Ottawa and Gatineau have very cold winters. In extremely cold years, coupled with little snow cover for insulation, the frost can actually drive six or more feet underground. Depending on its moisture content, when ground freezes it expands, causing structures such as fence posts and house foundations to heave or lift up. This movement must be eliminated as it can damage buildings and sunrooms, making it difficult to open doors and windows. Movement due to soil expansion can also cause sunroom glass to shatter. To avoid this problem, Ottawa, Gatineau, and other local building authorities require that any new building or addition such as a sunroom be anchored at least six feet below grade before a building permit is issued.
There are several foundation options which allow you to provide this necessary stability and meet building code requirements. The more popular of these include sono tubes, helical or metal posts, either poured concrete or cement block foundation walls, and full basements. A half wall with a finished cement floor is another option which can be used in those cases where the floor line is near or below grade.
In the last 5 to 10 years, metal posts have replaced sono tubes in eastern Ontario and western Quebec as the most common method for building a sunroom foundation. In general, over 90% of all sunrooms installed by Four Seasons Sunrooms in this area are placed on helical pier/metal post systems. Metal post are basically round steel shafts with a helical blade placed near the bottom of the shaft and then screwed by machine into the ground to the desired depth.
With piers the floor structure, often referred to as platforms, is usually constructed using 2 x 10 or 2 x 12 inch floor joists on 16 inch centres. Pre-engineered floor systems represent a second platform option. Two by ten inch systems generally use R30 to R32 fibreglass insulation placed between the joists. Sheathing is applied under the joists to keep rodents from disturbing the insulation and to keep it in place. Five-eights TG plywood is generally used for the sub-floor. In cases where ceramic tile is to be used, a second layer of 5/8th TG plywood is applied to provide rigidity and to protect the tiles from cracking. To prevent or reduce the growth of weeds or nuisance vegetation, the ground under sunroom platforms built on piers the ground is generally covered with a poly film with crushed stone or sand placed on top of the film.
Skirting, usually small PVC lattice, framing with parging, or other material, is often placed around the perimeter of the sunroom between the platform and about 4 inches above grade. Wire mesh buried 6 to 12 inches below grade to further deter rodents and other animals from entering this covered area can also be applied. The approximately 4 inch gap between grade and the bottom of the lattice or other skirting is to allow for ground expansion due to freezing in winter.
Foundations walls used for sunrooms are built using either 8 or 10 inch poured concrete or cinder block walls placed on a 16 or 20 inch wide by 8 inch deep footing. The building code requires that this footing be either sitting on bed rock, or 5 ft. or 6 ft. below grade. Variations with this depth can be accepted or required by city building inspectors often working with soil engineers. Ten inch walls on 20 inch footings are used if the home owner is choosing to go with brick siding for the sunroom with the extra 2 inch width being required to support the brick.
The decision to use a cinder block wall rather than poured concrete is usually dictated by access. In cases where a cement truck is unable to reach a foundation site, a pump truck can be employed to push the cement from the driveway at the front of the house to the foundation site, usually at the back, and generally adding $500 to $1,500 to the overall cost. Where this is necessary, the client’s choice is usually to use cinder blocks and avoid the additional pumping costs.
The sunroom platform sits on the perimeter cement wall with the earth below the platform left in place and not removed as is the case with a full basement. About 3 inches or more of SM Blue, or some other form of rigid insulation, is applied to the inside of the foundation wall to a depth of 2 ft., the purpose of which is to reduce heat loss from the crawl space. The space between the platform and grade, referred to as the crawl space, is normally accessible to the basement through a crawl space opening of at least 18 inches x 24 inches. The primary purpose of this opening is to allow for venting of the crawl space as required by code. In some cases, the crawl space is actually vented to the outside rather than to the basement, but this is not a preferred method. When vented into the basement, the utilities including the heating and cooling ducts for the sunroom as well as heat for the crawl space are usually routed through this opening.
Unlike platforms built on posts, which are insulated, half wall platforms are not. This is to allow heat from the crawl space to warm the sunroom floor, similar to the way in which your basement heats the main floor of your house. In cases where the crawl space is vented outside and not into the basement, the floor platform, as with a post floor, must be insulated.
As already mentioned, the majority of all sunrooms installed by Four Seasons Sunrooms in the Ottawa area are placed on post foundations. There are two main reasons for this-the relative cost of half wall installations, and their impact on the existing landscaping. In very general terms, and depending on room size and other variables, half wall foundations will often cost roughly $4,000 to $10,000 or more than a post foundation. The second deterrent to half wall or full basement foundations is the overall disruption and destruction to the landscaping caused by the excavation and backfilling equipment when installing the foundation. This is a particular concern in cases where homeowners have exceptionally well cared for and beautifully landscaped backyards.
There is one other consideration which may cause some potential clients to absorb the higher cost of half wall foundations and to accept the temporary disruption and landscaping costs. Half wall foundations, when vented into the basement, do provide a warmer floor. But at the same time it has to be recognized that properly insulated platforms built on posts, and when combined with in-floor heating, will create rooms that are every bit as comfortable as rooms built over a basement or foundation wall with a crawl space. Sunroom buyers seeking the comfort of a floor built over a foundation wall but without the added expense of that wall may wish to consider in-floor heating. Although this option does add some cost, it is still well below the cost of building on a foundation wall.