A party wall or common wall is built between semi-detached homes and it is built to address two concerns for both sets of homeowners: fire protection, and sound protection between the two units. To conform to Alberta Building Code, a party wall must achieve a Sound Transmission Classification (STC) of at least 50, and a Fire Rating of one hour. Also, there needs to be a one inch clear airspace when two separating walls comprise “the party wall.”
Different building materials and measurements can be used to achieve the bare minimum, which the code requires. Here are a few things to keep in mind about party walls:
First, materials – Some Builders use concrete to construct party walls and others use a double wall made up of 2×4 or 2×6 boards. While concrete is better for fire protection, alone, it actually transmits sound better than 2×4 or 2×6 boards. Standard 5/8″ fire-rated drywall provides 45 minutes of fire protection. Some builders double the drywall in a party wall so that the combination of boards and drywall exceeds the building code requirement for fire protection and also provides a better sound barrier.
Second, insulation – Spray foam of any kind is not allowed in between the walls of a party wall as it cannot be exposed to fire due to the very toxic fumes it emits. A product such as ROXUL Safe‘n’Sound soundproofing insulation bats can be inserted between either concrete or wooden party walls and the drywall in the stud spaces. A one-inch clear airspace is still required between the two homes.
Third, cost – Concrete walls are often considered a preference among Purchasers and a significant upgrade among Builders and they can easily run in excess of $10,000. This cost is often passed on to the Purchaser and reflected in the price.