Trained Eye


Adding Insulation


Today we have two related questions on Insulation. They will be addressed together.

Hi Ari:

I've had condensation problems with the cathedral ceiling in my living room for the past 25 years. There’s adequate ventilation and the ceiling was built to common specifications.

Is it possible to fill the 6-inch cavity with Cellu-fibre and treat the ceiling as an exterior wall? Consequently the insulation will be upgrades to R 40 and will no longer allow snow to be blow in from the North, such as happened in 1997.

I enjoy reading your column in the Free Press.


We recently purchased a new home and paid to have the insulation in the attic increased to R40. We now have possession of the home but I would like to check and make sure we actually got our money’s worth. How thick should the blown-in cellulose be for an R40 rating?



The first item to answer for both your questions is the issue of R-value for blown-in Cellulose Fibre Insulation. According to the Canadian Mortgage & Housing Corporation (CMHC), loose-fill Cellulose Fibre or Cellu-Fibre has an R-value of 3.0 – 3.7 per inch or RSI Value 21 – 26 (Metric equivalent). That means that for an R40 rating, blown in cellulose should have a thickness of 10.8 to13.3 inches. The large variability is mostly due to specifics about location of installation and settlement of the insulation. Newly blown-in insulation should have a depth closer to the high end of the range, which will be reduced, over time, due to settlement and moisture absorption.

Cellulose fibre is high quality insulation that is normally blown-in mechanically into an attic or poured in manually from large bags. It is a very environmentally friendly product, as it is made primarily from treated recycled paper. The other advantage, over other loose-fill insulation like fibreglass, is that it may actually improve its performance over time. The insulation will compact, over time, and may lose a portion of its R-value, but it will also increase its resistance to air movement. After being present in an attic for several years, the top surface of this insulation will form a “crust” that will help prevent warm air movement into the attic. Other blown-in insulations do not have this property. That is one reason that Cellulose is preferred by some building envelope and air sealing specialists.

The difficulty in your home, Mr. Lacasse, is that you don’t have a traditional attic space that would allow for 12 inches or more of blown-in cellulose to be installed with a large vented air space above. If you only have 6 inches of space to fill, you may only achieve about half that R-value. If you already have approx. 5 – 6 inches of other insulation, filling up the cavity may achieve near the desired R-value, but may create other problems with a missing airspace. A small, vented section above the insulated portion of your vaulted ceiling may be essential to maintaining a cool temperature on your roof deck. If this is not achieved, then ice damming will be a certainty and premature deterioration of your shingles, and probably condensation and leakage, will be the result.

Research and practical testing over the last two decades have discovered that controlling air leakage from heated living spaces into the attic is just as important, or more so, than controlling heat loss. Simply filling the entire cavity with Cellulose or Fibreglass insulation may not adequately prevent air leakage in an older home. There are too many small gaps around light boxes and ceiling protrusions to prevent this air leakage. A 25 year old home, such as yours, should have a Polyethylene air-vapour barrier installed, but it may be poorly sealed and have many holes from nails, screws or other fasteners that would allow warm air to leak into the insulated space above. The result of this will be condensation and frost, which may damage insulation and ceiling finishes, or in the worst cases cause rot and mould to grow in the wooden ceiling joists. This could have disastrous results, as you can imagine.

Your statement about your vaulted ceiling having adequate ventilation is partially incorrect. If you have had condensation problems for 25 years, then there is either inadequate ventilation or excess air and moisture leakage to this area, or probably both. This will cause the problems you are experiencing, condensation and ice damming. The solution is to strip off the roofing and sheathing and air seal the surface of the ceiling. It is now possible to accomplish this by using blown-in foam insulation at any joints, protrusions, gaps or points of possible air leakage. The majority of the cavity is then normally filled with cellu-fibre and a small, vented space is left under the roof sheathing. The entire cavity could be filled and the ridge vent (I am assuming that there is one that the snow blows into) could be eliminated, but the entire cavity may have to be filled with foam, at very high cost. If you filled the cavity completely with cellulose, it may settle and leave gaps, which could lead to some problems down the line.