Wet insulation: does it need to be replaced?

Why wet insulation is a problem

Insulation works by trapping still air. Fill those air spaces with water and three things go wrong.

• It stops insulating. Water conducts heat far better than air.
• It holds water in place. Wet insulation acts like a sponge pressed against studs, sheathing and drywall. The cavity stays damp long after the surface feels dry.
• It gets heavy. Soaked insulation sags, slumps down the wall and can bring a ceiling down.

Mold needs about 24 to 48 hours of dampness to begin. A closed wall with wet insulation in it can stay damp for weeks.

Type by type

Fibreglass batts and blown fibreglass

The glass fibres themselves do not absorb water or feed mold. But the batt holds a great deal of water between the fibres, the paper or foil facing on some batts does support mold, and dust in the batt does too.

• In an open attic, lightly damp fibreglass from a small leak can sometimes be dried if it is spread out, fluffed and ventilated quickly.
• In a closed wall or ceiling, it does not dry in any useful time. It is removed.
• Compressed or matted batts do not recover their thickness and are replaced.
• Contaminated by sewage or flood water: removed.

Cellulose

Made of recycled paper treated with fire retardant. It absorbs water readily, compacts, and loses its loft. The fire-retardant salts can wash out or corrode metal fasteners and wiring. Wet cellulose is removed.

Mineral wool

Rock and slag wool repel water better than fibreglass and drain more readily. Batts that were briefly wetted with clean water and can be taken out and dried may be reusable. In a closed cavity the same rule applies: open it up.

Closed-cell spray foam and rigid foam board

Closed-cell polyurethane foam, extruded polystyrene and foil-faced polyisocyanurate absorb very little water. They generally stay in place. The concern is water trapped between the foam and the framing or foundation, so the surfaces and the wood are checked with a moisture meter and dried.

Open-cell spray foam

Soft and sponge-like. It absorbs water and holds it. It can dry if exposed to air, but in practice it often has to be cut out to let the framing dry.

Expanded polystyrene

Bead board takes up some water over time. It is assessed case by case.

Vermiculite

Loose grey-brown or gold granules in older attics. Treat it as containing asbestos. Do not disturb, shovel or bag it yourself, wet or dry. See vermiculite insulation removal.

By location

Walls

Water inside a wall runs down and collects at the bottom plate. Insulation wicks it sideways and upwards. Standard practice after a clean-water leak:

1. Remove the baseboard.
2. Check with a moisture meter how high the wall is wet.
3. Cut the drywall above that line, commonly at about half a metre or a little over a metre so that standard sheets can be used for the repair.
4. Pull out the wet insulation.
5. Dry the studs and plates with air movers and a dehumidifier.
6. Re-insulate, restore the vapour barrier and close up once readings are normal.

Outside walls in Ontario have a polyethylene vapour barrier behind the drywall. It stops the cavity drying towards the room, which is another reason wet insulation in an outside wall has to come out.

Ceilings and floors between storeys

Insulation between floors is often there for sound. Wet, it holds water on top of the ceiling drywall. The ceiling is opened and the insulation removed.

Attics

• Find and stop the source: a roof leak, an ice dam, condensation or a disconnected bathroom fan duct.
• Remove insulation that is soaked or matted. Leaving it keeps the ceiling drywall and joists wet.
• Lightly damp areas can be raked open to dry once the source is fixed.
• Check the roof deck and rafters for mold.
• Replace to the original depth or better. Take the chance to seal air leaks in the ceiling first.

Basements

Basement walls are often insulated with fibreglass against the concrete behind a framed wall, which copes badly with any water. After a basement flood, the lower part of the wall is opened and the insulation removed. When rebuilding, rigid foam or closed-cell spray foam against the concrete is the more water-tolerant choice.

Crawl spaces

Fibreglass between the joists of a damp crawl space absorbs moisture, sags and falls. It is removed, the space is dried and the moisture source dealt with before anything is put back.

Can it be dried in place?

Sometimes, where all of these are true:

• The water was clean
• It is caught within hours
• The amount is small
• The cavity can be opened to air, or is an open attic
• Readings confirm the framing and insulation are dry afterwards

Restoration contractors do have ways to push dry air into wall cavities through small holes. These work better for drying the framing and drywall of an uninsulated interior wall than for drying insulation in an outside wall.

How to tell it is wet

• A moisture meter on the drywall and, through a small hole, on the studs
• A thermal camera, which shows wet areas as cooler patches
• A small inspection hole to look and feel
• In an attic, insulation that is darker, flattened or crusted

Insulation can be wet with nothing showing on the painted surface.

Handling it

Wear gloves, long sleeves, eye protection and a respirator rated for fine particles. Fibreglass irritates skin and lungs, and wet insulation may be moldy. Bag it in the room and carry it out sealed. In older houses, be alert to vermiculite and to asbestos in other materials you may cut through.

Insurance

Where the water damage is covered, removing and replacing wet insulation is a normal part of the claim.

More: water damage restoration.