[snic]

I have a 900 square foot attic. It's one of two attics in a large house that I bought a few months ago. It has a gable event (maybe 2 x 2 ft) with a fan at one end (the fan doesn't work, as far as I can tell - or maybe it's temperature controlled and it works only in summer). There is no other form of ventilation - no soffit vents, no ridge vent, nothing.

Unsurprisingly, this winter, I noticed condensation on the attic ceiling. The roof is also quite old and needs to be replaced in the next few years. So the question is, what sorts of attic ventilation should I add? I came across the attic ventilation bible:

http://www.airvent.com/index.php/ve...1-principles-of-attic-ventilation-course/file

...and made the following calculations:

For the 900 sq ft attic I need 6 sq ft of venting, ideally 3 sq ft intake (soffit) and 3 sq ft exhaust (ridge).

I can easily add about 4.5 sq feet (net free area) of intake venting by installing 10 8x16 soffit vents.

The length of the ridge is only about 22 ft, so at 18 sq in of venting per linear ft, that only gives me 2.75 sq ft of exhaust venting.

That wouldn't be so bad - I should just install the soffit vents, replace the roof, make sure a good ridge vent is installed, and be happy, right?

Except for one thing. I want to install this whole house fan:

https://airscapefans.com/products/airscape-sierra?variant=289587757061

The model I need to install (Sierra 5300, because of the size of my house) requires 10 sq ft of attic venting. So I figure I could enlarge the gable vent so I have, say, another 3 or 4 sq ft of net free venting area. But then I would be combining ridge and gable vents - which is apparently frowned upon:

https://www.jlconline.com/how-to/roofing/q-a-can-you-combine-ridge-and-gable-vents_o

On the other hand, I only have one gable vent, not two, so there would be no flow of air from one gable to the gable at the other end.

So at long last, here is my question: will it hurt to have this combination of vents: soffit, ridge and ONE fairly large gable vent? What could go wrong?

 

[Nealtw]

Are you in a place where you cold get snow on the roof? That fan will suck air from the house, so the first thing you need is sealing the ceiling . It is always better to have more intake than exhaust for the same reason, sucking air from the house. You don't have to wait for a new roof to install a ridge vent and a light colour roof may help a little with heat.

 

[snic]

Are you in a place where you cold get snow on the roof? That fan will suck air from the house, so the first thing you need is sealing the ceiling . It is always better to have more intake than exhaust for the same reason, sucking air from the house. You don't have to wait for a new roof to install a ridge vent and a light colour roof may help a little with heat.

Yes, I'm in NY. Getting the attic sealed and properly insulated is also on the list.

Re installing a ridge vent without installing a new roof - I'd consider that, but the roof is really on its last legs. To get every last bit out of the existing roof, one thing I was considering is to not install the ridge vent OR new roof at this point, but just install the soffit vents and expand the gable vent. That might be fine for a couple of years...?

 

[Nealtw]

Yes, I'm in NY. Getting the attic sealed and properly insulated is also on the list.

Re installing a ridge vent without installing a new roof - I'd consider that, but the roof is really on its last legs. To get every last bit out of the existing roof, one thing I was considering is to not install the ridge vent OR new roof at this point, but just install the soffit vents and expand the gable vent. That might be fine for a couple of years...?

I suppose, are the soffets common, as in a level that would have an area inside that would feed air into all rafter bays that have an air chute?

 

[Bud9051]

As you have encountered, the internet can be a confusing place.
Note, the date on that JLC article, June 1, 2003, way behind modern thinking.
gable vents and a ridge vent work fine together, as most air flow in an attic moves due to attic air pressure. Think of it as attic stack effect, but it takes pages to present the details.

Whole house fans can have their applications but in most cases plenty of insulation does the job. If you have any naturally drafted appliances like a water heater or furnace then you need to be sure the depressurization will not backdraft their exhaust. Example, range hoods in excess of 4,000 cfm ire an integrated make up air system. Just opening windows is not an approved solution.

So, ridge vent any time you want. enlarge the gable vent if needed. But be careful with the whole house fan.

Bud
Forgot to ask, are the two attics common in any way? When connected they alter the recommended ventilation area.

 

[carpdad]

Also where NY? I'm in nj and this winter was the warmest in history. So air not as dry. Venting the attic means taking in outside air and pushing out the stale air. Since your attic fan is temperature controlled, like mine, your fan does nothing in winter for pushing out. But since vent holes are open, warmer air, with more moisture, comes in, and adds to more condensation. That is one part of it, anyway. You probably have downstairs moisture going up to the attic as well. Attic stairs is usually leaky, among other things. Kitchen/bath vents and their ducts and ceiling lighting fixtures are main culprits.

There was a discussion about the whole house fan. I thought the fan had to be vented to outside directly, but it turns out it is also ok to vent into the attic. Not sure what the conclusion was, but search the forum.

 

[snic]

I suppose, are the soffets common, as in a level that would have an area inside that would feed air into all rafter bays that have an air chute?

Not quite. That's actually one problem. On one side all the soffits lead directly to the attic. On the other, part of the attic ceiling is actually the vaulted ceiling of the room below - that is, it's not the attic ceiling at all. So installing vents in those soffits wouldn't ventilate the attic. But I think there are 5 rafter bays on that side that are in the attic, so installing 5 on that side and 5 on the other should be OK.

As you have encountered, the internet can be a confusing place.
Note, the date on that JLC article, June 1, 2003, way behind modern thinking.
gable vents and a ridge vent work fine together, as most air flow in an attic moves due to attic air pressure. Think of it as attic stack effect, but it takes pages to present the details.

That's good to know. Although there are pages and pages instructing people to seal their gable vents after installing ridge vents.

Whole house fans can have their applications but in most cases plenty of insulation does the job. If you have any naturally drafted appliances like a water heater or furnace then you need to be sure the depressurization will not backdraft their exhaust. Example, range hoods in excess of 4,000 cfm ire an integrated make up air system. Just opening windows is not an approved solution.

Well, I won't be operating the furnace and the whole house fan at the same time. The gas water heater, of course, is a different story. But the whole point of a WHF is to crack windows open evenly across the house, so you're constantly drawing in fresh air. So if exhaust is getting backdrafted, at least it's not going to build up. Still, I'll ask my WHF contact about this issue.

Forgot to ask, are the two attics common in any way? When connected they alter the recommended ventilation area.

No, they are completely separate from each other.

 

[Nealtw]

Not quite. That's actually one problem. On one side all the soffits lead directly to the attic. On the other, part of the attic ceiling is actually the vaulted ceiling of the room below - that is, it's not the attic ceiling at all. So installing vents in those soffits wouldn't ventilate the attic. But I think there are 5 rafter bays on that side that are in the attic, so installing 5 on that side and 5 on the other should be OK.

If the vaulted roof is like a shed roof against a wall there should be a vent system at the top. You could look at what might be needed when you do the roof.

 

[snic]

Also where NY? I'm in nj and this winter was the warmest in history. So air not as dry. Venting the attic means taking in outside air and pushing out the stale air. Since your attic fan is temperature controlled, like mine, your fan does nothing in winter for pushing out. But since vent holes are open, warmer air, with more moisture, comes in, and adds to more condensation. That is one part of it, anyway. You probably have downstairs moisture going up to the attic as well. Attic stairs is usually leaky, among other things. Kitchen/bath vents and their ducts and ceiling lighting fixtures are main culprits.

There was a discussion about the whole house fan. I thought the fan had to be vented to outside directly, but it turns out it is also ok to vent into the attic. Not sure what the conclusion was, but search the forum.

Actually my attic fan does nothing in summer OR winter, because there are no intake vents in the attic. It is so stupid - I can't imagine what the previous owners were thinking. If they ran that in summer, they were just sucking conditioned air from the living space (through leakage) into the attic and blasting it right outside. They were throwing money away.

Whole house fans can vent directly outside, but I think the point of venting them into the attic is that then you cool your attic down as an added benefit! You just need to have adequate attic ventilation. (Which I think will address the concern Bud9051 had about gas appliance exhaust backdraft: if attic ventilation is insufficient, then some of the air you are trying to exhaust out of the house will recirculate back into the house. If you also have negative pressure because you are blowing some air out of the house, then you end up recirculating the exhaust gases - not good. But if you aren't doing that, you're getting a high number of air changes per hour, so even with backdraft you aren't going to have a problem. I think.)

 

[Bud9051]

Well, I won't be operating the furnace and the whole house fan at the same time. The gas water heater, of course, is a different story. But the whole point of a WHF is to crack windows open evenly across the house, so you're constantly drawing in fresh air. So if exhaust is getting backdrafted, at least it's not going to build up. Still, I'll ask my WHF contact about this issue.

There is no nice way to say this, but that is bad thinking. However it is clear you have no intention of listening so I won't upset you more.

Good thing you are not pulling permits as they might follow the rules.

Bud

 

[snic]

There is no nice way to say this, but that is bad thinking. However it is clear you have no intention of listening so I won't upset you more.

Good thing you are not pulling permits as they might follow the rules.

Bud

I wasn't trying to come across as resistant to your advice, sorry if I did. My comments that you quoted were just speculation on why I think the backdraft issue may not be a major one; they most certainly don't come from an informed point of view.

As I said, I do plan to talk to the guy who is selling me the WHF about this. They are very common on the west coast, where gas appliances are also common, so I'm sure this particular safety issue has been considered and there exist parameters that should be followed.

 

[snic]

I wasn't trying to come across as resistant to your advice, sorry if I did. My comments that you quoted were just speculation on why I think the backdraft issue may not be a major one; they most certainly don't come from an informed point of view.

As I said, I do plan to talk to the guy who is selling me the WHF about this. They are very common on the west coast, where gas appliances are also common, so I'm sure this particular safety issue has been considered and there exist parameters that should be followed.

So I googled a bit to learn more, and here's an explanation of what Bud is referring to:

https://carbonmonoxidemyths.com/co-warnings/whole-house-attic-fans-and-carbon-monoxide/

Several other sites say basically the same thing, which is essentially the same as my idle speculation: as long as enough windows are open and attic ventilation is adequate, backdraft won't happen. The problem arises when the WHF is improperly used when windows aren't open or the heating is on. I am not sure how to address that other than to make sure whoever might use the WHF (including guests) knows not to use it except with open windows.

I think one part of my strategy to deal with this will be to simply unplug the WHF during the winter, and put the wireless remote controls away.

Edited to add:
Looks like CA requires WHFs (at least under some circumstances) - but they also require interlocks to prevent gas appliances from operating at the same time:

https://www.greenbuildingadvisor.com/article/should-we-install-a-whole-house-fan

But I suspect that the vast majority of WHF installations don't have any combustion safety measures at all. Yet, as far as I can tell, there have been very few problems, so the risk is low.

 

[carpdad]

My previous town told me to add make up air vent next to the furnace/water heater. In nj and building codes getting stricter every year. I didn't understand the calculations and still can't really find a website with easy to understand info. It involves people, living space/not, btu and such. Maybe that could help you with back draft issues. I just had to drill a hole in the rim joist and add 50 cfm inline fan (4" diameter) with arrow pointing inside. Then for outside, vent cap with the door removed. Current house, I will add motor controlled inline baffle to block cold air coming in. Furnace had an outlet for humidifier so electric was simply adding a plug to the fan. When furnace turned on, fan turned on. I wouldn't necessarily trust the installer for this calculation. Whole house fan is fairly simple install and neither carpenter nor the electrician may understand what you need. You may have to call in fairly large hvac company.

 

[Bud9051]

I'll let you calculate your house volume but for this example I'll use 2,000 ft².
Assuming 8' walls that would be 16,000 cubic ft. With a 5,350 CFM exhaust fan it would exhaust the entire house volume in 3 minutes.

Now there will be restrictions getting all of that air to flow through the intake and exhaust openings but that still sounds like overkill. Point being, a much smaller fan would probably do the job just fine.

As an example, I use a large 3 speed window fan similar to this and use low speed (1,400 cfm) and cannot leave it running all night, too cold.

Once you resolve the passive attic ventilation and ensure it is well air sealed with lots of insulation, IMO, give it a year to see if the WHF is really needed.

Bud

 

[snic]

I'll let you calculate your house volume but for this example I'll use 2,000 ft².
Assuming 8' walls that would be 16,000 cubic ft. With a 5,350 CFM exhaust fan it would exhaust the entire house volume in 3 minutes.

Now there will be restrictions getting all of that air to flow through the intake and exhaust openings but that still sounds like overkill. Point being, a much smaller fan would probably do the job just fine.

Right - I chose that model because it is advertised as for houses up to 4500 sq ft, and my house is 4400 sq ft. In addition, there are 5 rooms with cathedral ceilings, so that increases the volume even more.

Even so, yes, at full speed, that's a lot of air changes per hour. The idea is that you run the fan at high speed as soon as it's cooler outside than inside. The air in the house will rapidly cool down, but the house and its contents take much longer to cool down, and continue to radiate heat. If you just turned the fan off, the inside air would gradually get warmer. So you turn the fan down to a low speed and let it run all night, balancing the radiant heat effect. (Then, of course, in the morning you close all the windows and run the a/c if necessary - hopefully it won't be necessary until much later in the day than if the house hadn't been fan-cooled the night before.)

One argument against WHFs on the east coast is that it doesn't often cool down at night to a comfortable level, and even if it does, the air is humid. So it will take your a/c system more energy to reduce that humidity the next day. But I've lived here for more than 10 years, and have found that we run the a/c only for a few weeks in summer. The rest of the time, we open all the windows and run fans all night, and grumble that it's so pleasant outside yet so hot inside. Running the a/c when it's cooler outside than inside just seems insane, yet about half our a/c usage seems to be under those conditions.

 

[Bud9051]

I'm in Maine and it is a rare occasion I need to set one of the window ac units. Even in NJ I used window fans. I suspect once you correct the attic ventilation and insulation the evening overshoot where that attic keeps cooking the house will be less.

Sounds like you are more concerned about comfort than energy costs, which is common, and having more options is good.

Enjoy,
Bud

 

[3onthetree]

Snic, I don't see anything wrong with your thought process and you've crossed your T's and dotted the I's. You won't get much love on this forum for Whole House Fans when the minutae of fixing porous buildings is prioritized. The fact is, they are very common and beyond the benefit of exchanging cool air for hot air, the physiology and psychology of cooling is what is desired - that is, the breeze effect on your skin and the senses impacted by fresh air. The presence of proper insulation/energy efficiency providing comfort during mechanical heating and cooling is a separate, albeit parallel, pathway. They are accepted/recommended by energy.gov, the IRC allows exhausting them into the attic, and the HVI sets standards for them.

The entire purpose of their installation is to open windows, so when that occurs backdraft is minimized. You can agree with the value of CA adding an interlock to gas appliances, but using that logic then everyone should also add an interlock on their gas stove to guarantee a pan is actually on the burner - but why would one light a burner without a pan as that is the entire purpose of a stove? Also, the arguments of bringing in humid or hot air is null, as that is not when users choose to use the fans.

To get the "breeze" effect, HVI actually recommends Whole House Fans air change every 2 minutes (for an occupied room) and CFM calculated to 3x square footage. This total CFM seems outrageous, but if you think about it, you size bathroom exhaust 1cfm/1sf and you feel no noticeable air movement. However, most fan manufacturers size for around 10-15 minutes, and in reality, the max CFM won't be realized because of varying how much a window is open, an incongruous path through a house, and attic ventilation.

Which brings me to attic vents. Using some combination of ridge, gable and box vents (in concert with soffit vents) at the same time does allow air to be sucked in at certain conditions. The confusion occurs attributing it has to do with old vs. modern thinking about energy efficiency and thermal dynamics. It has nothing to do with that, but with tried and true aerodynamics, the same principles Richard Petty and skyscrapers use. When wind passes over a ridge vent, the low pressure on the back side can pull air out of the vent. That's good, but if there is a box vent near it, it can suck snow from the roof into that box vent. Now, with a perfectly calm winter day or windless rain, there should be minimal concern.

So to answer if you should keep a gable vent for the Whole House Fan after installing ridge and soffit vents, the answer is it depends. Whether you can satisfy venting by other means. How much of an appetite do you have for an occasional intrusion of wind blown snow or rain.

 

[snic]

The presence of proper insulation/energy efficiency providing comfort during mechanical heating and cooling is a separate, albeit parallel, pathway.

Right - we have to use heating extensively in NY, so of course I am also interested in sealing the house and making sure the insulation is sufficient.

The entire purpose of their installation is to open windows, so when that occurs backdraft is minimized. You can agree with the value of CA adding an interlock to gas appliances, but using that logic then everyone should also add an interlock on their gas stove to guarantee a pan is actually on the burner - but why would one light a burner without a pan as that is the entire purpose of a stove?

I'm not sure I completely agree with the CA logic - I think it's a CYA measure. They pass a law saying you have to have a WHF in a new building. Someone does something stupid and gets killed. Then the state gets blamed. On the other hand, where the analogy to a stove breaks down is that you can see the flame if you accidentally turn the gas on, or hear the igniter click. Everyone knows an unattended flame or a stream of gas is dangerous. Turning on a fan is not generally considered dangerous. So there is an extra education step.

Also, the arguments of bringing in humid or hot air is null, as that is not when users choose to use the fans.

Exactly. The point is that there are PLENTY of nights where the humidity and temperature are both acceptable.

Which brings me to attic vents. Using some combination of ridge, gable and box vents (in concert with soffit vents) at the same time does allow air to be sucked in at certain conditions. The confusion occurs attributing it has to do with old vs. modern thinking about energy efficiency and thermal dynamics. It has nothing to do with that, but with tried and true aerodynamics, the same principles Richard Petty and skyscrapers use. When wind passes over a ridge vent, the low pressure on the back side can pull air out of the vent. That's good, but if there is a box vent near it, it can suck snow from the roof into that box vent. Now, with a perfectly calm winter day or windless rain, there should be minimal concern.

So to answer if you should keep a gable vent for the Whole House Fan after installing ridge and soffit vents, the answer is it depends. Whether you can satisfy venting by other means. How much of an appetite do you have for an occasional intrusion of wind blown snow or rain.

One strategy I'm considering:
1. Install soffit vents and enlarge the gable vent.
2. Install WHF
3. Wait a few years, until we have a new roof. During that time, assess attic temperatures relative to outside temperatures in summer.
4. Replace roof and install ridge vent at the same time.
5. If we get snow and rain coming in the gable vent, figure out a way to seal it up (e.g, by putting some plywood over it).
6. Assess summer attic temperatures to see if the attic is cooler with the ridge vent. If not, it could be because not enough air is being pulled through the soffit vents (because the path of least resistance is through the gable vent, which will be quite large), so the ceiling of the attic doesn't get cooled. To test that, seal the gable vent and assess again.

Basically, the idea is to test the strategy of opening the gable vent in summer and blocking it in winter.

 

[3onthetree]

Or, if it is close enough, just duct the WHF direct to the gable vent, so you are not using the vent for general attic ventilation.