Is attic insulation necessary if the space is not in use? How does proper attic insulation affect heating and cooling costs? Which insulation materials are the best choice: rock wool, glass wool, polystyrene…? Is it possible to repurpose the insulated attic for storage?
When discussing attic insulation, the focus is often on converting this part of the house into a livable space. However, it’s common for there to be no need to convert the attic for habitation, or the roof slope may be too low and unsuitable for conversion.
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In such cases, the primary goal is usually to install insulation to keep the rooms below the attic warmer and reduce energy consumption, both in winter and summer. It’s worth noting that studies suggest up to 30% of energy used for heating escapes through uninsulated attics.
Insulation Placement: Floor rather than the Roof
When insulating an attic that will not be used, the insulation is placed to protect the space beneath the attic, not just the attic itself. Therefore, insulation is installed on the attic floor rather than the roof structure. This approach significantly simplifies the process.
Start by laying a vapor barrier, or vapor-permeable membrane, on the attic floor. Vapor-permeable membranes allow controlled moisture transmission to prevent condensation but are permeable in only one direction.
Properly install the vapor barrier with the printed side facing you, with overlaps of about ten centimeters, and seal the joints with universal adhesive tape provided with the vapor-permeable membrane. When laying the vapor barrier, take care not to damage it. Wear appropriate footwear, preferably with soft soles, and try to minimize walking on or leaning against the membrane to avoid damage.
Optimal Insulation Thickness
Above the vapor barrier, lay insulation material. In general, the rule for insulation is: the thicker, the better. However, any thickness of insulation is far better than none. For the best insulation effectiveness, it’s advisable to use a layer of rock or glass wool insulation not less than 12 centimeters thick (aprox. 4,7 inch) .
Thicker insulation will reduce heating costs, and you’ll notice a significant difference, especially during hot summer months when the air conditioning will kick in much later.
If you’re using rock wool panels, ensure they are tightly fitted together, leaving no gaps or spaces. If using two layers of thinner rock wool panels, stagger the joints of the upper layer relative to those of the lower layer to further reduce the possibility of air leakage and improve insulation quality. Instead of rock wool panels, you can also use unfurled rolls of glass wool insulation.
When joining the rolls, again ensure the joints are compressed to enhance insulation quality. Another option is to use polystyrene boards. When using polystyrene boards, it’s advisable to tape the joints with universal adhesive tape as rigid polystyrene boards cannot form a tight seal between themselves like rock or glass wool panels.
Protecting Insulation from Moisture
Finally, it’s not mandatory but recommended to lay a layer of waterproof, vapor-permeable membrane over the insulation if there’s a chance of snow drifting onto the roof or water penetrating the attic through any means. Follow the same overlapping and sealing procedure as with the vapor barrier. Install the membrane with the printed side facing you.
This procedure is for attics that won’t even be occasionally used. However, there are attics that aren’t used as living spaces but are practical for storage. In such cases, insulation is installed differently. First, wooden beams are laid on the attic floor.
The spacing of the beams depends on the type of insulation you plan to use. For rock wool panels, the spacing should be one meter, matching the length of the panels. For glass wool rolls, the spacing should be 120 centimeters (aprox. 47 inch), matching the width of the rolls.
After laying the beams and floor, install a vapor-permeable membrane following the same overlapping and sealing procedure used for insulating attics not in use. Then, fill the spaces between the beams with insulation material, and once the insulation is in place, add another layer of vapor-permeable, waterproof membrane.
Finally, lay wooden floorboards over the beams and secure them with nails at beam joints. This way, you’ll have an attic suitable for storage, with the insulation layer protected from mechanical damage. Keep in mind that due to the absence of insulation in the roof structure, this attic may experience high temperatures in summer and below-zero temperatures in winter, affecting the items stored there.