How much attic insulation is needed to be the most energy efficient, and are there moisture issues with insulated attic ventilation?


How much attic insulation is needed to be the most energy efficient, and are there moisture issues with insulated attic ventilation?

Asked by M Glick

Concrete slab, brick home. Hollow block walls covered in mesh, concrete, then plaster. Carpet in bedrooms and living rooms, ceramic tile elsewhere.

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Elizabeth DiSalvo's picture


First of all I will discuss your attic, but then I will talk about the rest of the house.

The amount of insulation you need in an attic depends on the climate that your house is in. You are in southern Illinois where you may not get as cold as Chicago, but you still get pretty darn cold.

  • I would say that you are about on par with where I work in Southern Connecticut, which is Zone 5.
  • In Zone 5, the International Energy Conservation Code calls for R 38 in the 'ceiling'.
  • As designers of energy efficient homes, we try to achieve R values in the roof of about R49 or higher. (Code typically lags behind good practice.)

Your local code may not have adopted the new energy codes yet and may only require R30.

Insulate the roof

You are talking about insulating the underside of the roof (I assume) and not the attic floor. I am a fan of this practice as often we have equipment or storage in our attics. Especially for the equipment, it is best practice to keep things inside of the conditioned envelope.

So my advice regards insulation on the underside of the roof - not the attic floor.

If you insulate the attic floor you do not need to worry about venting the roof as you likely already have attic vents or a ridge vent that will remain open.

Venting the roof

To achieve an R 49 you can take a number of routes.

Yes you do have to be conscious of venting your roof and of moisture issues that may build up in between the roofing material and sheathing.

  • If you use closed cell foam insulation you can blow that right against the underside of the sheathing and not worry about venting.
  • The thermo-dynamics of this method greatly reduce any chance for issues from not venting and this is an accepted method by building inspectors.
  • If you use open cell foam or any sort of batt or blow in insulation you will need to vent the roof.
  • This can be achieved easily by installing venting baffles from the inside/ underside of sheathing.

These venting baffles are manufactured items that fit between the roof rafters and don't allow the insulation to be installed directly against the sheathing, thus leaving an air space under the sheathing that vents the roof. You have to make sure that air can flow top and bottom of these baffles. The baffles have names like Raft-r-vent.

Insulation options

The size of your rafters will limit the amount of insulation you can put into them.

Closed cell foam will get you the most R value per inch - about R 5.5 per inch.So if you have 2x8 rafters ...which are about 7.5 " deep you can get 7.5" x R5.5= R41.5. Very good. Not R49 but a better than code R38.

The open cell foam, blown in and batt insulation will get you more like R3.6 per inch. So 7.5"x R3.6 = R27. Not enough. You could add rigid foam boards across the underside of the rafters. An EPS board will give you about R5 per inch. So if you add an inch (R5) you get to R32. 2 " gets you R37.

You can price out options and decide which way to go. The closed cell foam also gives you somewhat of a vapor barrier and an air infiltration barrier. Don't forget to insulate the gable ends! And really focus on the eaves!

Insulate your walls

Now, regarding your main house. It sounds like you don't have any insulation in your floor or walls! Getting the roof done is a great start but your next focus might be the walls.

I am not sure how big your house is but one way to address your particular home (CMU with a slab on grade) might be to wrap the entire exterior of the house in foam insulation boards.

  • You could remove the existing exterior mesh (stucco?) and glue on 2 inches (or more) of foam board and then re-stucco.
  • This will help the walls a lot.
  • And if you dig around the perimeter of the house and allow the foam board to either go into the ground (straight down) about 3 feet or horizontally out (just about 6-12" under the dirt) for about 2 feet, you would actually help keep the slab warm.

This works because the earth is always at about 55 degrees in your region. Your slab gets colder around the edges because of surface frost, snow and frigid temperatures. This cold get sucked into the house via the concrete slab. The foam board insulation will help keep all of your concrete slab at least closer to 55 degrees which is much easier to heat.

If you have more question on insulating your walls or slab, please feel free to write me directly or ask the question here at the Green Home Guide.

Best of luck! Elizabeth