Wet Insulation: Dry It or Replace It?

With wet insulation, replace or dry depends mostly on the type. Cellulose and soaked fibreglass batts are replaced. Closed-cell spray foam and rigid foam board usually stay. Mineral wool and lightly damp fibreglass can sometimes be dried, but only if the water was clean and the wall is open. Any insulation wet with sewage or floodwater is removed, whatever it is made of.

Why wet insulation is a problem

Insulation works by trapping still air. Fill those air spaces with water and it stops insulating. That is the smaller issue. The bigger one is what the insulation does to the wall around it.

A wet batt is a sponge pressed against the studs, the bottom plate and the back of the drywall. It holds water there for weeks. In Ontario, exterior walls and finished basement walls normally have a poly vapour barrier on the warm side, directly behind the drywall. That sheet is very good at stopping moisture, in both directions. Water that gets into the cavity has almost no way back out. This is why a wall can look perfect from the room side and be growing mold inside.

So the question is usually less about saving the insulation, which is cheap, and more about getting the framing dry.

Wet insulation, replace or dry: a guide by type

• Fibreglass batts. The glass fibres do not absorb water, but the batt holds a great deal between them, and it slumps and compresses when soaked. A batt that is only damp, from clean water, in an open wall can be dried. A soaked one is replaced. Paper-faced batts are replaced, because the paper supports mold.
• Mineral wool batts. Denser and water-repellent, so water tends to drain through them and they keep their shape. In an opened wall with clean water they often dry well and can stay or be set aside and reinstalled.
• Cellulose, blown or dense-packed. It is made from recycled paper. It soaks up water, gets heavy, settles and packs down, and it dries very slowly. Wet cellulose is removed.
• Blown-in fibreglass in attics. A small damp patch under a minor roof leak can dry if the leak is fixed promptly. Areas that are matted, compressed or stained are removed and topped up.
• Open-cell spray foam. It is soft and sponge-like and can take on water. It is assessed case by case with a meter, and sections that stay wet are cut out.
• Closed-cell spray foam. It does not absorb water in any meaningful amount. It normally stays. The concern is water trapped between the foam and the wood, so the framing beside it is checked carefully.
• Rigid foam board. The same as closed-cell foam. The board is fine, and the job is to dry whatever is behind and around it.

When it comes out no matter what

• The water was contaminated: a sewer backup, an overflow with waste, or water from outside that crossed soil.
• There is visible mold on the insulation, its facing or the framing beside it.
• It has been wet for more than a couple of days.
• It has slumped, compressed or fallen away from the top of the cavity.
• It sits behind a vapour barrier in a wall that will not dry otherwise.

Insulation removed from a moldy cavity is bagged inside the work area, with containment and HEPA-filtered negative air running, so fibres and spores are not carried through the house.

How wet insulation is found

You cannot see it, and a meter reading on the face of the drywall does not always show it. The usual steps are a thermal camera scan, which shows wet areas as cooler patches, followed by pin meter readings low on the wall and in the bottom plate. If the wall is still in doubt, a small inspection hole behind the baseboard line settles it. Pull out a pinch of insulation. If it is damp, the cavity needs to be opened.

In an attic, look and feel. Wet loose-fill is darker, flatter and clumped. Check the underside of the roof deck above it as well, since that is where the water came from.

Basement walls deserve extra suspicion

The common finished basement wall in this area is a wood stud wall with fibreglass batts and poly, built against a concrete foundation. When water comes in at the floor, or seeps through the foundation, the batts wick it up and the poly holds it in. The concrete side stays cool and damp. Of all the walls in a house, this one is the least likely to dry closed. After a basement leak, plan on at least the lower section of those walls being opened.

What happens after removal

With the insulation out, the cavity is cleaned as needed and the framing is dried with air movers and dehumidifiers until it reads under 16 percent moisture content. The wall stays open and empty until then. New insulation goes in as part of the rebuild, which we refer to local contractors. That is also a good moment to ask your contractor about a more moisture-tolerant insulation for a wall that has been wet before, particularly in a basement.

If you have an open wall or an attic hatch, the simplest first check is a gloved hand. Insulation that feels cool, heavy or damp has a story behind it worth finding.

Common questions