Replies

1. 
   user-2310254 | Nov 20, 2017 03:25pm | #1

   I think you will be advised to airseal your attic and increase its insulation levels. Where are you located?

2. GBA Editor
   Martin Holladay | Nov 20, 2017 03:38pm | #2

   Mike F,
   The most common cause of ice dams is air leakage from the warm interior into the attic. The solution to this problem is better air sealing. After the air sealing work is complete, extra insulation is always a good idea.

   Adding a powered attic ventilator (a fan) will make things worse, since the fan will pull even more interior air into the attic through cracks.

   For more information, see these three articles:

   Prevent Ice Dams With Air Sealing and Insulation

   Ice Dam Basics

   Fans in the Attic: Do They Help or Do They Hurt?

3. 
   Jon_R | Nov 20, 2017 07:04pm | #3

   Listen to the above advice. But to answer the question, the right amount of ventilation blowing outside air into the attic will cool it directly and stop (not just reduce) the flow of interior air into the attic.

4. GBA Editor
   Martin Holladay | Nov 21, 2017 08:16am | #4

   Jon,
   If Mike installs a powered attic ventilator backwards, so that it pressurizes the attic instead of depressurizing it, it will, indeed, reduce the attic temperature during the winter. But this approach has two big disadvantages: (a) The fan requires electricity, so it will increase Mike's electricity bill, and (b) In light of the fact that Mike's ceiling is undoubtedly leaky, the backwards attic ventilator will force cold attic air into the house through ceiling cracks, increasing Mike's heating fuel bill.

   In short, the best approach is to air seal the attic.

5. GBA Editor
   Martin Holladay | Nov 21, 2017 12:41pm | #5

   Jon,
   If Mike installs a variable speed fan, and can adjust the fan speed so that the attic pressurization exactly cancels the stack effect leakage on the day the fan is installed, you're right -- but Mike isn't going to do that. Moreover, the day after the fan is installed, the rate of stack effect leakage will change, since stack effect leakage varies with outdoor temperature, wind speed, and the operation of furnace blower motors. Unless I'm totally confused, that means that Mike would have to continually adjust the variable speed fan.

   If this approach were possible, another question arises: Would the value of the heating energy saved by pressurizing the attic to perfectly match the pre-fan-installation stack effect leakage be more than or less than the cost of the electricity to run the fan? The answer will vary depending on the outdoor temperature, of course.

   Anyway, we agree on one point: sealing the air leaks is the best approach.

6. 
   Jon_R | Nov 21, 2017 01:54pm | #6

   Any increase in attic pressure up to and somewhat beyond the stack effect pressure will decrease Mike's heating fuel bill. Even if were incorrectly way over pressurized at times, is a pascal or two a "big disadvantage"?

   I agree that in terms of operating costs, "green" and simplicity, air sealing is the best approach. But for perhaps $40/year in electricity, $1000 (a wild guess) more for air sealing may not be attractive.

   I have no idea if there is a fan controller that would do a reasonable job for such a case - while pressures don't have to be exactly matched to cool the attic and save heating cost, the best design is more complicated than just "40 degrees".

7. Expert Member
   MALCOLM TAYLOR | Nov 21, 2017 09:46pm | #7

   Martin,

   Surely in country with as large a population and diverse interests as America, there is someone who wouldn't mind sitting in Mike's attic and adjusting the fan every so often?