What type of insulation do you recommend for a 1959 tri-level home in Illinois? Our house has cathedral ceilings without attics.

Asked by Yvette
Glenview, IL

We had a complete tear-down/ new roof placed w/ watershield throughout done 5 yrs ago & now are having buckling in roof & cracking, staining in our ceiling in our upper level. The roofer stated the ridge vent provides enough ventilation & the plywood is damaged on the inner side. Our rafters are 8 inches in height. We've gotten mixed recommendations from couple of local insulation companies & some are recommending foam w/ BASF closed-cell 6" R-41 applied after tearing down ceilings. Other is recommending fiberglass R-19 batts 6.5". What do you recommend?



This is a great question concerning a very common problem.

To fully understand the solution you’ll need to have a basic understanding of how the components of the roof structure work and contribute to the problem.

Roof components and their dynamics

Concept 1: Heat always travels towards cold. The energy in the warmed air in your home is always trying to leave your house. The greater the temperature differences between the indoors and outdoors the greater the energy (heat) drive.

Concept 2: Moisture always travels with heat. Any moisture that is in the indoor air of your home will travel with the heat energy towards cold. Both the heat and moisture are traveling convectively.

Concept 3: Dew Point Temperature. When the warm moist air comes into contact with a surface that is cold enough to allow the moisture to condense on the surface (dew point temperature) the moisture will collect on the surface and eventually fall like raindrops.

  • In your case the underside of the roof plywood is cold enough to allow the moisture to condense.
  • The resulting moisture is absorbed into the plywood, insulation (making it less effective) and finally the ceiling drywall. If there is enough excess moisture it might even drip from the ceiling.
  • Micro condensation conditions start at around 15-degrees difference in temperature. The warmer the air the more water it can carry.

Concept 4: Heat energy travels three ways:

  • conduction (through a solid object),
  • convection (in a liquid, air is a liquid) and
  • radiation (with light rays).

In a cold-weather state the greatest type of heat loss is convection. R-value is not an indicator of an insulations ability to stop convection. R-value only deals with conduction.

So the R-value isn’t nearly as important in this situation as the type of insulation.

  • You’ll want to select an insulation that is resistant to moisture travel (migration).
  • Fibrous materials (cellulose, fiberglass, mineral wool, etc.) are ineffective at stopping heat (and moisture) migration via convection.

Concept 5: Any structural cavity between outdoors and indoors must be constructed to reduce the potential for moisture penetration yet be constructed to allow the cavity to dry out when moisture is introduced.

In looking at your roof assembly, the Ice & Water shield is a water barrier. It was installed to prevent water from penetrating the roof assembly in the event that water bypassed the roof coverings.

  • This product is a very effective barrier to moisture.
  • While not installed as a vapor barrier it acts as one when the roof cavity ventilation does not remove excess heat and moisture that passes through the attic insulation.
  • In your case moisture enters the attic but cannot escape the attic because the roof ventilation (while adequate from a code perspective) is inadequate in performance.
  • Since the Ice & Water shield is a moisture barrier vapor cannot penetrate the material from the attic side and evaporate to the outdoors.
  • Visualize a terrarium with a glass top. The flat (ceiling) of the terrarium is always wet because the vapor that condensed on the cooler glass surface cannot escape to the outdoors.


Closed-cell insulation is the only consideration for the condition (and solutions) that you described.

I would caution that I would not recommend spraying the closed-cell insulation directly to the underside of the roof plywood. Doing so will create a different problem. Any moisture that penetrates the roof system could become trapped between the Ice & Water shield and the closed-cell foam. This could result in structural damage that would take years to show up. In addition if you ever had to change any of the roof sheathing you would also have to re-insulate since the closed-cell would be bonded to the plywood that is being removed.

The best option would be to install foam baffles on the entire underside of the roof before installing closed-cell insulation.

  • Conducted heat loss would then have an area to dissipate (ventilation) and any moisture penetration from through the roof assembly would have a path to dry.
  • Don’t be fooled into thinking any fibrous insulation material is effective at solving this problem in the long term.

Best of luck.

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