As our demand for tighter and more energy efficient buildings increases, we must pay attention to proper design and construction techniques to maintain adequate ventilation and moisture control.
The challenges with ventilation and moisture control are in part controlling the humidity levels within the home. Humidity levels within the home are in direct relation to the ventilation process and the living habits of the occupants. A 1200 ft. 2 home with a family of five occupants will result in higher humidity levels and more demand on the ventilation system, while that same family in a larger home, at 2400 ft.2 which has a larger volume of air and will require less ventilation and have a lesser humidity level simply because there is a greater air volume within the home to absorb humidity. The humidity levels in any home are, in part related to the home’s interior air temperature, commonly known as relative humidity. Whereas the warmer the air temperature, the more humidity or water vapour the air is capable of holding. As warm, humid air laden with water vapour passes across a cold surface, the humid air that circulates over the cooler surface releases its moisture, and this moisture is deposited on that cooler surface. This is evidenced by wet interior windows in the cooler seasons. This is commonly known as the dewpoint. Total interior moisture control and protection for housing can be difficult, our understanding and control of interior moisture is evolving as older technologies are being replaced by new technologies, and ventilation techniques continue to move forward.
Taking a quick trip down memory lane, I can remember our old farmhouse with a sawdust burner in the basement; I can clearly remember our father getting up in the middle of the night during the winter months to restock the sawdust hopper. This would keep us warm for a few hours until it was time again to refill the hopper. Then the old furnace was converted to oil, then eventually a natural gas mid efficiency forced air heating system. Through all of these heating upgrades, there were other upgrades and repairs done to our home. All of these repairs and upgrades were to make our home more efficient and hold the heat in longer. This proved to be quite successful, and by the time I was ready to leave and branch out on my own, we had a reasonably efficient and comfortable home for that time period.
Before the advent of high efficiency forced hot air furnaces, we relied on our low to mid-efficiency furnaces to keep us warm. With this type of heating system there needed to be a combustion air supply vent for the furnace and the mid-efficiency hot water tank. This combustion air intake vent would supply and replace the air used by these appliances for combustion and chimney draft. We have all seen this metal insulated pipe which terminated near the heating appliances and had a metal bucket like assembly at the end of it, and it always had a cold draft blowing through it.
You may have noticed that this cold draft was coming into your furnace room, even though the furnace or hot water tank was not operating. This cool air was coming into your basement through this combustion air vent simply because there was warm air leaving at some other point in the house, usually from the top floors which may have had a window or door open. Even with all the windows and doors closed up tight, as in a winter condition, there is still warm air leaving the building through small penetrations through the building envelope, fans, and, various vents placed throughout the home. This air movement which is the warm air leaving through the top portions of the building, simply because it has greater buoyancy than cold air, which is entering the building through the lower portions. This exchange of cold air in, for warm air out, is called the stack effect.
For the most part, this natural convection of air movement throughout the building kept a supply of fresh air circulating and helped to reduce a moisture buildup within the home. In some homes, this stack effect was more noticeable than in other homes. In homes where it was more noticeable, it was less comfortable as you could feel the cool air moving around your feet, or you noticed the furnace was continually cycling on and off through a cold spell. Not only was this uncomfortable but it was becoming costlier as energy prices began to rise.
Not so long ago the high-efficiency furnace and hot water tank came on the scene. These furnaces and hot water tanks boasted a 96 to 98% efficiency, they use no internal air from your home for combustion, and therefore a fresh air intake for combustion air was not required. No more cold air from outside being dumped into your furnace room through a 4-inch pipe through the wall. Your home is slowly becoming more energy efficient; the windows are of better quality and better sealed to the wall for a tighter building envelope. The new bathroom fans are becoming quieter, and they are exhausting, on average 50 to 100 CFM of air per minute to help keep the moisture out of your bathroom and kick it outside as quickly as possible.
The exhaust fans over your stove and cooking appliances are more powerful, and they are exhausting at minimum 200+ cubic feet of air per minute. The average clothes dryer exhausts approximately 250 CFM of internal air per minute out of your house. Imagine, at the end of the day, a family member has a shower with the bathroom fan on, someone is cooking dinner on the stove with the hood fan on, and there is a load of laundry in the dryer for approximately 40 minutes. That is an incredible 550 CFM of air leaving your home, every minute, all through mechanical, forced means. Where is this fresh air coming from to replace the air leaving? As it was before with the older mid-efficiency appliances, there was the combustion air intake venting in place. Not only did this combustion intake air vent supply fresh air to the burners in the appliances, but it also neutralized and kept the house in a balanced air pressure condition with the outside air. Now, with the high efficiency furnaces and hot water tanks and the home in a negative air pressure condition, the only places available to neutralize this negative air pressure condition is through small cracks and through wall penetrations in the walls, floor systems, around doors and windows, exterior and interior lighting fixtures, and back drafting through other venting appliances that are not in use. In some cases, this cold air infiltration enters the building in undesirable places where there is a temperature difference and a frosting up, and melting/wetting condition can occur.
One such situation comes to mind, a basement bathroom wall and ceiling had to be completely opened up because of a severe wetting and moulding condition. This was caused when the bathroom fan outside metal hood vent tailpiece (the section that goes through the wall to the inside of the home) was cut up the length for about 6 inches and folded inwards. This was done to allow for the exhaust fan extension pipe from the bathroom fan to fit over the top to complete the venting connection to the outside. Unfortunately, the bathroom fan extension pipe did not completely cover the cut that was made in the metal hood vent tailpiece. As the fan was running and drawing warm moist air from the bathroom to expel it outside, it was also leaking through this connection point and into the joist space above the wall. Also, when the bathroom fan was off, natural air convection allowed for cold air from the outside to be drawn inwards, and the cold air moved through this bad connection point and across the top of the wall system and up through the plumbing penetrations leading to the main floor vanity above. This is a perfect example of natural air flow through the building structure. Had the holes for the plumbing pipes above being plugged or blocked in some way the convection cycle may have been eliminated. But there still would be a percentage of cold air being sucked in through the poorly fitted exhaust vent below.
Consequently, this air flow movement through natural convection, and when cold air meets warm damp air, a heavy frost was created which subsequently melted when the temperature warmed up. This wetting condition, over time, within the floor system above the ceiling caused a considerable amount of mould and a great expense to the homeowners. The mould had to be remediated and the bathroom completely rebuilt. If this exhaust fan connection were sealed up correctly, the cool air would still make it through the fan exhaust pipe, and perhaps you may have felt some cool air exiting the ceiling fan above. The science behind air movement and humidity levels are to keep the two airflows separated with a proper vapour retarder and adequate insulation between them. When we tighten up our homes, which include removing the old fresh air intake systems from yesteryear, new challenges with humidity levels and air infiltration through cracks and small openings within the building cavity will arise. And ultimately, when your home is in a high humidity condition, with frost buildup and wetting during the winter months, the increase in moisture and humidity will eventually increase mould growth.