Rigid Insulation on Joists

Verify that installation area is free of:

active water leaks, fuel leaks (i.e., gas, oil, propane), and pest intrusions

energized knob and tube wiring

uncovered electrical junctions

improperly terminated devices (ventilation fans, dryers, plumbing stacks, condensate lines, combustion appliance flues/chimneys, etc.)

unshielded high-temperature devices (non-IC rated recessed lights, chimneys, flues, vents, etc.) unless they are zero clearance devices

insulation escape openings

Verify that installation area is:

intact, able to support insulation weight, and air sealed

Select insulation that has a flame spread and smoke development index of 75/450 or less and backing material that has a smoke development index of 450 or less when tested in accordance with ASTM E84 or UL 723

Select rigid backing material that:

is a permanent air barrier

will support installed insulation without failure

Select sealants that:

are compatible with their intended surfaces

allow for differential expansion and contraction between dissimilar materials

meet the requirements of the applicable fire safety code (e.g., thermal or ignition barriers)

Remove any obstacles, fasteners, or protruding objects that will prevent insulation from fully contacting the bottom of the floor joist

Install rigid insulation continuously over entire accessible area to prescribed R-value in full contact with the floor joists without gaps, voids, or misalignments

If installing multiple layers, offset seams at least 12" and seal the seams of each layer before applying the next

Where rigid foam plastics are used, in no case will the final thickness exceed the manufacturer's tested thickness used to determine the maximum 75 flame spread and 450 smoke-development index when tested to ASTM E84 or UL 723

Mechanically fasten a continuous rigid barrier to underside of floor assembly that is suitable to withstand weather, moisture, and pest contact, and with a fire-resistance rating equal to the resistance rating of the original floor assembly

Seal all seams, joints, connections, penetrations, etc. in the rigid protective barrier with compatible sealant

Stuff any gaps larger than 1/4" with copper metal mesh or steel wool prior to sealing

Cover exposed corners of the rigid protective barrier with reinforced hardware cloth or metal board bead

Apply a weather-resistant barrier to the exposed side of the insulation (e.g., exterior insulated finishing system and spray-on roof waterproofing over foam)

Separate all foam products from living space with a thermal barrier material (e.g., 1/2" gypsum wallboard) as specified by applicable building code

If code requirements are unclear, consult local code officials for clarification

Post a dated receipt signed by the installer that minimally includes: Installed insulation type, coverage area, installed thickness, and installed R-value