roof insulation vs. ATTIC INSULATION

Without high levels of attic or roof assembly insulation warm air will flow through the ceiling and roof to the outdoors in the winter; similarly, in hot weather conditions, warm air will infiltrate into the living are (attic temperatures may reach 140ºF/60ºC and more).

In other words: home comfort and energy bills depend largely on attic insulation.

High levels of attic (or roof assembly) insulation should be a top priority.

Installing insulation on the sloped roof Assembly (Roof Insulation)

Unless in air-conditioned attics, insulation is typically installed on the ceiling plane – not on the sloped roof assembly. It’s more effective and easier to install insulation on the attic floor than in the roof assembly..

If you do need to install insulation in the roof assembly take into account some rules.

Rrigid foam above the roof sheathing is usually preferable to using a spray foam between the rafters, or below the roof sheathing: it avoids thermal bridging through the rafters, and keeps the sheathing “warm”, an important feature in cold and mixed climates - warm sheathing prevents moisture accumulation and reduces the risk of sheathing rot. 

American building codes require at least R-30 of rigid foam in zone 1 (about 7.5 - 9 inches of expanded polystyrene, 6 inches of extruded polystyrene, or 5 inches of polyiso), R-38 in zones 2 and 3 and R-49 in zones 4 through 8.

You may install part of the foam above the rafters and the rest underneath it (in contact with the roof sheathing), to reduce the thickness of the insulation above the sheathing, as long as you do not reduce too much the top layer.

American building codes address this issue by fixing the minimum amount of rigid foam above the roof sheathing for each American climate zone. In other words, you shouldn’t use all the insulation below the roof sheathing (or between the rafters). You have to use a fraction of it above the sheathing.

Obviously, those who want to thicken the insulation layer under the roof sheathing beyond the minimum code requirements (for more thermal comfort and lower energy bills) will also have to thicken the above-sheathing layer proportionally.

Attic Ceiling Plane Insulation

As mentioned earlier, attic ceiling insulation is easier and less expensive than roof insulation. Installing (or adding) insulation to the ceiling plane is rather straighforward.

And you may use very different insulation materials.

Cellulose and other fibrous materials are good and inexpensive choices. They are a lot better than insulation batts and blankets (fiberglass...), or rigid foams. These ones don’t conform well to irregular spaces, and may become ineffective after some time.

Cellulose is an excellent material. Properly installed cellulose will form a seamless blanket, without voids and edge gap.

Highlights on attic and roof insulation
» Attic and roof insulation is critical to reduce heat loss in winter and heat gains in hot weather
» Consider very high levels of insulation: R-50-60.
» Air seal all the gaps before installing any insulation

Recommended Attic (or Roof) Insulation levels

Energy experts recommend attic (or roof) insulation levels of about R-50 or even R-60 (Metric System: U-0.1), whatever the climate.

R-60 amounts to about 20 inches of cellulose (43 cm), 12 inches of polystyrene (30 cm) or 9 inches (23 cm) of high-density polyurethane.

These values are well above code requirements. In the USA: R-30 in zone 1; 7.5 -9 inches of expanded polystyrene, 6 inches of extruded polystyrene, or 5 inches of polyiso), R-38 in zones 2 and 3 and R-49 in zones 4 through 8.

Attic and Roof insulation Improvements: a DIY job?

Aattic insulation improvements (at the ceiling plane) are a typical DIY job, though a messy and dirty one, with some risks, especially when using spray materials.

Attic Air Sealing
Before installing insulation in your attic you should inspect it and air seal all the existing openings and repair existing gaps or other leakage areas. You must caulk and foam small and medium attic gaps, and apply an air barrier in large attic openings. The performance of the insulation depends largely on a proper air sealing.

Attic Vs. roof insulation Improvement costs

Improving the roof insulation is usuallly difficult and expensive.

Improving attic insulation with cellulose from R-19 to about R-40 may cost you, say, 0.5-$1 per square foot - labor and materials included.

If you do it yourself, the cost can be significantly reduced.




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