How hot are you?

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brj

Re: How hot are you?
« Reply #20 on: 2 Aug 2018, 04:24 pm »
Be aware that many of the answers above may be more or less specific to where the poster lives.  It's worth reviewing the options again with one's own locale in mind.

(I already live in a semi-arid environment, so I don't want the AC to strip any more humidity out of the air.  Even when we have day-time highs in the upper 90s, we'll often have nighttime lows in the 60s or even 50s.  A whole house fan works very well in this environment, cooling the house substantially in just 20-30 minutes.)

Be careful of powered attic fans - they often sound like a good idea, but typically have 2 challenges.

First, many attics are not properly vented for either passive or active (attic fan) cooling.  Inflow area (usually soffit vents) must be equal to or larger than the exhaust area (ridge/turtle/etc vents).  The exhaust area should be sized according to the volume of your attic - look up 'net free vent area'.  When you run a powered fan, the intake area becomes exhaust area as well, as the fan overpowers the natural convection process, but that total still needs to be sized according to the volume of the attic, which is also what drives the sizing of the fan (airflow measured in CFM sized to turn over the total volume of air in the attic X times per hour).  You basically can't have too much exhaust area - as it will only improve the passive cooling and reduce the resistance that a powered fan has to overcome - but again, you have to make sure the inlet area matches or exceeds that, or you can have condensation issues.  (Don't over look this... condensation can lead to horrible consequences, from making blown-in insulation less effective, to mold infestations capable of causing serious health problems, etc..)

Second, even if you have proper venting and a properly sized powered attic fan, that fan is going to pull air from wherever it can via the paths of lowest resistance.  Your intent is for those paths to be the inlet vents to your attic, but it will happily pull air from inside your house through the sill plates of your walls, the attic access, etc. - air that you likely paid to condition.

Note that if you have a whole house fan, you need to do similar NFVA checks against the CFM of that fan, though the sizing of a whole house fan is driven by the volume of your house, not the attic.

When I had to redo my roof a few years ago thanks to hail damage, I ultimately decided that a powered attic fan was a horrible idea, and I had them go to town on passive venting instead, putting exhaust vent along every possible linear inch of ridge, and then adding soffit vents until that resulting ridge-installed NFVA was exceeded.  I then had a radiant barrier installed on the roof decking underneath the double batten system supporting the shingles to limit how much radiated energy can even enter the attic.  The double batten system also acts as both a thermal break from the shingles heated by the sun and provides an above-roof-decking vent path to further reduce the heat under the shingles that could then be conducted through even those limited batten points of contact.

Couple all of that with R60 insulation blown into the attic and a bunch of air sealing and other insulation work, and my house is much slower to heat up in summer, and thus much more easily cooled in the evenings via the whole house fan.  If I'm home during the day during a hot spell, I may eventually break down and run the AC as the house finally heats up enough to be uncomfortable, but I don't need to do so nearly as often now, and it occurs much later in the day.  (The roof system design is less of a driver for winter heating, though even more important to attic condensation management during that season, but the insulation and air sealing definitely improved my winter heating bill, not to mention eliminating cold/hot spots across the house.)

In short, you really do need to view your entire house as as system, and there are couplings between elements that may not always be apparent.  And the system should be tuned according to the environment in which the house is built - heating vs. cooling needs, humidity levels, etc..  (I ended up reading a bunch of US Department of Energy building envelope studies at the time, and found it really interesting to drill into the details of that system-level view.)

Elizabeth

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Re: How hot are you?
« Reply #21 on: 2 Aug 2018, 04:39 pm »
72F outside right now, at 11:30 AM  :)
Rained last night a little.
I am about 2 miles from the shore of Lake Michigan. So "Lake Effect" plays a part in my temps.
1,300,000,000,000,000 gallons of water at my 'doorstep' give or take a few hundred billions.  :thumb:
I have no need for a water saver in my shower...