Trained Eye


Fibreglass Over Cellulose In Attic


We would like to know if we can install pink fibreglass batts in our attic over existing blown-in cellulose. We were told that the weight of the batts would reduce the insulating value of the cellulose as it would compress it and reduce the air pockets. The value of the existing insulation is R40. What would you recommend? We were thinking of adding R20 to R30.

Thank you for your time.


There may be some concern over compression of the cellulose insulation in your attic, depending on how long since its installation. If the cellulose was blown in more than two or three years ago, there should be little concern. If the cellulose is newly installed, then the compression factor may be significant.

To put it simply, insulation works by trapping air within the material being used. The larger the capacity for trapping air, and preventing air movement, the higher the R-value or insulating capacity of the insulation. This is why loose insulation, like cellulose fibre, should have its maximum insulation value when it is not compressed. Shortly after the cellulose fibre insulation is blown into an attic, it has its maximum thickness, which will be reduced over time. Within a period of months and years, the insulation will naturally compress, and reach its normal level, if undisturbed. This level will be dependent on a number of factors, such as initial thickness, presence of air-vapour barriers in the ceiling, ventilation in the attic and Relative Humidity and temperature in the attic.

Placing additional weight, such as that from Fibreglass batt insulation on freshly installed cellulose is not recommended, as it may cause the insulation to compress unnaturally, and lose some of its effectiveness. Once the cellulose has been in place for several years and is settled, it normally develops a crusty surface, which may help resist further compression. Fibreglass batts are relatively light in weight and installation over this settled cellulose fibre insulation would distribute the weight over a large area. This should not have a large effect on the performance of the original insulation.

Like many building science principals and theories, there are inherent environmental problems when these theories are put in place in the real world. This may be true in this situation, if you attempt to add the Fibreglass batts over the original insulation, as suggested. The compression factor from the new insulation should be minimal, but compression from the installers working in the attic may be extensive. Once insulation is blown in to an attic to a level of R40, as in your home, it will cover then entire top of the ceiling joists, or bottom truss webs, by several centimetres. Gaining access to the attic to add batt insulation without walking over the old insulation, and disturbing it substantially, may be impossible. This disturbance may compress the cellulose much more than any other factor, and significantly reduce its effectiveness.

For this reason, Fibreglass batt insulation is not normally installed over top of loose-fill insulation. It is very common to see the reverse situation as additional insulation is blown in through attic vents, or the access hatch, on top of older fibreglass batts. This is normally done when the original insulation level is low to moderate, and will generally be attempted to raise the insulation level to R-40 or greater. I would suggest that this will be a better alternative than batts, but may be more costly, depending on the design of the home.

An attic insulation level of R-40 is considered to be adequate and additional insulation may lower the heating bills, but may not be necessary or cost effective, in the short term. In the longer term, the dollar saving from the additional insulation will pay for the cost of installation, but this may take many years. If the current insulation level was low, then adding more would be advisable, but this is not true here. If you do decide to increase the thickness of attic insulation, caution over one major concern should exercised.

Often, when additional insulation is added by homeowners, and sometimes by professionals, current attic vents are blocked or partially blocked. This is most common at the edges of the attic where soffit venting is normally located. When insulation is blown in, cardboard, plywood, or foam insulation stops should be installed in this area to prevent blockage of the soffit vents. These simple and inexpensive items are normally stapled to the rafters or trusses and prevent the insulation from filling the area above the soffit. When installed correctly, they allow at least 5cm of air space above the insulation, below the roof sheathing. This last point is critical, not only to stop blockage of the soffit vents, but also to prevent moisture damage to the roof sheathing.

Before investing in increased insulation in your attic, I recommend that you attempt a rough estimate of the heating cost saving for the added insulation vs. the installation cost, to see if it makes good economic sense. Information on this may be obtained from the Office of Energy Efficiency, Manitoba Hydro, or CMHC. Often, concentrating efforts on caulking and weatherstripping doors and windows, light fixtures, exterior wall outlets, and other areas of air leakage, will yield a bigger heat saving than adding attic insulation, at much less expense.