Heating and cooling a new home addition, whether it be a conventional shingle roof or glass roof addition, can be a tricky undertaking. Many factors can come into play including the size and orientation of the new room, its intended usage, the extent of shade cover from surrounding trees and buildings, the type and capacity of the current heating system, the location of the existing furnace relative to the new addition, the cost of ancillary heating and cooling units, the type of foundation, the extent to which the new addition will be separated from or incorporated into the existing home, as well as questions such as is the current basement ceiling finished or is it accessible for the installation of new duct work.
Extending Existing Duct Work
One option for heating and cooling, and certainly the simplest approach, is to extend the existing duct work into the new addition. In fact, the question as to whether the existing heating system can be extended efficiently and economically is often the critical factor from which all other heating considerations follow. If your furnace is operating at its load capacity in meeting your home’s current heating requirements, then extending the duct work to a new addition, regardless of its size, is simply out of the question without a major furnace upgrade. Also, even when the heating and air conditioning system has extra capacity, its location relative to the addition may preclude any extension into the new room.
Most new additions are added onto the back of the house. In these cases, the ideal situation occurs when the furnace has extra capacity and is situated close to and in direct line with the proposed addition, and with access to the basement ceiling for extending the ductwork. In this configuration the new addition is almost certain to enjoy a sufficient delivery of hot and cool air through the central heating system. As the distance between the furnace and the addition increases, the amount of heated or cooled air available for delivery to the new room diminishes until the addition can be uncomfortably cool in winter and possibly hot in summer. Heat ducts with several corners, although often unavoidable, also serve to restrict air flow. Similarly, a plenum serving several runs with possibly multiple heat registers will often have little air pressure by the time it reaches the new addition.
Building the addition with a floor line lower than the adjoining house floor, even by as little as 7 inches, will often require drop down elbows (i.e., two additional corners) in each duct, with a negative impact on the system’s efficiency. Homes with a rear patio door or large window that is to become the proposed access to the addition often have a heat register located close to that access. A fairly common mistake is where homeowners assume they can simply extend that particular duct to meet the needs of the new addition. This approach rarely works. Firstly, the new addition will most likely require more than one heat register. And secondly, the existing register already has a job to do and that is to heat or contribute to heating the existing room. Removing this duct and extending it to cover the new room would in all likelihood leave both the original and new room relatively cool.
The choice of foundation and the type of flooring to be used must also be factored into all sunroom heating considerations. With foundation walls the sunroom platform is not insulated, allowing heat from the crawlspace under the platform to warm the new floor, just as the heat in your basement warms your house floor. Foundation walls also have the advantage of making installation of the heating system much easier than it is for posts.
With metal post foundations, the platform is exposed to the elements and is usually insulated to achieve R30 to R40. Hardwood flooring will boost this R value slightly. From our observations (Four Seasons Sunrooms – Ottawa), rooms built on either foundation walls or posts seem to cost about the same to heat. This opinion, however, is based solely on anecdotal evidence and we are not aware of any verifiable examination of this question. On one point we are certain-floors built on posts are cooler in winter than floors built on a foundation wall, especially where ceramic tile rather than hardwood, laminate or carpeting is the choice of floor covering.
Extending existing duct work to a new sunroom is usually neither complex nor expensive. Heating contractors, since their experience is often limited to inexpensive glass roof sunrooms built with low performance and poorly insulated glass, are often skittish about sunroom heating requirements and too frequently recommend overkill. Their solution can be both expensive and unnecessary. When the effectiveness of extending duct work is questionable, our recommendation is generally to proceed, but with a backup plan. In such situations we usually either install one or more ConVectair electric wall heaters, or at a minimum, pre-wire for later installation if the room turns out to be less than comfortable. The installation of one or more cold air returns should not be overlooked, especially where the new addition is isolated from the existing home.
On occasion, as noted, extending the existing duct work is just not an option. In these cases, an independent heating system, such as a gas fireplace, a separate gas furnace with its own duct work, or sufficient electric heating represent the obvious options.
With additions that are incorporated into and open to the existing house, regardless of whether they are conventional shingle roof, or glass roof, it is usually easier to achieve and maintain a uniform, consistent, and comfortable temperature throughout the entire house, including the sunroom, especially if the furnace fan is run continuously throughout the winter.
One approach that has become increasingly popular over the past few years is the installation of electric in-floor heating in combination with post foundations. While there is additional cost involved in installing this heating, the combined cost of posts and in-floor heating is well below that of a foundation wall. And it has the added benefit of providing a warm tile floor without having to destroy or damage your landscaping with a foundation wall installation. Hot water in-floor heating is also an option. Although less costly to operate than electric, hot water systems are more expensive to install. There are two additional points that should be noted when considering in-floor electric heating. First, it will not heat your room. While it will contribute to some space heating, its main function is to keep the floor comfortable. The second point is that while it works extremely well with tile flooring, and reportedly works with laminate flooring, the results with hardwood and engineered flooring are, to the best of our knowledge, quite disappointing.
Heated Double Floor Systems
A second method of providing a warm floor on post foundation is to install what is known as a heated double platform. This approach calls for the installation of a standard insulated 2 x 10 or 2 x 12 inch platform with a second un-insulated platform using web joists built over the first. Web joists are designed to allow air to flow freely between each of the joists within the platform cavity. Vapour barrier is placed between the two platforms with insulation placed around the outside perimeter walls of the sealed upper platform. Hot air from a separate duct is then directed into the upper web joist platform where it is able to circulate freely throughout the platform cavity. Heating the cavity in this manner will serve to ensure a warm and comfortable floor in the sunroom. Duct work to carry warm air to the sunroom floor registers is installed within the web platform the same as it would be in a standard insulated platform.
Double floor systems will not work with all projects. All too often the space between grade and the house floor is limited, often to as little as 14 to16 inches, or less. This is barely enough space to accommodate a single platform and still leave a reasonable distance of about 4 inches between grade and the platform so as to allow for any soil expansion due to freezing. Allowing frozen and expanding soil to touch and lift the sunroom will likely cause significant damage to the sunroom. At least 25 to 30 inches between the floor line and grade is required for the installation of double floor system.