Question: I recently had a new furnace installed, as well as my old vermiculite attic insulation replaced with blown-in fibreglass insulation.
Will I have a problem with condensation in my attic due to higher humidity in my house because there is no vapour barrier on my ceiling?
I have a ’50s bungalow in Transcona and the old barrier, I am assuming, was the newspapers that lined the rafters.
My plan is to strap the ceiling with three-quarter-inch-thick boards and have a layer of thin Styrofoam between the existing ceiling and newspaper vapour barrier.
Then, I would drywall over that.
Is this all necessary, or is there any alternative?
Thank you for your time.
Answer: Adding insulation to an attic and replacing an old furnace with a new, high-efficiency model can be factors in raising the indoor relative humidity of a home.
While your proposed method will both prevent heat loss and warm-air leakage into the attic, there may be easier and less costly methods to prevent moisture damage in your home and attic.
There are often three main upgrades, which I refer to as the trifecta, that can create an ideal climate for moisture issues in an older home and attic: upgrading attic insulation; replacing an old, naturally aspirated furnace with a high-efficiency model; and replacing old, drafty windows with new ones.
These three factors can turn an older home that leaks air like a sieve into a relatively airtight structure. This is often manifested by increased condensation on window glass, uninsulated foundation walls and attic frost, in the heating season. These areas are typically colder than the surrounding environments and air with relatively high moisture content, or high relative humidity, can condense on these colder surfaces.
The reason these upgrades create conditions for higher humidity inside the home is that previously, the warm, heated indoor air easily escaped the building through these poorly sealed areas.
The newspaper in your attic and the vermiculite insulation would do little to prevent warm air from rising into this space. Since the attic was relatively warm, due to the lack of thermal insulation, this air may have escaped to the exterior through attic vents before it condensed.
Also, the air likely had low relative humidity, due to dry air that leaked into the living space through the old windows. You may still have some of this drying effect on the indoor air if you retained the older windows, but this will undoubtedly be less than before the other upgrades.
You have also closed up the vent from the older furnace, which was another way to release moisture from the home. Because of this, not only will the indoor air be wetter, it will now try to escape the home via the "stack effect" through other avenues — mainly the attic.
You are correct that there is more chance of condensation, frost and moisture damage in your attic due to these changes, and installing a layer of extruded polystyrene and new drywall on the ceiling will help prevent this. But that may be a fairly drastic step, which will be moderately expensive and quite labour-intensive.
A better approach may be to minimize this potential issue by pinpointing several problem areas and addressing those accordingly.
To do this, you should understand that the majority of air leakage into your attic likely occurs in a few areas.
The majority of the ceilings in your home are fairly well sealed against air intrusion due to multiple coats of paint on the old plaster. The areas of high air leakage are any openings or protrusions through these ceilings, and adjacent walls. So, air-sealing the tops of junction boxes for ceiling lights, the tops of wall plates, exhaust-fan housings and ducts and, particularly, the attic hatch may be an easier approach. This will likely require venturing into your newly insulated attic with a roll of 6mil poly, acoustical sealant, a staple gun and a few cans of blow-in foam. Regardless, this should still be much less work — and definitely less expense — than your other proposal.
On top of these air-sealing upgrades, two other items can be addressed to prevent moisture issues within the attic and inside the living space. Increasing attic ventilation, particularly at the eaves, may help prevent moisture problems in that area.
Even with increased air leakage into the attic space, if there is enough airflow to push that air out the roof vents before it cools too much, there may be little negative consequence.
In conjunction with that, the final issue may be the most important.
Taking active measures to reduce the relative humidity inside the home will prevent moisture issues inside and in the attic. If you maintain a low amount of dissolved moisture in the indoor air during the winter, there will be much less chance of condensation in both environments.
The most effective way to accomplish that is by installing and using exhaust fans in the bathrooms and kitchen. If you don’t already have good fans and a range hood in the home, now is the time to put them in. Adding timed switches to the bathroom fans is also a good idea to ensure you have adequate ventilation after showering or bathing.
Installation of a complete layer of foam insulation to the underside of your ceilings will be an effective method of preventing moisture issues in your attic, but it will be costly and will not prevent moisture issues inside the home.
Installation and use of bathroom and kitchen exhaust fans and other measures to reduce relative humidity in the indoor air, along with air-sealing select problem areas in the attic floor, may be just as effective with much less fuss and cost.
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.