Ventilation key when increasing insulation

Hey there, time traveller!
This article was published 09/12/2001 (8744 days ago), so information in it may no longer be current.

QUESTION — You performed a home inspection for us earlier this summer. One of the things that you recommended was that we add some insulation and ventilation to the attic. I am preparing to contact a few firms for estimates and thought I would seek your input. I am leaning towards cellulose blown-in insulation for environmental reasons, but the home currently has blown-in fibreglass. Is mixing the two materials acceptable or recommended?

Thanks again for your comments during the inspection. I am using the report you gave us as a guideline for prioritizing home renovations/maintenance projects.

I look forward to your reply,

— Michael Fernandes e-mail

Answer — Increasing insulation in an older home is a good way to reduce heating costs and make the home more comfortable, but will take several years to pay back the initial cost. If a home’s attic has very minimal insulation, adding insulation will be highly recommended to prevent heat loss and damage to roof components due to heated air escaping into the attic. Care must be taken to adequately increase the ventilation when the insulation level is increased. Blown-in insulation is often the most cost effective and least disruptive way of adding insulation to an existing house. Loose fill insulation can often be blown-in through roof vents from the exterior or from an attic access hatch inside the home.

The two most common types of insulation are cellulose and fibreglass. The fibreglass has a wool-like appearance and is often white, pink, or yellow and is a made from glass fibres. It is fire resistant and withstands moisture well. Cellulose fibre insulation is usually grey in colour and is made from cellulose fibre, mainly from recycled paper and newsprint. This is the environmental reason for choosing this insulation, stated by Mr. Fernandes in his question. Cellulose is often treated with a fire retardant and is not very resistant to moisture. Both kinds have similar R-values per inch and will have some settlement over time, which will reduce their effectiveness somewhat.

I see no reason that mixing the two types of insulation will have any detrimental effects and have often seen cellulose blown-in over existing fibreglass batts. I have not seen any research done on mixing the two types, and also have not read any notices of any concern on many Web sites, forums and literature dedicated to building science. The only drawback I can think or is that cellulose may be more dense that the fibreglass and may compress the older fibreglass more than adding more loose fibreglass.

Before any extra insulation is added, the area in the attic above the soffits should be inspected to make sure allowance has been made to allow sufficient airflow after installation. Cardboard or foam air chutes should be installed which will prevent the insulation from touching the underside of the roof sheathing between the rafters or trusses in this area. If installed properly they will also prevent the insulation from filling up the soffit area as well. A minimum airspace of two inches below the roof sheathing is recommended to allow air from the soffit vents to enter the attic space.

Additional roof or gable vents may also be required to allow better airflow in the newly insulated attic. As always, get more that one quote for the work and ask friends and neighbours who have had insulation increased for recommendations on hiring specific contractors.

I received an interesting follow-up e-mail about my November 4th column that may provide a solution to that person’s question. Thanks to Ms. Cels for her input. Here is her response:

“A few weeks ago I read in your column about a problem someone was having with an odour coming from his kitchen sink. We had a similar problem earlier this year. The odour came from the kitchen sink, and only the kitchen sink. We tried all the remedies you mentioned in your column without success. Finally, we disconnected our sink from the trap and washed out the pipes and trap. When we went to reconnect everything, we discovered that the neck of the drain basket in the sink had corroded. This allowed water and waste to get between the layers of the basket. The bacteria and debris trapped in this space was the source of the odour. We changed the basket and the problem was solved. Please suggest this to your reader.”

— Charlotte Cels e-mail

Ari Marantz is owner/inspector of Trained Eye Home Inspection and is the PR Rep. for the Canadian Association of Home Inspectors — Manitoba (www.cahi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com