Requirements for Fire and Sound Resistane: Permeance of Materials, Degree Days and Ventilation
Topic 2 – Requirements for Fire and Sound Resistance; Permeance of Materials, Degree Days, and Ventilation
Fire Resistance
Many buildings are constructed to prevent the spread of fire from one portion of a building to the next. Walls, openings, and floor systems are designed with a specific fire rating which is designated by hours of fire resistance. The NBC specifies wall assemblies and their fire resistance ratings:
***In table 9.10.3.1-A of the NBC, find the minimum assembly allowed to achieve a 1-hour fire rating in a non-loadbearing wall constructed of a single row of wood framing.
Sound Transmission Loss
Transmission loss is a term that describes the number of decibels lost as sound is transmitted through a wall or floor system. The greater the value, the more effective a structure is at reducing the amount of sound that is transmitted through it. The amount of airborne sound that is able to transmit through walls is directly affected by materials and methods the structure is composed of. When sound waves hit a structure, some of them are reflected back or absorbed into the materials. Others find their way through cracks and holes.
To quantify a structures transmission loss, a Sound Transmission Class (STC) rating is assigned. The greater the STC rating, the better a structure is at reducing sound transmission. STC ratings are given with a number value which represents the reduction in decibels. For example, a wall with an STC rating of 40 would reduce a sound from 85 decibels to 45 decibels.
The primary factors that will influence a walls STC rating is the materials it is constructed with. Construction methods such as dividing a wall into independent layers and staggering the studs will also increase its STC rating. Additional methods can also be used:
The part of the wall most likely to transmit sound is the openings. Airtight installation of windows is essential in sound control. Solid core doors equipped with gaskets at the jambs and auto-bottoms will produce a higher sound transmission loss than a hollow core door.
Vapour Permeance
Materials that are meant to restrict the transmission of water vapour are rated in terms of their permeance. The lower the perm rating of a material, the better vapour barrier it is. For example, 6 mil poly has a permeance rating of 0.06 whereas drywall with 2 coats of oil-based paint has a permeance rating of 91-172. As the 6 mil poly has a lower rating, it is a much better vapour barrier.
Degree Days
Degree days is a tabulation of data which is used to express the severity of an area’s climate. For each day that falls below 18°C, an average between the high and low is taken and the difference from 18°C is calculated and added to the total degree days for that year. For example:
Let’s say on April 8, Regina experienced a high of 15°C and a low of 3°C. The average temperature for that day would be = 9°. If we want to calculate degree days, we will take 18° and subtract the average of 9° to give us the degree days value. In this instance, 9 degree days would be added for April 8 (18 – 9 = 9).
Using this process, the higher the degree days value for a municipality, the colder that climate is. For more information, see Appendix C in the NBC Volume 1.
Ventilation
While air tightness in a building’s envelope is essential to prevent heat loss through convection, it also creates the problem of moisture buildup in a building. There are many sources of moisture in a building such showers, boiling water, and even the breathing of the inhabitants. Generally, two types of ventilation are used: natural ventilation such as windows and mechanical ventilation such as exhaust fans and HVAC systems.
Review Questions
Use the CHBA Builder’s Manual and the NBC to answer the following:
- In the NBC, find the degree days below 18° and ground snow loads for the following cities:
- Prince Albert
- Regina
- Saskatoon
- What is the difference between air-borne and impact sound?
- List the STC ratings and Fire Resistance Ratings for the following walls (non-load bearing):
- 2×4 studs at 16” centers, ½” drywall both sides, no insulation.
- 2×4 studs at 24” centers, resilient metal channels one side, ½” drywall both sides
- 2 separate 2×4 walls, 1” apart, separate plates, 3 ½” insulation one side, 5/8” fireguard drywall both sides.
- Wood floor trusses or joists @ 16” centers, 5/8” OSB subfloor, joists insulated, 5/8” fireguard drywall on underside.
- What are the requirements for venting roof spaces according to the NBC?
Topic 3 – Insulation, Air/Vapour Barriers, and Weather Barriers
Insulation
Insulation describes materials that are effective at reducing heat transmission. Most insulators work by using tiny air pockets to prevent heat transmission. These air pockets are a poor conductor of heat and the thicker the material, the more effective it is at reducing heat loss. Insulation comes in five varieties:
Air and Vapour Barriers
Because insulation is a porous material full of air pockets, it readily absorbs moisture from the air. Once that occurs, the insulation loses its thermal resistance as the air pockets fill with water. To prevent insulation from absorbing moisture it needs to be protected by an air/vapour barrier.
While the air/vapour barrier primarily protects the insulation, it also prevents heat loss through convection as it produced an airtight seal inside the exterior walls and ceiling. Polyethylene is the most commonly used material. It is a sheet plastic that is placed on the warm side of all exterior surfaces of a building. It is sealed at each joint with caulking and tape.
Weather Barrier
The weather barrier is placed on the exterior of a building and is used to prevent water from infiltrating into the building envelope. Because the air/vapour barrier is intended to be air tight on the warm side, the weather barrier must be breathable to allow any vapour to diffuse out from the exterior wall cavity.
Review Questions
Use the NBC and CHBA Builder’s Manual to answer the following.
- Loose fill insulation is permitted to be installed in a ceiling sloped not more than?
- If loose fill insulation is installed into an attic space with vented soffits, what must be installed to minimize air flow over the insulation?
- What is the purpose of a vapour barrier in a building?
- What is the purpose of an air barrier in a building?
- Gaskets are used to seal the joints in a building such as under a sill plate. List some materials that can be used for gaskets.
- What is the weather barrier and where is it installed?
- How does a weather barrier differ from a vapour barrier?
- When insulating a roof that is flat or slightly sloped and framed with joists such as 2×10, should the space between the joists be completely filled prior to finishing the ceiling? Why or why not?