Final answer:
The type of membrane required for open web truss or I-Joist floor assemblies on the underside is either a vapor barrier membrane or a thermal insulation membrane.
Step-by-step explanation:
The type of membrane required for open web truss or I-Joist floor assemblies on the underside is a vapor barrier membrane or a thermal insulation membrane.
A vapor barrier membrane is a material that prevents the movement of moisture vapor from the interior of a building to the floor assembly. This is important in order to prevent condensation and moisture issues that can lead to structural damage or mold growth.
Alternatively, a thermal insulation membrane can be used to provide insulation and improve energy efficiency. These membranes can be installed on the underside of the floor assembly to help reduce heat loss or gain.