Buildings are ventilated in many different ways.
In essence, the process is that of allowing air in and out of a building – much
the same way as a person would breathe. Buildings are never constructed as
complete ‘air tight’ structures. There is always some kind of air flow moving
through the interior. This air flow or ventilation happens as a result of
situations like a bathroom fan being turned on or a door being opened. The
concepts and terminology with regards to the way a building is ventilated can
be complex and difficult to understand and apply. This is why it is important
to understand the basic mechanisms at work when referring to ventilation – air
in and air out.
Mechanical Ventilation
Read Appendix A-9.32.3
For many years, houses were constructed
without mechanical ventilation systems and relied on natural air leakage
through the building envelope for winter ventilation. However, houses have
become progressively more airtight through the introduction of new practices
such as:
- Panel type sheathings like plywood and wafer
board replacing the more traditional use of sawn lumber plank siding.
Plank siding would allow more air to infiltrate through because of the
increased amount of joints between the pieces
- Tighter windows and doors
- Improved caulking materials
- Friction fit batts with polyethylene film
have replaced paper backed bats
Following the energy crisis in the 1970’s,
considerable emphasis was placed on reducing air leakage in order to conserve
energy. At that time, Electric heating systems were encouraged and higher
efficiency furnaces were developed. The combined effect of these efforts,
coupled with new building materials worked to reduce the air change rates in
buildings. As a result, concerns became focused on indoor air quality as a
result of the reduced flow of natural air through the building.
To some extent, it can be suggested that
the objectives set out following the energy crisis of the 1970’s have been
achieved. Buildings are more air tight and require less energy to heat and
cool. If it were not for the fact that people use these buildings it would be
possible to achieve much greater efficiencies because ventilation would not be
an issue.
In the1985 and 1990 versions of the Ontario
Building Code the issue of ventilation in buildings was recognized and
addressed with new requirements that all houses be required to have mechanical
ventilation systems capable of exchanging the indoor air for outdoor air at a
specified rate. In 1985 this rate was set at 0.5 air changes per hour (0.5 ACH)
and in the 1990 edition of the code this rate was set at 0.3 air changes per
hour (0.3 ACH)
With reference to the Building Code
requirements, the following descriptions direct the designer to either Part 9
or Part 6. Ventilation systems that meet any one of the following conditions
must be designed according to Part 6
Systems serving more than one dwelling unit
Systems serving any occupancy other than
residential
Storage garages for more than 5 cars
Natural Ventilation
Question 1: Does the dwelling
have electric service?
Question 2: Is the dwelling
intended for occupancy on a continuing basis in winter?
If the answer to either one of the
following questions is no, a single dwelling unit may be ventilated by Natural
means only according to Articles 9.32.1.3 and Subsection 9.32.2. Natural
Ventilation means that the infiltration occurring through any number of sources
contributes to the circulation of air throughout the dwelling. These sources
would include spaces under doors, opening windows, cracks in walls, exhaust
fans in bathrooms or any random source of air flow occurring as a result of
age, decay, or workmanship. Small air paths created by the inevitable imperfection
of the workmanship or decay over time contribute to the infiltration in any
building. The building code allows for this circumstance as an acceptable
measure for adequate ventilation in small houses under Part 9 buildings only.
Consider the effects of a windstorm on a
newly constructed house. Although unperceivable, the house is moving and
racking in small amounts as a result of the wind forces. This racking causes
differential movement in any number of situations around doors, windows,
soffits et cetera. When winter turns to summer many of the materials used to
seal a building expand and contract. Over time this back and forth expansion
leads to cracks and crevices in the building. This is especially true with
caulking materials around windows and doors. The result of all these forces and
changes in the building is air flow in and out of the building through means
that had not necessarily been designed or anticipated.
When trying to come to grips with the requirements
for ventilation in a house, infiltration (the flow of air) can be a complicated
and often misunderstood or misinterpreted value when assessing the health and
safety of the occupants. Infiltration is measured and expressed as a unit value
of Air Changes per Hour.
Infiltration
The following example of an ‘Energuide’
report by Natural Resources Canada prepared for a house in Toronto prior to a
renovation in 2004 is used to illustrate one of the realities associated with
infiltration.
The house was built in 1905. One of the
tests performed for this report measured Air Changes in the house. This test was
done by placing an exhaust fan at the front door (blowing out). The windows and
doors were closed and the fan was turned on. While this was happening it became
evident where air was being drawn into the house by way of walking around with
a ‘smoke candle’. A smoke candle is a small apparatus that emits a small stream
of smoke which gives a visual indication of air currents and velocity. In some
instances during this test, a smoke candle was not necessary as the wind could
be heard whistling through a couple of places like a crack in the wall of the
workshop in the basement or at many of the electrical outlets.
The test showed that the house had 16 air
changes an hour at 50 Pascals. This is equivalent to a 500 square inch hole in
the house open all the time. The following is a list of items identified as the
major sources of air infiltration during the test. Note that these were the
‘major’ sources of the infiltration.
- Basement south wall - at header
- Basement north wall - crack in wall
- Basement windows - all
- Main floor through fireplace
- Interior sliding doors
- Electrical outlets (significant
infiltration)
- Bathroom pot lights and exhaust fan
These were the major sources as mentioned.
Combined, the total amount of air changes per hour for the house was 16 ACH.
The Building code requires a specified rate of 0.3 ACH. Note that the rate is
‘specified’ and not a minimum. Specified means that the anticipated rate wants
to be no more and no less. Any less and the indoor air quality would not be
sufficient for the future health of the occupants. Any more and the house
becomes too expensive to heat and operate. 16 air changes per hour makes for a
house with more than enough air circulation.
The Code requirement of 0.3 air changes per
hour for a house would cost much less to heat. Some would argue that the air
flow through a house of this nature might not be sufficient to maintain
adequate indoor air quality. It can usually be anticipated that a piece of
furniture will find its way over or in front of one of the supply or return air
grilles once the construction is finished and the occupants move in. Large
wardrobe cabinets and boxes covering vents are not factored into the
calculations when determining ventilation capacities. As a result, the
inevitable complexities of furniture layout and blocked air flow leads to a
house that does not operate or function in the way one had expected. To some
extent, this is one reason why various cracks and small holes scattered
randomly throughout the house can be a good thing. Using the example above it
could be noted that by responding to the Natural Resources of Canada Energy
audit and fixing the things that one can readily see and repair that the
resulting air change rate could be reduced to around 3 or 4 air changes per
hour. These items would include the caulking around the doors and windows, sealing
the electrical outlets, replacing the exhaust fans in the bathroom, replacing the
wood burning fireplace with a direct vent gas fired unit and sealing the
chimney.
In 1993, revisions were made to the
Building Code to replace a so-called ‘hole in the wall’ system with 4 new
prescriptive mechanical ventilation options. The various options available
under part 9 are generally restricted to small houses. The next path that is
referred to is either Part 6 or then a reference to acceptable and qualified
sources such as ASHRAE. In terms of content and reference to the small
buildings exam, one can expect questions that refer to information found in the
code. This information might be best presented by illustrating when in fact a
mechanical ventilation system is required and why.
Classification of Combustion appliances
Part 9 requirements for ventilation systems
depend upon four categories or ‘types’ of houses. The Building Code identifies
four house types according to various characteristics. This determination is
important to make at the outset of an inquiry onto the requirements of a ventilation
system because the determination of house type leads to different answers. More
importantly, an incorrect determination of house type, can lead to either
increased construction costs or on the other side, a compromised indoor air
quality environment for the occupants.
The Building code uses four house types:
House Types
Type I
This category includes houses containing
fuel fired combustion appliances that are direct vented or mechanically vented
induced draft. Type I houses cannot contain solid burning fuel fired appliances
(such as a wood burning fireplace or a kiln). Only direct vented fuel fired
fireplaces are permitted in this category. Type I houses cannot have any
electric heating.
Type II
A Type II house is any house with type I
characteristics that contain solid fuel fired appliances (such as a wood
burning fireplace or a kiln).
Type III
This category includes all houses that
contain natural draft fuel fired appliances or mechanically vented induced
draft non solid fuel fired fire places. The ventilation system for a Type III
house must be designed according to Part 6
Type IV
All houses that contain electric space
heating are Type IV. The exception to this is a Type III house meaning that if
a house is designated as Type III and has electric heating it is still
designated as Type III. If the house does not meet the requirements for a Type
III designation and contains electric space heating then the house is a Type IV
Combustion appliances
Part 9 requirements for ventilation systems
depend on house type. The designation of house type in turn is also determined
by the classification of any fuel-fired combustion appliances present in the
dwelling. Combustion appliances are classified in three ways according to OBC
requirements
- Direct vented – if combustion air supply,
the combustion chamber, and the combustion products vent are
aerodynamically uncoupled from the house, an appliance is said to be
direct vented. This means that combustion air is provided via a sealed
passageway directly to the combustion chamber from the outdoors. The
products of combustion are exhausted directly from the combustion chamber
directly to the outdoors via an independent sealed vent. The combustion
chamber is sealed from the house
- Mechanically vented induced draft – a fan
transports combustion air from the space to the combustion chamber and
forces the combustion products from the combustion chamber to the outdoors
through a dedicated sealed vent
- Natural draft – natural forces (such as
warm air rising, or air flow created by a fireplace) transport combustion
air and combustion products through the combustion chamber and out of the
house via a chimne
Ventilation Requirements
How much ventilation space is required
for an office space in the basement of a house?
If the office area is within the dwelling
unit, (ref.table 9.32.2.1) such as dining rooms, living rooms, bedrooms,
kitchens, combined rooms, dens, recreation rooms and all other finished rooms,
the min. unobstructed area is 0.28 m2 (3 ft2) per room or
combination of rooms.
Mechanical ventilation:
Where a basement incorporates rooms of the
types designated in the table, the assigned ventilation capacities for each
room shall be specifies for those type of rooms. Basement areas used for other
purposes that exceed 2/3 of the total basement floor area shall be mechanically
ventilated with the fan blower having a capacity of 10 L/s (21.2 cfm). Those
that are less than 2/3 of total floor area may be assigned 5 L/s (10.6 cfm)
.(ref table 9.32.3.3). Glass area for rooms of residential occupancy: For the
office room (185 sq.ft) with no electric lighting, the min unobstructed glass
area will be 4 % of area served.
Crawl Spaces
The maximum height of a crawl space is 1800
mm 0r 5’-11”. (Ref.sec.9.18.1.1)
Insulation requierements:
Insulation, air barrier system and vapor
barrier shall be installed in the walls of heated crawl spaces in accordance
with section 9.25.2.1. The min thermal resistance of insulation is to be
installed based on the Degree Day Zones.
Access required:
According to sec 9.18.4.1…….where equipment
requiring service such as plumbing cleanouts, traps and burners is located in
crawl spaces, an access way with a height and width of not less than 600mm (23
5/8”) shall be provided from the access door to the equipment and for a
distance of 900 mm (2ft 11in) on the side or sides of the equipment to be
serviced.
Ground cover:
Ground cover in heated crawl spaces
(sec.9.18.6.1)
Ground cover in unheated crawl spaces
(sec.9.18.16.2)