In a previous post, I described how ice dams are formed. In this article, we will look at some strategies for creating a “cold roof”.
Keeping Heat Out of Your Attic
In short, you want to keep your roof as cold as possible. Cold roofs = less snow melt and fewer, less serious ice dams. If you could keep your roof as cold as your garden shed, you wouldn’t have any ice dams!
New construction lends itself to a number of effective, easy-to-implement cold roof strategies. But what about the majority of homeowners dealing with older homes? -or even some newer homes where the builder didn’t do their homework…
In order, focus on three things: air sealing, insulation, and attic ventilation.
In terms of bang-for-your-buck, nothing tops tracking down and blocking air leaks into an attic. Even the smallest of holes admits a shocking amount of warm air into the attic. The best tools for the job are cans of spray foam, caulking, rock wool insulation, Zip Tape, and weather-stripping.
Here are some places to look. Focus on any areas where there are penetrations of the attic ceiling. If your attic floor is boarded, the task is considerably harder but worth the effort. Otherwise, carefully move the existing insulation out of the way and plug any gaps you find. A smoke pen is useful to find air leaks. Some common air leakage sources include:
- Holes drilled in framing for electrical wires
- Holes made for plumbing pipes
- Bathroom exhaust fans
- Around ceiling light fixtures
- Attic hatches / Doors to the attic (not weather-stripped)
Air leaks are readily identified as areas of dirty insulation. Look for dark spots / stripes on your insulation and there is likely an air leak to be plugged. As air moves through gaps and holes, fiberglass insulation acts like a filter, trapping dust particles. Cellulose and rock wool also trap dirt the same way though both are better at resisting air flow.
Use mineral wool and spray foam to fill construction penetrations. Zip Tape is excellent for sealing around exhaust fans. Around chimneys, the ONLY product meeting code would be mineral wool insulation or fire-rated intumescent foam. Check with your local building inspector or qualified contractor.
If you see something like the picture below, there is a serious air leakage problem.
In my area around Boston, MA., attic ceilings should be least R-38. R-50 would be even better. If your insulation is installed only between the attic ceiling joists, the next layer (if you are using rolled fiberglass batts) should be applied 90 degrees opposite, across the joists. Be sure to use unfaced batts to avoid trapping water vapor between layers of insulation.
Of special note is the intersection of the exterior wall to the roof rafters. In this area, because the space above the ceiling joist and below the roof sheathing is limited, it is difficult to obtain R-38, let alone R-50. Of course, it’s exactly this area where you would want the highest R-value. In these areas, consider having a professional spray a high R value material such as closed-cell foam. Remember to install baffles to maintain an air space, preferably 2”, providing for air flow from the soffit for proper attic ventilation.
One of the best attic ventilation systems employs soffit and ridge vents. Both should be continuous runs along the length of the house. The ridge vent creates a natural convection, exhausting [rising] warmer air and bringing in colder, outside air through the [lower] soffit vents. Critical to the process is a 2” space between the top of the insulation and the underside of the roof sheathing. There are a variety of ways to create this space, including pre-formed baffles or hand-made baffles made from foam insulation boards.
It is very important to install baffles in such a manner to protect the attic floor insulation from air washing in from the soffit vents. Properly installed, attic ventilation will create a constant flow of cold air washing the underside of the roof sheathing. Bear in mind this ventilation effect is rendered useless if the attic has not been properly air sealed and ventilated.
I’ll touch on exterior treatments in a separate post (example: Ice and Water Shield). Anything on the exterior is a band-aid or part of belt & suspenders solution. First and foremost, the goal is keeping warm air inside the house. Air-sealing, insulation are your top priorities. Ventilation is also important but not nearly as effective (or as easy) as air-sealing and insulating.