What's the verdict on attic fans?

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

What I've decided to do is a combination of things: one very small (8") attic fan at the rear, on a thermostat and humidistat

This might be more effective regarding humidity than those passive roof turbines. I remember a swamp-cooler factory in my hometown claimed their swamp-coolers led the roof turbines of the service area of a Fiat dealer to slow down due to the temperature decrease.

A ridge vent would have been ideal, but it isn't in the cards this year.

If you get the ridge vent, remember to use a bug screen or some other trap to prevent vermin to get in.

[aerohead]

I'm late to the discussion, so sorry if I'm repeating info.
When in the West Texas Solar Energy Society, the common metric for adequate attic ventilation was that, when measured directly above the insulation surface, the dry-bulb temperature should not exceed 10-degrees above that of the ambient outdoor dry-bulb temp..
If it's 100- F outside, 150-F inside the attic under the roof, but 110- F just above the attic blanket, it's okay. The insulation is 'blind' to heat stratification that your head experiences when standing there. Measure at your feet.
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If the HVAC system and ductwork are in the attic, then long-wave, infrared radiation is an issue ( its in an oven now ), and a radiant barrier can be just as much an issue.
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If all the HVAC system is in a furdown and closet, below the ceiling, or under the house, it doesn't matter so much about the radiant load, unless you have finished living spaces right below the roof.

[Ecky]

Originally Posted by aerohead

I'm late to the discussion, so sorry if I'm repeating info.
When in the West Texas Solar Energy Society, the common metric for adequate attic ventilation was that, when measured directly above the insulation surface, the dry-bulb temperature should not exceed 10-degrees above that of the ambient outdoor dry-bulb temp..
If it's 100- F outside, 150-F inside the attic under the roof, but 110- F just above the attic blanket, it's okay. The insulation is 'blind' to heat stratification that your head experiences when standing there. Measure at your feet.
--------------------------------------------------------------------------------------
If the HVAC system and ductwork are in the attic, then long-wave, infrared radiation is an issue ( its in an oven now ), and a radiant barrier can be just as much an issue.
--------------------------------------------------------------------------------------
If all the HVAC system is in a furdown and closet, below the ceiling, or under the house, it doesn't matter so much about the radiant load, unless you have finished living spaces right below the roof.

No ducting in this house, and there's no cooling load either. It's a rare day indeed that I set the mini split to cool.

The house is a long ranch style and is heated at one end by a pellet stove, and at the other by a mini split. The temperature delta on the coolest nights been room temperature and outside air can (rarely) be as much as 100 degrees. More typically it's 60-70 degrees.

[Piotrsko]

Originally Posted by cRiPpLe_rOoStEr

If you get the ridge vent, remember to use a bug screen or some other trap to prevent vermin to get in.

Leaves are a big deal also

[cRiPpLe_rOoStEr]

Originally Posted by Piotrsko

Leaves are a big deal also

Of course, but my main concern would still be vermin, or birds nesting. They can do some real mess.

[hayden55]

How exactly do you know if you have adequate attic ventilation? I'm used to thinking about homes where the air changes per hour and humidity ranges have standards. Besides not seeing rusty nails or sweat on surfaces or mold it seems odd to talk about. Although, if you guys could see the houses they build in the south it pretty much shows they have no standards or inspections on attic ventilation.
This brings to mind one of the standards of one of the energy efficiency groups where they build house to have 1 air change per hour at 50 pascals of pressure. Which means the home is super efficient, it just needs humidity control, a hepa filter, and an air exchange system installed.

[redpoint5]

Originally Posted by hayden55

How exactly do you know if you have adequate attic ventilation? I'm used to thinking about homes where the air changes per hour and humidity ranges have standards. Besides not seeing rusty nails or sweat on surfaces or mold it seems odd to talk about. Although, if you guys could see the houses they build in the south it pretty much shows they have no standards or inspections on attic ventilation.
This brings to mind one of the standards of one of the energy efficiency groups where they build house to have 1 air change per hour at 50 pascals of pressure. Which means the home is super efficient, it just needs humidity control, a hepa filter, and an air exchange system installed.

There are rules of thumb for ratio of ventilation per attic area (not volume), like 1:150 or 1:300, split 50/50 between intake and exhaust.

I've read other rules of thumb too, like the attic temperature should not vary more than 20 degrees from outdoor.

Doesn't seem like builders actually do any math or stick to any standard of ventilation.

[aerohead]

Originally Posted by hayden55

How exactly do you know if you have adequate attic ventilation? I'm used to thinking about homes where the air changes per hour and humidity ranges have standards. Besides not seeing rusty nails or sweat on surfaces or mold it seems odd to talk about. Although, if you guys could see the houses they build in the south it pretty much shows they have no standards or inspections on attic ventilation.
This brings to mind one of the standards of one of the energy efficiency groups where they build house to have 1 air change per hour at 50 pascals of pressure. Which means the home is super efficient, it just needs humidity control, a hepa filter, and an air exchange system installed.

For air-conditioned homes, if the temperature in the attic, directly above the ceiling insulation, is no more than 10-degrees above the ambient outdoor temperature, then you have 'adequate' ventilation for both summer and winter.
100-degree dry-bulb temperature outside:
157-degrees outside on top of asphalt shingles in full sun.
146-degrees under the roof decking.
110-degrees just above the insulation, you're 'DONE.'
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If you have un-vented gas heating in the home, you'll be loading the interior air with water vapor. It will do all it can to permeate floors, walls, and ceiling.
Any surface below dew-point that it can reach will produce condensation, bacterial activity, rot, and attract termites. It's why 'TYVEK' or related breathable barriers are mandatory in all new construction.
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If a contractor has installed blown-in insulation, make sure they haven't smothered all the soffit inlets. You've got to have convection-driven air circulation up there.

[cRiPpLe_rOoStEr]

Originally Posted by aerohead

If you have un-vented gas heating in the home, you'll be loading the interior air with water vapor. It will do all it can to permeate floors, walls, and ceiling.
Any surface below dew-point that it can reach will produce condensation, bacterial activity, rot, and attract termites. It's why 'TYVEK' or related breathable barriers are mandatory in all new construction.

As construction methods in my country often differ a lot compared to what is done in the USA, Tyvek and similar materials are more often placed below the roof tiles, and it's still not much common. But anyway, besides preventing the moisture from being trapped inside, another matter of concern for me would be the air renewal, exhausting CO² too.

[hayden55]

Originally Posted by cRiPpLe_rOoStEr

As construction methods in my country often differ a lot compared to what is done in the USA, Tyvek and similar materials are more often placed below the roof tiles, and it's still not much common. But anyway, besides preventing the moisture from being trapped inside, another matter of concern for me would be the air renewal, exhausting CO² too.

You would be impressed how much a home leaks air. They still build them 1970s style in Arkansas to this day. No code. no inspection. Code wise on energy standards i bet we are tied for last or in the running for last usa wise. Even Texas follows a pretty recent code but we don't lol. Its impressive how bad they are. But typically they are set up so badly they will typically have a blower door score of 7-12 air changes per hour. You can look into ACH50 and it will lead you down a rabbit hole. So they get plenty of fresh air exchanges above required through just air leaks (typically all wall penetrations and top and bottom plates as they don't seal up spec homes). Usually the only problem we might face in relation to indoor air quality is like he said above, dust and humidity from the outside will typically get inside your house and cause problems. Mostly just make your indoor air uncomfortable, make you run your air more, and probably slightly more allergies.

Interesting Aerohead. I know from my human senses that typically my attic in the summer is much warmer than 10 degrees over ambient ( i would honestly have to say probably more like 135 degrees actual temp). I am installing rafter vents though now and sealing up my top plates and electrical connections this week to get ready to update my insulation up top. Should help a lot with venting. When the vents aren't blowing the loose fill insulation out of place towards the center of the house, the insulation is usually blocking them. lol
Mold wise the house has been mold free for 17 years now so i guess we are good.

Water heater and central heating wise we are fully vented. Too inefficient to meet code to go ventless. Only sightly though.
Aerohead, what are your thoughts on stapling up infrared heat barrier to the 2x6's like they are doing on the new energy efficient builds? Gives a 6" air gap and is supposed to cut down a bit on energy transfer. Seems cheap enough.