The Framed House

Walls
Roof and attic
Envelope below grade

Most Edmonton detached homes are stick framed, the dominant method of detached home construction in the USA and Canada.  Framed construction was developed in the USA, probably the mid west in the early to mid 1800’s.  Framed home construction design principles have not changed in over 100 years, what has changed are materials used for framing, at first mostly solid wood, over time replaced with manufactured wood products and synthetics.

Framing uses wood or wood products manufactured or milled to standard sizes.  Framed walls floors and ceilings, nailed and sheathed with readily available mass produced materials enables the rapid construction of small to medium sized buildings.  Doors, windows, cabinetry etc., are mass produced to fit the standard dimensions of framing materials.  Framing is well suited for cold climates.  Framed exterior walls, floors and ceilings over or under unheated space are mostly hollow, providing spaces that can be filled with insulation.

If wood is kept dry it can last for thousands of years, on the other hand, wood with a moisture content of higher than 20 percent rots rather quickly.  In Alberta, a log house, with the bottom logs in contact with the soil could only be expected to last about 40 years.  By placing a house made from wood on a dry foundation above the soil, and ensuring that the wood is kept dry, a house can last as long as it is needed to last.

The wall

Wood framing is kept dry by siding placed over sheathing, fastened to the frame.  The sheathing may also help prevent the frame from racking, ie,  the top of the framing shifting laterally.  The hollows between the framing can be filled with insulation if the building is to be heated.  Occupied heated spaces are finished with a wall covering, plaster originally, since replaced with drywall.

The typical Edmonton house exterior cladding or siding for at least the past 100 years was either overlapped, wood, later metal or vinyl siding, or stucco, a cement based coating that completely covers exterior walls.  Less common but also used was brick veneer, where the exterior of  a framed house was covered with bricks.   The siding gives the house its appearance, and is subject to prevailing fashion and cost, but its function is  protection, mostly from weather, but also within the limits of the materials used, fire and impact.

As most sidings will allow some water to penetrate, behind the siding is a weather resistant barrier placed between the siding and sheathing, at first water proof paper, and more recently, house wrap, a synthetic fabric that prevents liquid water from penetrating, but allows air containing water vapor to pass through it.

Exterior sheathing was originally wood planks, later replaced with sheets of plywood, more recently sheathing is made from oriented strand board (OSB).  Except for some types of exterior grade plywood treated with wood preservatives, sheathing needs to be kept dry as it will be damaged by prolonged exposure to moisture.  Interior walls were originally finished with plaster, a cement based product applied over thin strips wood nailed to the framing.  Plaster has to be applied manually with a trowel, it has since been replaced with drywall, which comes in sheets that can be fastened to the framing,  only the seam between sheets and the heads of fasteners need to be finished,

Framed walls sheathed on both sides are mostly hollow, hollow walls provide slightly more insulation than solid walls, but filling the hollows with insulation provides much higher level of resistance to heat loss or gain, and the ability to maintain a comfortable temperature.   Warm air is able to hold more moisture than cold air.  The more air is cooled, the less moisture it can hold, when it reaches the dew point (100% humidity), the moisture it was able to hold at warmer temperatures is released as condensation.  If condensation occurs inside a wall containing materials that can be damaged by moisture, it can be assumed that nothing good will happen.  For this reason, indoor air needs to be prevented from entering an exterior wall.  In new homes in Edmonton this is typically done by fastening and carefully sealing a fairly thick sheet of plastic between the framing and the drywall.

The air barrier, or what used to be called the vapor barrier should be placed to separate warm air from cold, in climates where air conditioners are used more often than furnaces, the air barrier is placed on the exterior side of the insulation.

A plaster wall has no seams, plaster was usually at least 3/4 of an inch, or 21 mm thick, when painted with a high quality oil paint (which probably also contained lead), the combination was a fairly effective vapor barrier. When drywall replaced plaster, a lighter gage plastic sheet called a vapor barrier was placed behind the drywall and less care was taken to seal it, sheets were lapped without taping or sealant, and penetrations such as electrical outlets were not sealed with molded plastic covers taped and sealed into the air barrier.

Less than perfect air or vapor barriers that allow some indoor air to enter the wall is dealt with by allowing drying by outside air, which is free enough to enter through siding, sheathing and building paper and dry any moisture inside the wall. Problems will occur if the exterior is sealed so that drying is not possible. In most cases the amount of air able to pass through a wall enclosure from the outside is sufficient for drying, older homes leaked on both sides, however, sometimes homes are ‘upgraded’ on the exterior, or by adding insulation to attic spaces that can interfere with drying by outside air.

New homes require a much tighter air barrier than was mandated by building code or best practice in the past, but even so if warm moist air does leak past the air barrier, and gets inside the wall and condensation forms, there has to be a way that the interior can dry out.  Using plastic sheets which are easily cut or penetrated may not be the best way to achieve this, but remains the dominant choice of home builders. Some commercial building or multifamily building designs are using alternative methods to control air movement through exterior walls, such as placing insulation and air barrier assemblies on the outside of the building and vapor barrier paint on the inside.

Windows and doors

Windows and doors are necessary penetrations of the building envelope. Needed to provide a way to get in or out, bring in daylight, ventilation during warm weather, windows and doors are also breaks in the building envelope and potential leak points.

A primary factor of insulation effectiveness is thickness, insulation twice as thick is twice as good an insulator all other things being equal. Windows and doors are generally much thinner than the walls they are installed in, so also a greater source of heat loss.

Windows let in visible light, and may also let in invisible solar heat rays, or prevent heat rays generated indoors from escaping. Windows that can be opened may not be as air tight as windows that can not be opened.

Doors, being thinner than walls, are not as well insulated as walls even if they are insulated with materials with higher resistance to heat loss than walls simply because they are substantially thinner. Doors and windows that can opened may not be as air tight as the rest of the building envelope because they need more clearance for ease of use.

Windows are made of glass, glass is not a very good insulating material, and a pane of glass is only a few millimeters thick. However, glass is air tight, and air can be a very good insulator, so using two or more panes of glass to enclose air between is how windows can be insulated.

Well built Edmonton framed homes have always used two or more panes of glass enclosing an air space to keep windows warmer in winter. Older homes used wood framed windows with removable storm windows, which had to be removed and replaced with insect screens in warm weather. Later sliding double windows using aluminum framed panes that could be slid open and closed instead of removed or replaced with the seasons were used. For about the last 30 years new homes and replacement windows for older homes use double and triple glazing, two or more glass panes sealed together as a single unit. When a heavy gas is used instead of atmospheric air the insulating property of the air space can be improved. However even the best combination of triple glazing containing heavy gas is still no better than half the insulation value of a two by four batt insulated exterior wall.

Double glazed fixed windows that could not be opened first appeared in the 1950’s, as the picture window that every modern 1950’s living room needed to have. Later, double glazed glass in wood frames became a more expensive option to double windows. Today double and triple glazed vinyl windows are practically the only residential window used for Edmonton homes.

Windows and doors are ordered to size and made in a glass shop or factory. Window and door openings are rough framed slightly larger than the size of the manufactured window or door complete with frame so that they can be installed and leveled, which also means there will be gaps around the frames that could leak. The newer the home the more attention would have been given to ensuring gaps between the framing and window or door assemblies are sealed and insulated. New home vinyl windows come with fins that will be used to seal the windows into the weather barrier, gaps are filled with expanding foam insulation.

The ability of glass to either prevent or enable radiant solar heat from entering or escaping a building can be modified with coatings. Low emission coatings are more about preventing solar heat gain during warm weather, making cooler easier, however, there are coatings that will also help retain radiant heat inside the building enclosure, which ordinary uncoated glass will do as well.

Exterior doors are typically no more than 50 mm or two inches thick, limiting a door’s ability to insulate. Doors can be solid wood, or some combination of wood, metal and insulation. Exterior Doors also need to be able withstand a certain amount of impact damage. Doors are also made off site complete with frames, and have to be fitted and gap sealed like windows. In order open and close easily in all weathers, doors need clearances that can leak air, weather stripping is used to slow air leaking between door and frame, there will be a trade off between effectiveness of the air seal and ease of use.

Roof and attic

Ventilated attic

The typical Edmonton detached home has a pitched roof with shingles. This type of roof is designed to shed water, a pitched roof with shingles is not water tight. By far the most common type of shingles used in Edmonton are asphalt composition shingles. Also fairly common in higher end houses are softwood shingles, and shakes. There are also some flat roofs, and some some home owners have replaced shingles with metal roofs but these are comparatively rare. Other than metal shingles, shingles wear, life spans, can range from fifteen to fifty years or more depending on the type of shingle, exposure to sunlight and wind, roof pitch, etc..

Shingle life may also be affected by the design and condition of the attic. The usual Edmonton attic is unheated and ventilated, not suitable for living space, a hatch is provided for inspection. The unheated attic is generally ventilated to the exterior through vents in the soffits (the under side of eaves) and ventilators near the ridge (top of the roof), taking advantage of natural ventilation, hot air rising. Good ventilation keeps the attic cool in summer, which is important for shingle life as well as comfort. Ventilation is also important for drying should indoor air leak into the attic through penetrations or a poorly sealed vapor barrier. A poorly ventilated attic may also form ice dams in winter, ice dams occur when snow from the top of the roof melts but freezes at the eaves before it can drain off the roof. As the roof is not water proof, an ice dam that holds liquid water that could leak past the shingles and damage the sheathing or leak through the roof. For this reason good roofing practice dictates a water proof membrane is installed over the sheathing at the eaves. It should be noted that this is a safety feature in case of ice damming, a well ventilated attic should be able to prevent ice damming from occurring.

Unheated attics are usually insulated with loose insulation, wood chips between the ceiling joists were the most common type used well into the 1970’s. The ceiling under the attic is finished like a wall, with drywall over a vapor or air barrier, like walls, older vapor barriers were not tightly sealed as they are in new construction. In the late 1970’s loose fibreglass or cellulose insulation replaced wood chips. As most attics are relatively easy to access, many if not most older attics that were insulated with wood chips have had loose fiberglass or cellulose added on top.

Indoor air leaking into attics in winter through a failed air barrier may cause fairly serious moisture damage in very cold climates such as an Edmonton winter. Condensation will form as frost if attic temperatures are below freezing. Frost does not dry as readily as liquid water, and may build up during extended cold periods. When there is a thaw, the built up frost can be inches thick, and if it melts quickly, will not be dried by ventilation before it leaks through the ceiling or at penetrations such as light fixtures.

Some Edmonton homes have unventilated attics or attic space. A flat roof or a low pitch vaulted ceiling may have the drywall or interior sheathing applied to the bottom of the rafters, this type of ceiling is basically a horizontal wall, the rafter is sandwiched between the roof sheathing and interior sheathing. Where an attic is used for living space the attic may be partially ventilated, and partially enclosed like an exterior wall. The attic as living space in the semi bungalow or one and a half story can be more prone to ice damming, unless care is taken to ensure there is a tight air barrier and ventilation at each unheated section of attic space. Alternatively the roof can be insulated between the rafters like a vaulted ceiling, if this is done the spaces behind the knee walls and above the ceiling must also have access to indoor conditioned air to prevent cold areas that can form condensation.

The building envelope below grade

Most Edmonton homes have a basement. The reason for basements below houses in Edmonton as opposed to a crawl space or a slab on grade starts with buildings that need a foundation. A foundation supports the building, keeping it upright, and also ensuring that all parts of the building more or less remain connected in the way the builders intended.

Edmonton homes are built on soil, requiring a foundation wide enough to support the building and resist movement. The foundation should be placed on undisturbed soil below the frost line, where temperatures remain above freezing year round. In Edmonton the frost line is roughly four feet or 1.2 meters below the surface. Foundation walls are placed on top of concrete footings. Framed homes are mostly made of materials that need to be kept dry, so foundation walls generally extend above grade by at least 8 inches (200 mm), usually more like two to three feet. The material chosen for foundations have to be able to tolerate being buried and exposed to moisture. Most Edmonton homes have concrete foundation walls, some older homes use masonry foundation walls and a few homes have rot proof pressure treated wood foundations.

With foundation walls for Edmonton framed homes being at least 6 feet high when the above grade portion is added on, and that most homes do not have very large footprints, it made sense to excavate inside the foundation and create a basement. The early 20th Century method of central heating using coal burning gravity furnaces that use the principle of hot air rising to heat a home needs a basement so that the furnace and a Winter’s worth of coal can be located below the living area. .

Early basements were not intended for living space, typically they had less head room, few windows, and whether or not they leaked a bit of air or water was not a huge concern. As the old gravity style coal furnaces were replaced by furnaces burning natural gas with an electric blower to move the air, people were able to use basements as living space. Home buyers wanted and got more basement headroom with more and larger windows.

With basements used for living space, including bedrooms, or even separate suites that could be rented, keeping basements warm and dry became essential. The building envelope below grade has to deal with a completely different environment than above grade. Soil temperature is more consistent year round, not subject to sudden changes in temperature like the air surrounding the house above grade. Soil can be wet, Edmonton soil is clay based which can hold a lot of water. Wet clay expands and can put a lot of pressure on a foundation. Concrete is not water tight, it can absorb water like a sponge. Concrete frequently cracks, and cracks can leak water or gases.

Fairly recently it has been discovered that homes can have high levels of Radon, a radioactive gas in found in soils. Radon has been linked to lung cancer. Radon, if present in the soil, enters homes through the basement building envelope. The current Alberta building code has been changed to require that foundations in new homes are sealed against soil gas entry.

If there are sufficiently high levels of water in the soil around a foundation, a drainage system is installed around the outside of the foundation.

Foundations and the methods used for drainage and waterproofing basements are designed for the soil and ground water conditions of the site. In Edmonton foundations have gravel and weeping tile placed beside the foundation to collect water and drain water at the outside of the foundation. The weeping tile is either directly connected to a storm drain, or passed into a sump pit, where a sump pump can pump the water to a storm drain, or on to the lot surface where it can drain away from the house.

Eavestroughs collect all the water from the roof of the building, which can be considerable. Downspouts take the roof water into the storm drain or on the lot surface. Sump drain or eavestrough extensions should be added to place the water collected well away from the house. The lot should be kept graded away from the house at all points so that surface water from rain or snow melt is kept away from the foundation.

New home built to the code requirements for preventing ground gases from entering the basement typically use a layer of heavy plastic placed on top of a layer of gravel under the concrete floor slab. Penetrations in the slab, such as the sump pump cover, plumbing stacks and access covers are made air tight to prevent ground gas entry. A rough in for Radon gas mitigation is placed in the slab, if Radon does enter, the a mitigation system can be added by the homeowner.

The exterior of the foundation wall are coated with damp proofing or water proofing, usually bitumen (tar).

New homes have fully insulated basements. Older homes came with uninsulated basements, or only insulaton above grade. Year round average soil temperatures below grade for Edmonton are 5C. Bare concrete is a poor insulator, insulating basement walls below grade will save energy, given that the basement is inside the building envelope. As insulating basements is easy to do, most older homes have insulated basements by now as well.