Question: I am in an old 1950s house in the forest of New Brunswick and it is -19C outside. The thermometer in the attic registers -13C. That isn't too bad of a difference, considering how cold it is out?
The thing is, the sun heats up the metal roof during the day and the temperature in the attic goes to 4C. Then at night, the cold air comes in through the soffit vents and condensation forms. I get a very thin layer of frost in some places. Sometimes it melts and looks a bit wet. It is not thick frost. Is there really much I can do about it?
If I get obsessive on sealing everything, it can only help so much. I put Styrofoam on the back of the attic hatch and outlined it with weather stripping. That is where the melt was visible, above the hatch area. Is a roof turbine a good idea? Thanks, Ewan
Answer: A small amount of frost on the underside of your roof sheathing, or on the framing, in really cold weather is not normally something to freak out about. Sealing up your attic hatch, and other gaps that may allow warm air to leak into the attic, may help minimize the frost. Improving living space and attic ventilation should also help, but installing devices like wind turbines are not necessary.
I frequently get inquiries from worried homeowners who have the idea that looking into their attics in the dead of winter will provide some valuable insight into the health of their home. They will most often find evidence of something of concern, which prompts the call or e-mail. Often, seeing snow-like frost in some areas of the attic is the issue. While not desirable, a moderate amount of this in an older attic in January or February is quite typical. As long as it is no more than a couple of centimetres thick, and not covering every visible surface, there should be little to worry about. Despite this assurance, it is always a good idea to try and address the causes of the frozen attic moisture.
To accomplish this, we must understand the actual factors leading to the frosty roof sheathing. Firstly, the extreme cold temperatures outside the home will quickly cause water vapour trapped in the attic to condense and then freeze. The condensation normally occurs on the colder surfaces in the attic, namely the underside of the roof sheathing, the framing in contact with this surface, and roofing nails. Because these rigid surfaces are the closest to the outside frigid environment, they will lose any heat energy they contain very quickly. Especially metal fasteners like your roofing nails, which are very good conductors of heat, may be the first to show frost development. Unfortunately, we have little control over these components, or the weather, so there is nothing we can do with these in regards to preventative measures.
The next issue to address is adequate ventilation inside your attic. In an attic that has areas where airflow is limited, caused either by design or limited space between the insulation and the roof components, condensation is much more likely. Since we know that condensation from dissolved moisture in the air is the cause of your troublesome frost, improving airflow should help with our efforts. A good attic will have enough soffit and gable ventilation to passively allow dry, outside air to blow into your attic. This airflow should be enough to force warmer, wetter air to rise up and ultimately out of the roof vents near the peak, before it cools enough to condense. In a perfect world, and a well designed and constructed attic space, this should be enough to eliminate any frost buildup. This is not normally the case with older homes, which may have had additional attic insulation installed that can blocks these vents. So, adding more passive vents, when the weather allows, in these two critical areas may help with next winter’s issues. Whirlybird roof vents are a gimmick, which are costly and often don’t work as designed, so additional regular passive roof vents, especially near the middle of the roof, may do the trick.
The next, and often most important, factor to address is the warm, moist air that gets into the attic. This comes from the heated living space, for the most part, so efforts to prevent this leaking into the attic will be paramount. As you have already noticed, the area of worst frost and melting was above the attic access hatch, where the largest amount of air leakage likely occurred. Your recent hatch insulation and weatherstripping efforts are exactly the right thing to do. That type of air sealing should also be done at any ceiling light fixtures, junction boxes, exhaust fans, or other components that penetrate the attic floor. Caulking, weatherstripping, and blow-in foam from a can are tools that can be used in this effort. Also, reducing the relative humidity inside the home, especially in the dead of winter, will help minimize the attic condensation issue. This can be done by increased use of exhaust fans, HRV’s, or other mechanical ventilation inside the living space.
Brutally cold winter conditions in our Canadian environment may almost certainly lead to some frost buildup in your older attic, but that is not something to panic about. Air sealing your attic hatch, light fixtures, and other ceiling protrusions along with improved attic ventilation is the right approach. Lowering the relative humidity inside your home should be the final piece of the puzzle to help prevent a typical, minor seasonal issue from becoming a more serious problem.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (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 trainedeye.ca.