1-Minute Building Health Advice: Don't Let Your Roof Cook! Simple Ventilation Tips Inside
Building insights in under 1 minute
Jens Johanson
Jun 26, 2025
Building insights in under 1 minute
Picture this: Pitched roof, asphalt shingles, "cold roof"
(A cold roof means insulation on the ceiling or drywall lid of the home, but an open attic with outside air flow into the attic for venting, and "mixed climate location" meaning cold winters and hot summers. This is 80% of the 30-year-old buildings we see.)
Attic and roof ventilation is achieved by drawing in air at the low edge of the roof through soffit vents or "bird blocking," and allowing it to exit at the high edge through ridge vents or "turtle vents" at the roof’s peak. If the roof gets too hot, the shingles will curl and degrade prematurely and the underside of the roof sheathing will condense the warm interior moist air on the bottom side of the cold roof surface, causing mold on the attic sheathing. Here are the most common reasons why:
- Insufficient insulation on the ceiling. This allows warm air from the interior of the unit to leak into the attic space and warm up the attic.
- Blocked eave vents with too much blown insulation and no "baffles" to keep the insulation away from the soffit vents.
- Lots of "can" lights and other penetrations in the ceiling. Similar air leakage as #1 above, and the existing insulation getting pushed around or even removed by the light installer, making things even worse.
- Code says 50% of the attic ventilation is to be achieved from the lower edge of the roof and 50% from the upper roof or peak. This is just a math question of the net square inches of vent size as compared to the square footage of the attic space. We find a lot of under-ventilated attics due to lack of proper ventilation, either blocked vents or not enough vents.
- Ducting leaks into attics. The attic is supposed to ventilate as its own area, not have spare bathroom fan ductwork terminating or venting warm shower air exhaust into the attic space. Sometimes these ducts come loose, but many times they are just blowing into the attic, making it a mold fest.
- Complicated roofs with different roof lines. Each roof area has to work on its own with respect to vents and natural air flow. Some of the more complicated roofs don't have good air flow.
A quick way to check your attic for proper ventilation is to just stick your head up into the attic space and check:
- Can you see daylight at the eaves and ridge (meaning, you can see the vents)?
- Is it excessively hot, or can you actually feel the air flow? It may be warmer, but it should not be a sauna.
- Do you see any random ductwork pipes laying on the insulation and not routing up through the roof?
- What does the underside of the roof sheathing look like? Is it orange like plywood at a big box store, or is it black?
If you see anything out of sorts, call to let us crawl through the insulation for you and do the vent math. We can also stick our heads into the attic if you don't like spiders. We know tricks to make Difficult Attics Great Again.
Flat or low-sloped roofs, vaulted ceilings, or "warm roofs" (full of insulation, or rooftop decks) are different beasts and require different taming techniques to get them to perform well. There is usually no easy way to definitively confirm proper ventilation—invasive testing must be done. One of the best indicators of problems is spongy soft sheathing when you are walking on the flat roof. If you have that, just call us.
If you'd like to see examples of all these roofs and vent types, watch our Do-It-Yourself Inspection Series Episode 6 and Episode 18.