Professional Articles Written by Randy Foege
Published in multiple sources


 Attic Ventilation – Part 2             

By Randy Foege

Brookdale resident since 1982 - State Licensed & ASHIâ Certified Home Inspector since 1989


From last month…Next time, the other (partially written half) of this article: A little more on venting, preventing Ice dams and the scoop on powered attic ventilating fans.


Last month I mentioned poor attic ventilation causes the following:

  • Condensation in the attic.
  • Moldy attics (because of condensation trapped there in winter).
  • Deteriorating roof surface (decking) from the mold.
  • Deteriorating roof framing structure (from being wet all winter).
  • Decreased shingle life due to the attic being too hot with temperatures exceeding 160F.
  • The attic doesn’t cool reasonably soon after the sun goes down (A/C costs).
  • Ice damming (water leaks into the attic and or soffit and damages the home during roof snow melting).


Bathroom vent fans are blamed for many really moldy attics but in my experience, they typically cause only localized mold. However, all bets are off if you have four kids that each takes a 40 minute shower everyday - during winter. Brookdale home bathroom fans were typically vented into the attic. The best place I know to vent a bath fan is across the attic floor (using insulated, unspliced flexible ducting) and then down through an exterior soffit. That is apparently current code in Naperville and I rarely see problems utilizing that method of direct outdoors venting.


Poor ventilation encourages ice damming. Ice damming occurs when some portions of the roof are warmer than others. This enables a thaw-freeze cycle that allows accumulated snow on the roof surface to melt in some areas and then refreeze as it runs down to any colder portions of the roof. This freeze/thaw cycle creates an ice dam (on top of the shingles. This dam traps the water that is trying to run down the roof. On typically sloped roofs, if a puddle develops on the shingles that is about 1” or more deep, the water will seep (upslope) under the shingles. Captive (melting) snow seeping under the shingles is what creates roof leaks. Proper ventilation ensures the roof deck temperature remains consistently cold (or warm) so the water will stay frozen, or will run off.


Ice and water shield is a self adhesive roofing membrane material that is installed on top of the bare roof decking (under the shingles) especially in any areas prone to leaks. This includes where water is directed against the siding while running down the roof or in places that have experienced, or are likely to experience ice damming. This always includes the lowest edge of the roof surface, just above the gutters. Ice and water shield is a very effective preventative/corrective measure. These days, ice and water shield is (almost) always applied when old shingles are removed and a new roof is installed.


Power roof vents have been studied by the University of Illinois (and others). Research revealed that the cooling load in the house was not reduced sufficiently to pay for the operation of the fan.  They recommended alternately investing the fan money in more insulation and/or improving the passive attic ventilation. Passive ventilation has no cost of operation.  In homes where I find an existing attic fan during a home inspection, I mention that it makes sense to disconnect power to the fan. Most fans fail in a relatively short time anyway. It is (at best) inconvenient to annually lubricate the fan motor. Also, roof visiting pests bend the screening against the fan blades which prevents blade movement and burns out the motor.


A powered attic fan can only be as effective as the positioning and number of complementary intake vents from which it draws.  If the roof vents are too close to the attic fan, there is short circuiting of the vent stream.  Power vent fans draw from the source with the least resistance such as the vents below (at the soffit) or above at the ridge. The fans also draw through openings between the home and attic that really shouldn’t be there. When I encounter powered vent fans during home inspections, I often see mold on the lower sections of roof decking. Because of the fan, fresh air is not drawn from and across the soffit. Since there is no air movement there, humidity in the attic condenses on the lower decking and facilitates mold growth.


An Unintended Effect of a Powered Attic Fan:

(Total Fan Capacity) – (Attic Ventilation Ability) = Household air drawn into the attic.


Since air from the house will be drawn into the attic by the power vent fan, the following can be expected:

  • Potential back-drafting of the water heater(s), dryer or furnace.
  • Air leaking into the attic - around recessed (can) lights & bathroom vent fans, electrical conduits, chimneys, flues, plumbing DWV pipes and even the attic entrance cover.
  • Increased outside air infiltration around doors, windows, electrical & plumbing penetrations due to the negative pressure in the home.
  • Elevated indoor humidity and Radon gas levels. This is caused by drawing basement, crawlspace and below-slab moisture and soil gas (Radon) through the living space due to negative indoor airpressure exerted by a powered attic fan.


Controlling Attic Temperatures and improving ventilation:

  • Provide passive ventilation to equal or exceed building code specifications to prevent attic moisture accumulation and to prevent ice damming.
  • Seal any furnace ductwork located in the attic using a commercial grade duct sealer and insulate with a minimum 2” insulated duct-wrap with a vapor barrier.
  • Eliminate air leakage between the attic and living space. Common leakage points are penetrations made by plumbing and electrical conduits and fixtures, the top plates of the interior walls, attic access panels and HVAC and plumbing chases. Use expanding foam or caulk for plumbing and electrical penetrations and weather-stripping for attic access panels.
  • Insulate the attic floor to a minimum R-30. In ~1985 I upgraded from R-19 (the standard when my home was built) to R-38. That is my personal best ROI of both time and money, ever. I will upgrade to R-48 (if my new bride and I stay in Brookdale). R-38 is the standard for the newest homes in Naperville.
  • Next time, use light colored shingles to reduce roof surface temperatures.