**[FONT=Humanist521BT-Bold][size=7][FONT=Humanist521BT-Bold][size=7][size=5]VentingOf Attics & **[FONT=Humanist521BT-Bold][size=7][FONT=Humanist521BT-Bold][size=7][size=5]Cathedral Ceilings
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http://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam/
**The following comments from this site **http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/83-questions-answers.html](http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/83-questions-answers.html) Talks about attic venting and the importance of the vapour barrier.
Question 10
Is venting of an attic space the only way to avoid condensation problems?
Answer - M.Z. Rousseau
No. The purpose of ventilation of an attic is to flush out the moisture that enters from the living spaces. In winter, outside air in most geographical locations is close to the saturation point, and therefore ventilation of an attic with saturated air may not be very effective in removing moisture from the attic spaces. The effectiveness of ventilation in removing moisture depends on the quantity of moisture which appears in the attic, the capacity of the outside air to absorb extra moisture, and the flow of air which actually occurs by ventilation. Ventilation openings in attics do not ensure a continuous flow of air; there must also be an air pressure difference. Even then, sufficient air may not be passing through the attic.
Since the source of moisture is likely to be the inside space of the building, it is best to control attic condensation by providing appropriate barriers to moisture movement at the ceiling plane. The ceiling must have an air barrier and a vapour barrier to prevent moisture from entering the attic space.
Attic condensation control is a mixture of efforts; 1) careful air sealing and detailing of the ceiling plane to minimize the entry of moist air into the attic space and 2) provision of ventilation openings to let out some of the moist air. This movement will be more productive in spring, when the outside air has a greater drying potential.
**Elsewhere in the above article He talks about the RH limits within the home during winter be maintained between 30% to 35%. In Edmonton the mean temperature of the ground 6 feet deep is 6 C and room air at 21 C with 30% RH has a dew point of 3 C to 4 C. RH’s above 40% will condense of surfaces with less then 5.6 C. Thus in basements where the air flow and heat flow is restricted and the surface temperature is the mean temperature of the soil (6C), IE under stair wells, may become wet from condensation and if this moisture is continuous for more than 48 hours mould growth commences. Longer periods of continuous moisture may create mould problems. Although going from the attic to the basement may seem unrelated it all come relates to the RH levels in the home. Too high and condensation in the attic, on the windows and surfaces below the dew point become a problem. **
**He states in answer to queston 21 that the dew point at 20C Rh 40% IS 5.6c. **
My numbers are estimates by reading the Psychrometric chart.
Do you not insulate your basements out there?? Why are the interior surfaces so cold and not closer to say 15-18-20*C?
Very good piece!
Thanks
Older basements were not insulated until the code called for it. The builders than only did the minimum, the wall above grade. Now the walls are all insulated but the floor is not. So the floor under the stair well is susceptible to condensation and mould growth because the ground at that depth has a mean temperature of 6C.
I wish you would reread the sentence before your red high light.