Article and photos by Greg Havel
A century ago, fuel prices were low and there was little concern for insulation or for making houses and other buildings air-tight. Most buildings had no insulation. In some lumbering communities in the northern states, sawdust was sometimes added as insulation inside the walls of balloon-framed buildings from the attic and into the space between the attic floor joists to reduce the amount of firewood or other fuel needed for heat. There was no shortage of fresh air for combustion in stoves or space heaters since these buildings were not sealed, and sometimes snow blew in through the cracks and around windows with the fresh air.
As fuel costs increased during the 1930s and 1940s, insulation became more common in newer buildings, with vapor barriers being added inside the walls to prevent moisture from inside the house from migrating into the insulation and freezing.
During the energy crisis in the 1970s, with the price of heating fuel more than doubling in a single winter, utility companies and building contractors began advertising insulation and sealants as the remedy for high fuel costs. Insulation would reduce the heat loss through the walls and roof; and sealants around windows, doors, and in cracks would reduce “infiltration” of air that had not been heated or cooled, depending on the season. Many older homes and commercial buildings had insulation blown into the walls and attic to reduce heat loss; had caulking applied around window and door frames and at other joints in the exterior walls; and had weatherstrip applied between window sash and window frames, and between doors and door frames; all to reduce the amount of unconditioned air moving into and out of the structure, and to reduce the amount of fuel needed.
It was at this time that carbon monoxide (CO) became a frequent topic for discussion during heating season. Since buildings were insulated and sealed better than they ever had been, infiltration of outdoor air had been significantly reduced. Furnaces, water heaters, and space heaters that previously depended on infiltration for combustion air now began to use up the oxygen inside the building, sometimes creating an oxygen-deficient atmosphere, leading to incomplete combustion of fuel and production of carbon monoxide (CO) instead of carbon dioxide (CO2) and water vapor. Vents and chimneys no longer worked as efficiently as they had in the past, and CO and other flue gases could back up into the building, causing illness in the occupants. Photo 1 shows the draft hood on the top of a typical residential gas-fueled water heater, without a “closed combustion” feature. These water heaters draw their combustion air from the interior of the building, and are still sold today. The draft hood was a break in the vent that reduced the possibility of the pilot light being extinguished by a downdraft in the chimney, often caused by a wind gust. Heating furnaces and boilers operated in a similar manner.
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Newspaper articles and letters from fire marshals at this time began to recommend keeping a window open in the room with the furnace, fireplace, or other fueled heater, to prevent CO accumulation and to supply combustion air.
As fuel prices continued to rise, building codes were further revised during the 1980s and 1990s. Today’s buildings have:
- 50 percent or more insulating value in the walls and roofs than 30 years ago, and in some parts of North America, 2×6 studs are needed to contain it
- A tighter seal against infiltration than 30 years ago, often including both a moisture vapor barrier on the inside of exterior walls, and a breathable infiltration barrier on the outside of exterior walls
- Windows and doors with double- or triple-paned glazing that are sealed into the openings in the walls
- Window sash and doors equipped with weatherstripping
- CO detectors outside sleeping areas in residences and near attached garages (IRC section R315), and outside mechanical equipment spaces in commercial occupancies
- The International Residential Code, 2009 edition, Section N1101; and other model, state, and local codes have similar requirements.
As a result of well-insulated and infiltration-resistant construction and the addition of plastics and synthetics in room finishes and furnishings, the growth of room-and-contents fires is more rapid than in the past, fires burn hotter than in the past, and the burn time before flashover has been reduced. The well-insulated walls slow the transfer of heat from the fire through the wall to the exterior of the building or to the next room and reflect more heat back into the room, resulting in a more rapid increase in temperature in the room of fire origin than in the past. The reduced amount of infiltration results in a fire that is even more ventilation-controlled than in the past.
A modern house or other building is so well insulated and so resistant to infiltration that building and heating, ventilating, and air conditioning (HVAC) code IRC Section G2407 now require that either the furnace or heater be “closed combustion” (with a pipe or duct to bring outdoor air from outside directly into the combustion chamber; and no break in the vent for combustion gases) (Photo 2), or that a combustion air supply be provided into the room with the furnace or heater. Other codes have similar requirements.
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Because a modern building is so resistant to infiltration, building and HVAC codes may also require mechanical ventilation or air exchange through calculated infiltration inside the occupied space, to prevent smells and accumulation of toxic or otherwise unpleasant vapors from the synthetic materials used in construction and home furnishing.
Because fuel prices continue to increase, and because many homeowners and building occupants do not understand the reasons for sealed combustion heaters, combustion air, and mechanical ventilation, there are potential problems:
- If either the air intake pipe or vent pipe for a sealed combustion unit is obstructed, this will cause malfunctions and incomplete combustion; and if the heat exchanger is defective, CO will find its way inside the building
- If the combustion air supply is shut off to prevent cooling of the room in winter, the atmosphere inside the building may become oxygen-deficient, and combustion will produce CO which may spread throughout the building
- If the mechanical ventilation is turned off to reduce the energy cost to condition the air, this can cause the atmosphere inside the building to become stagnant, with accumulated odors and vapors from plastics and other synthetics. If there is a problem with the heating unit, CO will accumulate rapidly.
To prevent carbon monoxide accumulations, and hot, fast-burning fires, homeowners and building occupants must:
- Maintain their heating appliances and water heaters according to manufacturer’s recommendations and building and HVAC codes
- Maintain and test the CO alarms and smoke detectors in their homes and buildings
- Maintain openings and ducts for combustion air, and mechanical ventilation, as recommended by the appliance manufacturers and building and HVAC codes
- These recommendations become even more critical in today’s “green building” environment, where more insulation is provided than is required by code; where higher efficiency is required of heating appliances; and where infiltration is further reduced and replaced by required mechanical ventilation.
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