Attic hatch in garage ceiling an issue

Hey there, time traveller!
This article was published 23/10/2010 (5438 days ago), so information in it may no longer be current.

QUESTION: A previous column of yours has me thinking about my own garage. It is not attached, the walls are 10 feet high and I have finished insulating them with R12 fibreglass. The ceiling joists are covered with three-quarter-inch sheeting to provide a floor in the attic storage area. I have no intention of insulating in between the 2x6s forming the top cords of the homemade trusses below the roof sheathing. The roof has two vent caps. Access from below is through a five-foot-square opening that I plan to fit an insulated, two-part door-like affair into. I have started to insulate the ceiling with R24 and I will cover that with vapour barrier and panelling. The walls are covered with vapour barrier and panelling already.

Sometimes I heat the garage with my small construction heater. With what I’ve done already, that takes the chill off enough to work in there in all but the coldest weather. I rarely park in the garage, but that may change. When all is done, I may use it more often in the winter.

Is this arrangement acceptable without constant minimal heating? I could install a fan controlled by a dehumidistat if you foresee a problem. — Peter Knip

ANSWER: Heating an uninsulated garage can cause significant moisture problems and premature deterioration to the roofing. Heating a properly insulated garage with adequate attic ventilation should not be a problem, but there still may be some concerns to address. I will elaborate on those and offer some guidance on proper insulation and ventilation techniques for your garage attic.

Adding insulation and air/vapour barrier to the ceiling of your garage will create the conditions for a proper attic, rather than just an open-truss situation below the roof. If this garage area is unheated, the attic insulation will help keep the garage a little warmer due to natural daytime heating on sunny days. This may only be a minimal advantage but should pose no additional problems for condensation as long as the attic has adequate roof and soffit vents installed.

If you add temporary heat, there is always a chance of moisture issues from condensation after you turn the heat off, but that should be easily managed. Opening the vehicle door and the passage door for a short period of time after you turn the heat off will remove much of the warm, moist air built up during the heating cycle. It will also cool the interior of the garage more quickly, allowing you to notice and wipe up any excess condensation forming on windows or other areas.

The attic should be mostly unaffected by periodic heating as long as it is well sealed, which leads me to the biggest concern with your plan. Insulating and sealing an attic access hatch the size you are planning may be difficult. (I am assuming you are using the large opening for easy access of larger items that may be stored in the attic.)

There are a couple of issues with that proposal. Firstly, the homemade trusses installed may not have sufficient strength to store large, heavy items that can fit through the large access hatch. If overloaded, this can cause bowing and structural damage to the garage roof and ceiling. If you are only storing light items like Christmas ornaments, storm windows and other seasonal items, that should not be a problem.

The main concern I have is with the difficulty in properly air-sealing such a large opening. You must ensure that the entire perimeter of the opening and any gaps in between the dual doors are well sealed with proper weatherstripping, or you may get excessive air leakage in through those gaps. The top of the access hatch should be covered with an adequate thickness of moisture-resistant insulation like rigid polystyrene, which may create very thick, heavy doors. If you are painting or doing other moisture-producing activities in the garage in the heating season, that moisture will find its way through any gaps or poor insulation.

Also, if you open the large access hatch during these activities to get storage in or out of the attic, you will be pumping a large amount of warm, moist air instantly into the attic. This warm air will quickly condense on the underside of the roof sheathing and create frost when the hatch is closed if the temperature is below freezing outside. Having a small, well-sealed hatch is preferable because it limits the potential for air leakage, especially if you do a good job of installing your air/vapour barrier on the rest of the ceiling.

The final consideration for your attic is to avoid a common DIY mistake, and often one made by professional insulators as well. When you install insulation in between the ceiling joists, you must ensure it does not block the area above the soffits. Too often, this area is stuffed with insulation, as amateurs mistakenly try to eliminate air intrusion into the attic at all points. You definitely want to let as much air as possible in through the soffits, which will be necessary to create a good convective air flow to push any warm air that leaks into the attic out the roof vents. The attic insulation must cover the top of the exterior wall plates but should never extend beyond that point.

The easiest way to ensure that this area is not blocked with insulation is to install air chutes and/or insulation stops around the perimeter of the attic. These can be homemade out of leftover plywood or OSB sheathing, or purchased at a local home centre. These should be installed in between the rafters around the entire perimeter of the attic, ensuring that a minimum space equal to half the width of the rafter 2x4s is left above these components.

If all this is done well, and the hatch well-sealed, the current roof vents should be adequate to handle normal moisture that may gain entry to your attic from your periodic heating before it condenses and creates moisture problems.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors-Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at (204) 291-5358, or check out his website at www.trainedeye.ca.

trainedeye@iname.com