Follow the basic rules in garage insulation

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This article was published 15/07/2007 (6649 days ago), so information in it may no longer be current.

Q. I can’t be the only person with this question.

I have an uninsulated detached double garage that is about 10 years old. It is a standard lumber yard garage package. I would like to insulate it. Rather than running a bunch of drywall and vapour barrier under the trusses, however, I would like to leave that area open. I would like to have more head room and extra storage space among the trusses.

Therefore, I would be putting the fibreglass batt insulation at the top of the trusses right up against the roof. Then I would install vapour barrier and maybe some wood strips to hold it all up since I’m not concerned that it looks fancy.

Does this present any special problems or concerns?

— Dave Murdoch, e-mail

A: Yes, indeed, this will cause major problems with your roof system if done as you suggest. I will elaborate on this further and offer one alternative that may help satisfy your desire for open trusses in an insulated garage.

The primary consideration for a healthy and long-lasting roof system is to attempt to keep the roofing and components underneath as cool as possible in the winter months. Anything that allows the roof sheathing and covering to remain warm when it is covered with snow will lead to ice formation and premature deterioration.

Typical

This is typically done with a large air space in the attic above the insulation, cooled by passive roof, gable, and soffit vents that let cold, winter air flow underneath the roof. When done properly, this conformation will prevent excess melting of the snow on the roof and ice formation. The installation method you suggest will be completely contrary to this healthy situation.

What you are proposing is to install insulation directly underneath the roof sheathing, which is the worst possible location. Fibreglass insulation works by trapping warm air, helping prevent it from escaping to the unheated area above. Because of this property, the insulation itself can remain fairly warm throughout much of its thickness. In normal installations, this is desirable as it can prevent condensation of the moisture in this warm insulation, which may lead to moisture problems and mould growth. When there is a large, vented air space above this insulation, most of the moisture that may condense within the insulation will eventually evaporate and be removed from the attic by the air movement through the vents.

In your proposal, any warm, moist air that leaks through the vapour barrier will remain trapped in the insulation, likely condensing and freezing when it hits the cool underside of the roof sheathing. This moisture will have nowhere to go, but into the wood fibre-based roof sheathing. This will certainly lead to rot and mould growth, and the air-vapour barrier installed underneath will prevent this moisture from escaping back into the garage as the weather warms, making the situation worse. Your method will also warm the roof sheathing and the shingles above, causing the snow to melt and run down to the cool eaves, where it will freeze and cause ice dams. These ice dams will cause excessive, premature wear of the shingles at the bottom of the roof, which will eventually lead to leakage and further damage to the roof sheathing and fascia and other wooden roof components.

To make a long story very short, your method of insulation is the absolute worst thing you can do in your newer garage. I would assume that you want to insulate the roof to allow heating of the garage for the cold months and prevent heat loss. If you are not heating the garage, the damage will be lessened, but then there is little reason to insulate. If you want to heat your garage in the winter, proper insulation of the walls and ceiling is a very good idea to prevent heat loss, but it must be done properly.

One method to insulate the underside of top cords of the trusses, which may give you the desired room you seek, is to insulate with rigid foam insulation rather than fibreglass batts. While not ideal, this may be accomplished by installation of this rigid material on the underside of the top cords of the trusses, not underneath the roof sheathing. This will provide a minimum of three and one half inches of space for reasonable ventilation above the insulation. While not ideal, this method may work if the garage is to be periodically heated and the insulation is well-sealed, covered and continuous soffit and roof venting is installed.

If you try and insulate with the later method proposed it is absolutely critical that the entire soffit areas be vented, between each set of trusses. Also, a continuous ridge vent will be required for installation at the peak of the roof to ensure proper air movement between each truss from the soffit vents. A polyethylene air-vapour barrier should also be installed under the insulation unless extruded polystyrene sheathing, or other specialty insulation sheathing, is used and very well sealed at all seams and gaps. Air sealing this material with either method will be difficult due to the design of the web trusses, but may be reasonably well done with lots of patience. Once complete, the insulation and vapour barrier should always be covered with sheathing such as drywall, plywood, or OSB for safety in case of fire. Burning foam insulation can be quite toxic and sheathing installed underneath will slow ignition time, should a fire occur.

To address your inquiry, directly, insulating in the way you suggest will only lead to problems. If your desire is to heat your garage, to allow a warm workspace in the winter months, the attic and wall insulation and ventilation must be properly installed to prevent major moisture damage and deterioration to the roof system.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.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.