Can You Do That? (Part 2)
Installing fuel gas appliances within the building envelope
One of the ways the 2012 IRC defines the separation of interior building spaces and the outdoors is the Building Thermal Envelope. This is defined as “The basement walls, floor, roof, and any other building elements that enclose conditioned space or provides a boundary between conditioned space and exempt or unconditioned spaces.” One way that the thermal envelope provides separation is with Air Barriers. The 2012 IRC defines this as “Material(s) assembled and joined together to provide a barrier to air leakage through the building envelope. An air barrier may be a single material or a combination of materials”. Some migration of air will occur regardless of how tight a building or structure is designed and built, one way that this occurs is Infiltration. The 2012 IRC defines infiltration as “The uncontrolled inward air leakage into a building caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density or both.”
All these requirements create conditions where special consideration must be given to fuel burning appliances. In many cases these tight buildings require whole house mechanical ventilation systems to provide air changes as well as the required air for combustion, dilution, and ventilation air. The 2012 UMC states in section 701.3 Makeup Air. "Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.” The 2012 UMC also states 701.9.1 Exhaust Fans. "Where exhaust fans are installed, additional air shall be provided to replace the exhausted air.” So we have exhaust requirements for bath rooms, laundry rooms, kitchen range hoods, as well as clothes dryers and possibly fireplaces.
A popular method to create a tight structure or building is to utilize spray foam insulation methods. In the installation of batt or blown-in insulation in the ceiling of a building or structure are insulated with a minimum R-30 insulation. In the case of spray foam insulation there are two types, they are open cell or semi ridged foam and closed cell ridged foam. The difference is primarily in the permeability of the spray foam or SPF insulation material. The advantage SPF has over conventional forms of insulation is that SPF can be an air and vapor retarder, which fiberglass and cellulose cannot. When an architect or builder considers an insulation product, they must consider how the product will perform in all climate conditions for the building’s physical location. SPF has high R-values. For example: 6 inches of open cell spray foam insulation on a roof deck will perform better than R-value of over 38 and 3 inches of closed cell spray foam insulation will perform better than R-value of over 38.
In most buildings and structures the attic space is ventilated. There are many good reasons to ventilate attic spaces. This prevents buildup of heat and moisture within the attic space. If this moisture is not properly vented it can form condensation which can promote the growth of mold and cause damage to the roof and building structure and in colder climates this condensation will freeze and cause ice dams in roofs causing further damage. The 2012 IRC section R806.1 Ventilation Required states “Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow.” The application of SPF insulation is applied to the underside of the roof structure. This creates an unvented attic. Section R806.5 Unvented attic and unvented enclosed rafter assemblies states “Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted if all the following conditions are met:
1. The unvented attic space is completely contained within the building thermal envelope.”
So if we refer to the definition of the “Building Thermal Envelope” as being “building elements that enclose conditioned space or provides a boundary between conditioned space and exempt or unconditioned space.” we now have an area within the attic space that is inside the “Building Thermal Envelope.” In some cases ventilation louvers are placed between the occupied spaces and the enclosed attic.
This in effect creates a shared space, the 2012 UPC states in section 506.2 Indoor Combustion Air.“The total required volume shall be the sum of the required volume calculated for appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors and through combustion air openings sized and located in accordance with Section 506.3 are considered a part of the required volume.”
So what does this mean for the installer? It means that a fuel burning appliance can be installed within the “Building Thermal Envelope” with the exception of bedrooms and bathrooms, as long as the required volume of combustion, ventilation and breathing air is supplied to those spaces or the combination of spaces need to equal the required volume of air. But is it a good idea to install category I or II appliances within the “Building Thermal Envelope?” The 2012 UMC defines these appliances as category I as “An appliance that operates with a nonpositive vent static pressure and with a vent gas temperature that avoids excessive condensate production in the vent.”, and category II as “An appliance that operates with a nonpositive vent static pressure and with a vent gas temperature that may cause excessive condensate production in the vent.” In other words, these appliances depend on the difference in temperature of the flue stack gases to vent properly. Any negative pressure within the structure or the appliance enclosure will cause negative draft to occur. These appliances depend on proper draft to produce a proper fuel air mixture and proper combustion of the gas. If incomplete combustion occurs and a negative draft occurs these combustion byproducts can be drawn into the “Building Thermal Envelope.” This makes it imperative that these types of fuel burning appliances are located in areas which are either separated from and outside of the “Building Thermal Envelope”, such as in a dedicated mechanical enclosure, supplied with the proper combustion air, or the ventilation design is such that the combined ventilation, exhaust, breathing air, and combustion air are provided for.
An even more efficient means is to utilize high efficiency “direct vent” appliances. Direct vent water heaters and furnaces can be installed within the “Building Thermal Envelope” without special consideration for makeup air and are more efficient to use than less efficient Type I and II fuel burning appliances. The 2012 UMC defines a direct vent appliance as “Appliances that are constructed and installed so that air for combustion is derived directly from the outdoors and flue gases are discharged to the outdoors.”
While most commercial applications, though not all are being designed to meet these code requirements, many if not most residential single family homes are not. SPF insulation is a great product to seal buildings and structures. One of the benefits of moving the “Building Thermal Envelope” to the roof, especially in parts of the country where it is common to place fuel burning appliances in attic spaces, is that systems can be sized for lower heat loads, ductwork is now located within the conditioned space rather than attic spaces that can easily reach over 100 degrees Fahrenheit. This means less air conditioning loads in the summer and less heating loads in the cooler months = less energy use and lower heating and cooling cost. One consequence of this is that the air conditioning and even heating cycles may need to be extended in length to dehumidify the air in the building or structure. Since the cooling and heating loads are reduced the cooling and heating cycle may not run long enough to properly dehumidify the interior of the building or structure. Newer fan units are available that utilize variable speed fans or slower fan speeds to move the air more slowly over the cooling coil to dehumidify the circulated air.
With rising energy cost and awareness of “Green Construction,” owners, architects, engineers and builders are competing to build the most energy efficient and comfortable homes and buildings for consumers. This means many different materials and construction techniques are being employed to achieve these results. But as a consequence it is all the more important that Architects, Engineers, Builders and Inspectors be aware of and knowledgeable of current codes and standards of the industry. No one wants a “sick building’ or worse yet and unsafe building or home for the occupants who utilize and live in these buildings and homes.