Commentary on the Code
NBC Clause 18.104.22.168.
Questions have been raised as to why sub-clause 1) says “Combustible insulation, other than foamed plastics” and sub-clause 2) says “Foamed plastic insulation” whereas sub-clause 3) and sub-clause 4) only say “Combustible insulation”.
Some have taken this to mean that sub-clause 3) and sub-clause 4) excludes foamed plastic insulation. This is incorrect.
When the Canadian Codes Centre of the National Research Council of Canada was contacted, they advised that we should read the clauses as written and not try to add anything to them. They pointed out that sub-clause 1) allows combustible insulation with a flame spread rating of not more than 25 to be used without a thermal barrier whereas foamed plastic insulation, even though it may have a flame spread rating of not more than 25 (sub-clause 2) still requires a thermal barrier.
Sub-clause 3) and sub-clause 4) does not make this distinction as ALL combustible insulation (which includes foamed plastic insulation) which has a flame spread rating of more than 25 but not more than 500 shall be protected by a thermal barrier. The Canadian Codes Centre advised that "Combustible insulation" is not a defined term in the NBC, but the adjective "combustible" is. Words not defined in the NBC are to be understood in the dictionary sense.
So foamed plastic insulation whether the flame spread rating is not more than 25 or more than 25 but not more than 500 require a thermal barrier. The Code does not provide a reason why foamed plastic insulation has been singled out, normally all products are treated equally.
The difference between sub-clause 3) and sub-clause 4) is that 3) deals with exterior walls and 4) deals with interior walls, within ceilings and roof assemblies.
It is interesting that in the NBC Clause 22.214.171.124.2 (dealing with buildings less than 3 stories) it specifically removes any limitations on flame spread that may be in material standards. As an example, in clause 126.96.36.199.2 it requires spray polyurethane foam to meet the CAN/ULC S705.1 material standard. This standard has a requirement that the flame spread be not more than 500. Clause 188.8.131.52.2 essentially removes this requirement.
Any building built in accordance with Part 9 of the building code could have any product which is deemed to provide some sort of insulation to be used no matter the flammability of the product. As there is a thermal conductivity value to napalm, this material could theoretically be used as a thermal insulation in a home or small building. Not something I would want in my personal house.
Keep in mind that the NBC is a model code only. The authority having jurisdiction shall be contacted to provide their opinion on the requirements of NBC Clause 184.108.40.206. The authority having jurisdiction can incorporate additional requirements or have a different opinion than the Canadian Code Centre. The reason we contacted the Canadian Codes Centre of the National Research Council of Canada was to determine the original intent of this clause and provide that information to the user.