When installed properly and with sufficient R-value, almost all insulation will reduce condensation. For insulation like fiberglass to be effective in cooler climates, a vapor barrier is installed on the warm side of the wall. Only a few types of insulation will stop condensation completely without additional barriers.
What Causes Condensation?
Condensation is the process that changes the water vapor in the air into liquid water. In a house, this occurs when warm moist air meets a cold surface or cold air.
Warm interior air that gets into stud cavities passes through most insulation as it moves toward cooler areas. Condensation forms on the interior of the wall sheathing. OSB sheathing compounds the problem because a 1/8th” expansion gap is left between sheets. Cold air that penetrates the exterior finish can enter the stud cavities.
Condensation can be absorbed by insulation–reducing its effectiveness. It can also run down the wall, puddling on the framing and floor. Wood that is constantly wet provides The condition for mold to grow on insulation and the framing around it. Eventually, rot sets in.
4 Types Of Insulation That Stop Condensation
The most effective method of preventing condensation is to stop warm air from meeting cold surfaces. Insulation that fills or seals gaps and cracks is the best choice. To be effective it also must be waterproof.
Spray Foam Insulation
Closed cell spray foam insulation does not absorb moisture. When applied properly, it seals all gaps, cracks, and penetrations in the exterior sheathing. Moist interior air cannot pass through the foam to the cold exterior sheathing–eliminating condensation.
Open cell spray foam insulation seals gaps and cracks but will absorb moisture over time. It is not quite as effective as closed cell foam.
Rigid Foam Insulation
Extruded polystyrene insulation and polyisocyanurate insulation are waterproof. Installing rigid foam board insulation between the studs prevents moist air from reaching cold exterior sheathing. It is only effective if the gaps are sealed with spray foam from a can or acoustic caulking.
Rigid foam is also installed on exterior walls to form an insulation blanket and cover gaps. It may not be quite as effective because cold air could get behind it and into the stud cavities.
Extruded polystyrene rigid foam absorbs moisture–making it slightly less efficient.
Cementitious Foam Insulation
Cementitious foam insulation does not shrink and is completely waterproof. When sprayed into stud cavities, it fills all gaps and cracks. It does not sag when installed under slight pressure–eliminating air gaps. It is best used for insulating wall cavities without removing drywall.
Installing a thin layer of spray cork insulation in stud cavities before fiberglass batts seals the gaps and cracks and prevents moist air from reaching the cool exterior sheathing. The cork provides a waterproof barrier that will not shrink or slump.
Cork boards up to 12” thick can be installed on exterior walls to stop warm air from meeting cold surfaces. The boards are shiplap but sealing them with spray foam or flexible caulking prevents any cool air from leaking through.