Replies

1. Expert Member
   Dana Dorsett | Sep 13, 2016 01:54pm | #1

   Ice dams are a symptom of heat leaks. While roofing ventilation can mitigate the symptom, fixing the heat leaks would have more value. If the ceiling is air leaky enough that allergens and humid air is getting driven into the house, that's going to be a huge heat leak when the stack effect reverses in winter.

   Air sealing the attic floor/conditioned space ceiling is critical for blocking stack effect drives from moving air between the conditioned space and attic. The more humid air that leaks from the conditioned space to the attic over the winter, the higher the average humidity in the attic over time,. since some of the moisture gets taken into the structural wood as adsorb, which can sometimes take months to dry.

   The free area of the ridge vent should be no more than about 2/3 that of the soffit vents to limit depressurizing the attic relative to the rooms below. (If the ridge vent has more area than the soffit vents it creates an additional stack effect drive.) When the soffit venting is bigger than the ridge vent, the pressure difference at the ceiling plane is lower.

   See: https://www.greenbuildingadvisor.com/blogs/dept/musings/air-sealing-attic

   IRC code-min for zone 5A attics is R49.for open blown or batts thermally bridged by joists or trusses, R38 if it is continuous, no thermal bridding. A vented R49 roof will have very little ice dam potential in Indiana, but might still have issues in extreme-snowfall locations (such as the western NY lake effect snow-belt).

   Any ducts or air handlers up there we need to know about?

   Also, the percent relative humidity isn't a particularly useful piece of information unless accompanied by the temperature to which it is relative. At the same absolute humidity you could have a higher relative humidity simply by being at a lower temperature. The absolute humidity can be expressed either as dew point, or wet bulb temperature. It's unlikely that your location is chronically and significantly more humid than nearby Indianapolis in absolute terms unless you're at the foot of a waterfall or something. It's more likely that your location is a few degrees cooler than the city in summer, due to the "heat island effect" found in cities.

2. GBA Editor
   Martin Holladay | Sep 13, 2016 02:15pm | #2

   Edward,
   Your question is unclear. Most building codes require attic ventilation, unless you are trying to create an unvented conditioned attic.

   As Dana pointed out, a vented unconditioned attic works very well, as long as (a) the ceiling is relatively airtight, (b) the insulation on the attic floor meets the minimum R-value required by building codes, and (c) there are no ducts or HVAC equipment in the attic.

   For more information on these issues, see:

   Air Sealing an Attic

   Creating a Conditioned Attic

   All About Attic Venting