Modern Insulation

Air / Vapor Barrier

Air infiltration is almost as important as R-value in the thermal performance of a building. Infiltration of unconditioned ambient air means that heating and cooling systems must expend more energy to compensate for the infiltration.

A common misconception is that vapor barriers are required in all insulation installations. With Modern’s Cellulose insulation, a vapor barrier is not recommended except in very cold weather or high moisture areas such as spas or pool buildings. However, many building codes do require an Air barrier or Vapor retarder to be installed. Check your local building codes for compliance.

Insulation inhibits the movement of air from warm to cold (and vice versa) by trapping air or gas between its fibers or pores. The higher the density of the material used, the better it is at inhibiting the movement of air. Cellulose insulation has a high density compared to fiberglass.


Blow-in Cellulose insulation seals houses better by limiting the air flow, not only through the insulating material, but also around difficult to insulate areas such as the gaps around electrical boxes, wiring and plumbing. Cellulose insulation can also handle non-standard or off-center wall stud spacing areas better than batts.

Field tests have shown that Cellulose insulation can provide a building envelope that is 36% tighter than a fiberglass insulation seal.

To prove the point, Canadian engineers conducted a test on a new cellulose-insulated home for air tightness. First the engineers measured the air leakage with a polyethylene air/vapor barrier around installed before siding had been installed. The engineers then slit the polyethylene air/vapor barrier in approximately 20 places and retested air leakage of the building. There was absolutely no change in measured air leakage. Additional Smoke Pencil Testing at the slits showed “not a breath” of air leakage.