers for cold climate applications are designed to reclaim a portion of the heat in the outgoing warm indoor air and use it to pre-heat the incoming cold air, thus reducing the load on the home's heating system. Most exchangers accomplish this by using very thin foil passages to allow heat to transfer from the warm to the cold. Homeowners are often led to believe that the better heat exchangers can achieve efficiencies of 80% or more. This is somewhat deceptive: if the heat exchange device were perfect (had an efficiency of 100) it would only be able to capture 50% of the heat in the outgoing air. As the warm air looses heat to the cold air and the cold air becomes warmer, one can imagine that the two air masses are approaching a temperature half way between the cold and the warm. If they reach that mid-point in temperature, then there is no further exchange of energy: the outgoing warm air will not continue to heat the incoming cold air back up to the original temperature of the indoor air - physics will not allow that. Therefore, a heat exchanger that advertises itself as 80% efficient should actually by labelled as 40% efficient. It can reclaim 80% of half of the heat in the outgoing air, or 40%.

One important issue with heat exchangers for cold climates is the nature of the control systems that turn the exchanger on and off. There is no need for the heat exchanger to run continuously - that would result in a needlessly high number of air changes per hour. A lot of energy would be wasted by introducing much more outdoor air than necessary to ensure good indoor air quality. The question becomes: on what basis does the control decide to operate the heat exchanger? Most current heat exchangers offer at least two options. One choice is to allow the controller to monitor indoor air humidity and to begin exhausting air when the humidity reaches the level chosen by the homeowner. The device might include the instruction that one should watch for moisture to show up near the bottom of windows and then adjust the controller until the moisture no longer appears. This is an attempt to maintain indoor humidity at the highest possible level, which is a desirable situation for most people - it is well known that relative humidity near 30% is comfortable during winter months.

A second choice offered on many controllers is a set schedule, for instance, running the exchanger 15 minutes out of every hour, plus one defrost cycle every 24 hours. It is often difficult to choose between the limited program offerings, and the procedure for setting and choosing options can be counter-intuitive at best, but some people prefer this set schedule option. Clearly, whether humidity is being monitored or a timed schedule is being followed, the objective is to run the heat exchanger just enough to achieve good indoor air quality and no more. Any extra run time is just dumping good warm indoor air outside and wasting energy.

Some builders and ventilating tradesmen within the passive solar community are experimenting with sensors that monitor the carbon dioxide level of the indoor air and operate the heat exchanger when the level rises to a pre-set level. This may prove to be a better control method that more closely attends to the bodies need for the proper mix of carbon dioxide in the air we breathe. Such a control regime would also automatically adjust itself to the number of occupants in the home, increasing the air changes per hour when many people were in the house and reducing it when only one person was home.


Orientation of Windows

The same basic rules for locating and orienting windows apply to an energy-efficient home as to any other home, but the consequences for errors can be greater. East windows are desirable for bedrooms to catch the morning sun, north windows are favoured by visual artists for studio lighting, south facing windows tend to add heat to the interior space (summer and winter), and west-facing windows add heat in the late afternoon. However, because the interior space is so well insulated, overly large windows to the east and north may compromise the low energy input otherwise needed to heat the interior. As well, south- and west-facing windows can overheat the interior at certain times of the day. The use of a concrete slab floor will moderate some of the over-heating situation and turn it into a net benefit during the heating season.

The relatively new science of glass coating can help to alleviate some orientation problems. Certain coatings can improve the amount of solar energy accepted by the window and other coatings can reject a portion of the solar gain. Coatings can also increase the amount of heat that is bounced back into the room rather than passing through the glass to the outside.

An unfortunate consequence of urban building lots is that windows must often be positioned to suit the street, windows on neighbouring homes, and the back yard -there simply isn't any opportunity to address which direction a window faces in terms of solar heating or other factors.



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