Cost of adding insulation to a low-pitch roof is steep

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This article was published 04/12/2005 (7247 days ago), so information in it may no longer be current.

QUESTION — When we purchased our home three years ago, our home inspector suggested beefing up our attic insulation. He rated it at approximately R20. I climbed up there the other day, and discovered that I only have about two feet of room between the existing insulation and the peak of the roof, due to the low pitch of the roof. This, of course, decreases as one moves out to the eaves. There is batt insulation up there, but it appears not to be a nice smooth blanket, but rather bunched up and uneven. Do you have any ideas how I can get some more insulation up there without crawling around on my stomach for the entire weekend? I have fairly new shingles so I’m not keen to rip off the roof deck.

–Cam Bennet, Dauphin

ANSWER — This is a timely question with cold weather closing in and Manitoba Hydro’s newly announced incentives for upgrading home insulation. You have a common dilemma with your low-slope roof and I will offer some guidance, but a solution may be very difficult in your case.

Low-slope roofs are quite problematic to insulate to today’s standard of R40 or higher. This is due to the limited space between the underside of the roof sheathing and the ceiling. You may have approximately two feet at the peak, but this will be reduced to a matter of inches by the time you approach the eaves. This makes insulating after the roof has been completed a difficult to impossible task, as you have observed. That is likely the reason that you have around R20 or approximately five to seven inches of insulation only in your attic.

Typical batt or loose-fill insulation such as fibreglass or cellulose fibre may have a rough R-value of R3 to R 4 per inch, which would required approximately 10 to 12 inches of insulation to achieve the desired R40 attic insulation level. Not only would you have difficulty with installation in your attic, you could create a potential problem by adding this much insulation. When you increase the depth of the insulation in the attic, you normally require additional ventilation. This is to compensate for the reduced space now available for cool air movement in the attic. Also, the attic temperature should be reduced after upgrading and removal of moist air will be more difficult. Adding more soffit and roof vents is often simple in a home with a typical roof pitch, but may not be possible in your situation. If you increase the depth of insulation to 10 inches at the eaves, there will be little or no room for airflow from the soffit vents, where there may be adequate space right now. Even with the addition of continuous soffit venting and air chutes, the insulation may be compacted in this area and not be effective.

Block air

The real problem will occur if insulation is stuffed or blown in and touches the roof sheathing near the eaves. This will not only block air from soffit vents, but may also trap moist air in the insulation and cause moisture damage to the roof sheathing. This may also reduce the life of the roofing and cause premature curling and ice damming. There may be a couple of solutions to your situation, but neither is simple or non-invasive like typical insulation upgrades.

The first option is to add more insulation on the underside of the ceilings. This can be accomplished with the use of rigid foam insulation added to the bottom of the ceiling joists or trusses. To accomplish this, the ceiling plaster or drywall will have to be removed prior to insulating. This is a difficult and disruptive method for insulation upgrading and is often only employed when the ceilings have moisture damage or excessive cracking. Once the rigid insulation is installed, the ceiling must be drywalled and taped to finish the job, which requires additional cost and labour.

The final option is to remove the existing batt insulation and have blown-in foam insulation installed in its place. This material has a much higher R-value than fibreglass and provides an excellent air-vapour barrier, which may not be present in your home. It may be possible to install enough of this type of insulation to achieve the desired insulation level for your purposes. Also, the virtual elimination of air movement into the attic will minimize the additional ventilation required with traditional insulation upgrades. The main drawback of this method is that portions of the roofing and roof sheathing will likely have to be removed for access for installation. The cost of this material is substantially higher than typical insulation, and labour will be considerably higher due to the removal and replacement of the roofing.

I wish there were an easier solution to your problem, but the best one may be very costly. You should be applauded for your desire to reduce the amount of heating required by adding attic insulation, but this must be weighed against the cost of the upgrades. In your case, the costs of upgrading may outweigh the benefits, at least until your roofing wears out and needs replacement.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property 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