The cost of attic insulation varies based on several factors, including the type of insulation material, its thickness, and the finishing layer to be applied over the insulation. The accessibility of the attic also influences the cost – it differs whether it is a house or an apartment on the seventh floor without an elevator.
The complexity of the work can also be a factor; insulating a simple attic with flat surfaces is different from a complex roof with many angles and dormers. On average, installing mineral wool insulation costs up to $2 per square yard, including the installation of vapor-permeable foil.
Styrofoam insulation is cheaper, costing around one dollar per square foot. This only covers the insulation itself; additional finishing work, such as drywall, costs about one dollar per square foot.
What thickness of insulation is needed?
For insulation, the general rule is – the more, the better. With mineral wool, there are several options. The most expensive version, which provides the level of insulation needed for passive houses, involves 10 inches of insulation between the roof beams and an additional 4 inches on top.
This requires a large amount of insulation and structural changes, as beams usually aren’t thick enough to accommodate this amount. Though effective, this solution might not be practical as it reduces valuable attic space.
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Another option involves 6 inches of insulation between the roof beams plus an additional 2 or 4 inches on top. This is a costlier but better compromise, especially for protecting the attic from overheating in summer.
The standard solution is 4 inches of insulation between the roof beams plus an additional 2 inches on top. While the previous options offer better heat protection, this standard solution is the most commonly used in private homes. It is the most cost-effective, requiring less material and fewer structural changes, and it preserves more headspace in the attic.
Can mineral wool be installed over styrofoam and vice versa?
Technically, yes, but some combinations are not optimal. If styrofoam is already in good condition, adding another layer of styrofoam might be better. The biggest advantage of mineral wool over styrofoam is its vapor permeability, but if styrofoam is already in place, this benefit is lost, making the additional investment less justifiable. However, mineral wool is a better sound insulator, so it might be worth the investment in some situations.
Adding styrofoam over mineral wool is not optimal as it reduces the attic’s moisture-wicking ability. It is recommended to use mineral wool again for additional insulation over existing mineral wool.
Why is vapor-permeable foil important? Can it be cheaper without it?
Skipping the vapor-permeable foil or replacing it with regular plastic is not advisable. Vapor-permeable foil plays a crucial role in controlling moisture passage, protecting the insulation from condensation, which can be very harmful to mineral wool.
Can additional insulation be installed on the exterior of the roof?
If the attic is already adapted and inhabited but lacks sufficient thermal insulation, one option is to add insulation on top of the roof. This involves removing the roof tiles or covering material, but it is often a better solution than bringing workers into a finished attic and causing damage to add insulation. Adding insulation from the exterior also preserves headspace in the attic.
Photo: Rockwool