Will positive pressure work to vent an attic (i.e., blowing air in)? Attics exhaust fans are a bad idea. But what about blowing air in (using positive pressure).
My father is interested in removing heat from his attic to try to keep the upstairs of his house somewhat cooler. I have advised him that generally speaking, attic fans are a bad idea, which he has come to terms with. However, with that understanding that creating negative pressure in the attic can cause all kinds of problems (sucking out air from the conditioned space, possible back-drafting of combustion appliances, etc.), he has come up with the idea to blow air into the attic instead (i.e., creating positive pressure) to force the hot air out. I have already discussed with him that it is the structure that is heating up, and radiating heat into the attic, so an attic fan will not likely help much. So that aside, since he is determined, will he actually be able to create any air flow out of the attic by using positive pressure? Is there a fan powerful enough, and if he finds and installs it, what are the pros/cons that we may not be thinking of yet?
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When a House is Air conditioned...the absolute pressure in the upper portion of the house will be negative with respect to the outside(WRTO).
If there are any leaks in the air barrier between the attic and the house...hot attic air will "spilll" into the house.
Pressurizing the attic would only make things worse.
The best money spent would be to make the attic floor airtight and add ample insulation above.
Thanks John -- that's what I pointed out when he was considering an EXHAUST FAN (hence the "bad idea" part of my original question). But in reference to your response, I'm not sure how creating POSITIVE pressure in the attic (via a fan blowing IN) would allow conditioned air to "spill" into the attic. This seems like a different issue to me.
My feeling was he would have trouble finding a fan powerful enough to create an air flow through 1,000 sf attic space. And if he did find a fan that worked, it still would not resolve the heat issue, and there may be some unexpected consequences.
Just some further background info -- the house is in the Boston area, so hot, humid summers. There is no central air conditioning, just a single portable AC unit. He has been diligently working at air sealing in the attic.
Thanks!
Erica,
When you install a fan to pressurize the attic, the attic will become pressurized with respect to the outdoors and with respect to the conditioned house below. The pressurized air in the attic will force its way through ceiling cracks into the conditioned house.
In other words, the fan will make the ceiling leak at a higher rate than it otherwise would. You'll end up with more hot air leaking through your ceiling during the summer than if you had no fan.
Hi Erica,
I didn't say conditioned air would spill into the house
I said Attic Air would Spill into the House
(if the house were air-conditioned and it is hot outside)
If you pressurize the attic...
the hot air will flow/spill into the house even faster
thanks Martin...I was typing while you answered
Thank you both -
(John, apologies, I just realized I misread your response!)
Will pass along the info to my father...
Erica
I realize this is an old post....but it pops up. The idea that attic fans are a bad idea is nonsense. And so was the study from the school in Florida. Removing hot air from an attic works. I would like to know who on this site is familiar with manual j load calculations? You can get confused with radiation, convection, and conductance, but the issue boils down to temperature Difference and resistance (r value/ u value). If your attic is 140 or 90 that will make a huge difference! If you are worried about co gas made sure you also remove your bathroom fan, stove vent etc. A little information is dangerous and wasteful. Cheers
Sandy,
If you have developed an easy way to lower the temperature of your attic from 140°F to 90°F, that method would lower cooling costs -- but only if the method used to lower the attic temperature required less energy than the amount of energy you expect to save, and only if the method doesn't have negative ancillary effects (like depressurizing the house or sucking conditioned air through ceiling cracks).
Researchers haven't yet discovered a way to lower attic temperatures that is inexpensive enough to justify widespread implementation. For an article discussing all of the drawbacks of powered attic ventilators, see Fans in the Attic: Do They Help or Do They Hurt?
I assume that you were being ironic when you suggested that readers should "make sure you also remove your bathroom fan [and] stove vent" -- but if irony was your intention, your point escapes me.
Q. "I would like to know who on this site is familiar with Manual J load calculations?"
A. Many long-term readers and contributors to GBA are familiar with Manual J load calculations. If you are interested in learning more (rather than merely implying that this site's readers need to study more), you may be interested in reading these four articles:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
Sandy,
I'm interested in any argument on a subject that overturns what is accepted wisdom - but you really haven't provided any basis on which we could change our minds. Why not flesh out your reasons for using attic fans so we could see why they are a good idea?
I know it's an old post but it fits the bill for what I've been working on for 8 years.
A bit about our house.
1970, Ranch Style ( u shaped ), single story, 3600 sq ft. There is no radiant barrier or foam insulation under the rooftop sheathing between the trusses. Consider this a very basic house with nothing extra or modern to help with insulation.
I have a set of GoVee temperature sensors and gateways around the house totaling 30 sensors I use to monitor temperature and humidity. They are installed in every bedroom/bathroom/kitchen/living room/dining room as well as the garage and all 4 corners of the house externally at the yard and 6 locations in the attic to monitor the temperature over the garage, north ridge, center ridge, south ride and master bath ridge.
Prior to the 2019 Dallas Tornado that politely ripped our rooftop completely off the house, we had attic had temps of 150+. There were 2 solar powered attic fans pushing 1000CFM but they were not working as expected.
While replacing the roof, I had the solar powered attic fans removed, removed a nonfunctional gable vent ( just installed for aesthetics, literally boarded up from the inside ), ridge vents installed on every horizontal ridge and installed soffit vents every 4 feet around the house. The entire attic was then passively vented with ample ventilation per code for how a house SHOULD be vented to pass code.
The new roof along with the soffit vents brought our attic temps down from 160~ in the peak of the summer to now 125~.
This year we had our worst heat wave since 1980 with temperatures outside hitting 109 and heat indexes around 115-120. UV was 11+ with winds randomly going from 5mph from the southeast to the 15mph from the southeast. Our attic temperatures remained 120-130 however the radiant heat continued to cook the attic floor which slowly made its way into the conditioned living space. The temperature began creeping up to 76-78.
I installed a 1500CFM Garage Attic Fan, made by QuietCool, which lowered the garage attic space from 125 to 110 ( Not quite the difference I wanted but 15 degrees is 15 degrees ). Even though the ridge vent let most of that air out, it "pushed" some air across the living area attic space which brought down those temps by 5 degrees.
I just installed a 1500CFM Whole House Fan on our back porch and front porch, also by Quiet Cool, and directly the air into the main attic space over the living areas. Yes, I know Whole House Fans are designed for indoor but I needed airflow to test this and the fans are rated to support it. This dropped the attic space by 15 degrees.
So overall I've spent $1500 on fans and it made enough of a different to keep the attic temps low enough to maintain 74 degrees during peak hours.
I'm now working on a system using 8 x 500CFM fans positioned in the soffits around the house to help "pull" cooler air from the soffits and "push" into the attic space. Each fan is roughly 20 watts which is less than .2amps. Overall I believe all of the fans combined are less than 20 amps total, running on 2 separate circuits.
In theory, this would create a positive pressure attic space which might push air into the conditioned living spaces. However we have on 80 feet of ridge vents which eliminates any chance of positive pressure in the attic.
All of that being said, while my attic space are now reasonable to prevent 80 degree internal temperatures, I am also getting price quotes on installing closed cell foam insulation under the roof sheathing which will block all of the radiant heat from ever making it into the attic space, and lower the attic space to around 105 degrees in the summer. I don't know the cost of this insulation project yet, and while the ROI ( return on investment ) of spending $10k on closed cell foam insulation is not going to be 1,2 or even 5 years, it will keep the living quarters cool no matter what the outside temperature and not required ANY fans for active ventilation. This should allow all of the ridge vents and soffits vents to handle all required ventilation.
Now I get to figure out what to do with all these bad ass 1500cfm fans by QuiteCool!
I was thinking you have a very inventive mind and may be a genius because I haven't quite put my finger on why your plan wouldn't work. But when you said you were planning on putting closed cell spray foam under the roof, and your reason for doing it - blocking all radiant heat, it moved me closer to thinking you might be a crackpot. I hope you have a very good reason for wanting to do that, such as HVAC equipment in the attic (not just ducts). If you have read anything on this site, you should have been exposed to the risks of 2-part spray foam and its damage to the environment in its traditional formulations. It's also hard to deconstruct spray foam installations if future changes are in order, and hides water damage to roof sheathing if you have it. If you haven't read that info, then you should do it right now.
Inventive, yes. The laws of thermodynamics explain everything. Crackpot? Very polite. My apologies for politely saying “bless you” in return. ☺️
Closed cell foam insulation 6” thick is proven to eliminate most radiant heat. I’m not saying no heat will penetrate the attic. I’m saying it will block the majority of the heat. By blocking most of the heat, the soffit and ridge vents will cool the attic to temps below or up to the outside temperature.
As far as why? You nailed it. I live in Texas. Most HVACs are installed in a suspended fashion in the attic. So temps of 120, 150 or 180 basically bakes your equipment taking your 36 degree coil and increasing it to 50 thus eliminating the majority of your cool air.
If I could redo it all I would install the HVAC in an air closet in the garage for noise reasons or have a 2 story house with air closets under the attic space in ventilated rooms. Both options take the radiant heat on the unit outside of if the equation. Unfortunately that’s not cost effective at this point.
Damage to the environment?
* Freon leaks damage the environment.
* Loss of energy through a baked attic lead to more energy usage which equally damages the environment.
* throwing away worn out equipment damages the environment.
* Yes, closed cell foam is bad for the environment.
* radiant barrier spray is bad for the environment because of the chemicals.
* radiant barrier aluminum can be energized during a lightning strike resulting in a fire hazard.
* Humans have screwed up the environment.
There is a cost for everything. Dig a hole and build your house in a mountain 6ft under the topsoil at a constant 62 degrees and you’ll never have to heat or cool your house.
I can argue pros and cons of everything all day long. I’d love to build a brand new air tight pier and beam house with 2x12 walls, insulated with green insulation and a green roof. Setup a proper fresh air ventilation system and install the AC in the garage with ducting in the crawl space. But based on that you can also say it’s not cost effective. We do what we can with what we have.
If I win the lottery or strike it rich with one of my I.T. patents trust me I’ll do it as green and perfect as possible. Until then……. it’s a giant science experiment weights risk to the environment against cost.
This post might be spam as we've been seeing a bit of an increase in that as of late, unfortunately. On the assumption that this is a legit post, I offer the following:
1- blowing air from your garage into your attic is a problem. Attics are REQUIRED to be isolated from living spaces AND ATTICS per fire code. This is why you need a fire rated attic hatch or stair assembly if you have an attic access from your garage. There are two reasons for this:
(a) protection against fire spread from the garage (from a vehicle, for example) to the living space, and to provide occupants enough time to get out of the house in the case of a garage fire.
(b) protection against potentially harmful fumes in the garage (like vehicle exhaust) from being drawn into the living spaces and causing a health risk for the occupants.
Note that (b) is an especially big problem here, since you are using the fan to ACTIVELY draw garage air into the attic, pressurizing the attic, which ensures that MORE garage fumes will get INSIDE the home. This is a Bad Thing.
2- Is the energy consumed by all those fans at least offset by the energy saved on air conditioning your living space? If the fans use more energy than they save, you are net negative here, and this plan is COSTING you money, not SAVING anything. Ideally you want to use only passive venting and convective airflow so that you don't need any outside energy source to ventilate the attic.
3- Spray foam under the roof sheathing isn't really a radiant barrier, it's insulation. You'd be better off using something like Reflectix here if you just want a radiant barrier, as it's much cheaper than spray foam and there have been some reports of it helping to reduce attic temperatures. Spray foam here is going pretty overboard, and you'd need to see if the cost of the spray foam will at least be offset by air conditioning savings over the lifetime of the home. Since you have a vented attic, the spray foam you are considering here will do nothing to help you in the winter, so you would ONLY have potential air conditioning cost savings available to offset the installation cost of the spray foam.
I think you would be much better off doing some basic air sealing of the attic floor to minimize air leaks and stack effect in the home, bringing the attic floor insulation up to at least code level, and ideally more, and taking care to cover the exterior wall top plates as much as possible. I would use loose fill (blown) cellulose for this purpose. I would ensure there were adequate (code minimum is 1", I prefer 1.5" or more) vent channels in EVERY rafter bay, and I would put in one of the continuously vented soffit systems (not the "every so many rafter bays" round or rectangular vent screens). I would make sure there is a proper ridge vent along all roof peaks, and those "proper ridge vents" are the kind that have a ridgid vent piece that gets shingled over, not the "shingle over foam" type.
I would not use fans, and I would not depressurize the attic -- try to make sure you have a little more intake (soffit) vent area compared to the exhaust (ridge vent) area.
Bill
I assure you my reply is not spam.
I’ve spent 8 years in this house in Texas, replacing both AC units, old leaky ducts, non sealed/IC recessed cans with new canless wafers, smart switches for better energy consumption and fresh air intake systems used to pressure the living space. Our electric usage at 72 degrees in the summer per day in this 3600 sq ft house is down from 180kw to 120kw.
I’ve calculated the air ventilation and we have installed 1.5 times the amount of soffit ventilation intake to amount of ridge vent exhaust. It reduced the temps by 20-30 degrees which was a huge improvement.
We blew in enough insulation to reach R39.
The cost of the fans I’ve installed to run comes to $15 per month and the KW per day reflects a net cost savings.
The solar system installed covers 1.3x our energy usage and net metering makes it a wash with me only paying an energy delivery fee monthly from ONCOR, our utility provider.
The garage is not for cars. The garage is a workshop I use for woodworking. And the garage attic fan is fire rated, check out the links in my last comment. Our city and home inspector both signed off on it. if or when I sell the house the fan will be removed to not pose a risk for the next homeowner who will probably park cars in it.
Also, our water heaters are in an exterior water closet separate from the garage so the exhaust vents directly to the outside.
Have I spent way more money on this house than a typical homeowner would to increase energy efficiency and comfort? Absolutely. We’ll be here another 25 years at least so I have plenty more projects including ripping out every inch of aluminum wiring but that’s a whole different conversation.
It sounds like you've been doing a lot of good things on your house. It's definitely an unconventional approach but not as misguided as I originally thought. I still think you are mixing up concepts though. As Bill said, spray foam is not equivalent to a radiant barrier. It's just a form of insulation and the physics of insulation and RBs are very different even though the final results can overlap, at least in hot weather.
Overall, it seems like you enjoy learning about things, including green building. You could probably profit from reading this forum more and would probably enjoy the educational aspects of it. I do. One is never too young or old to learn new things. It also helps in avoiding reinventing the wheel.
100% agree. RB and foam are very different.
My uncle used foam under the roof of his 100k sq ft concrete shop and it dropped his temps from 150 to 90. Granted it was installed nearly 18” thick.
A friend here in Dallas had his old house RB sprayed and had similar results. His new house they installed the RB foil and overall he said it’s not comparable to the spray.
I budgeted $2k for this 1st experiment dealing with airflow to monitor changes in temp. It does support more flow = more cooling = lower temperatures. But as Bill said, I do think continuous soffit vents around the entire perimeter will do the same. I’m measuring length today to calculate original intake ( pre fans ), to new intake with fans and proposed intake with continuous soffits and no fans.
I’ll spin up a new post later on with all my specs and numbers. You guys have been on this forum much longer than I so I’m always open to learn.
Here are the links to the fans I installed.
1500 CFM Garage Attic Fan = https://quietcoolsystems.com/garage-fan/
* This is setup for proper code and has a temperature sensitive link that will melt if exposed to 165 degrees preventing a fire from the garage to spreading to the attic or a fire in the attic from being fueled by fresh air from the garage.
1500 CFM Whole House Fans = https://quietcoolsystems.com/whole-house-fan/energy-saver-whole-house-fan/
I installed these on smart switches controllable by Apple HomeKit and Google Assistant.
They are triggered with thermostats in the attic that turn on when the temperature exceeds 105 degrees.
There is a lot of misinformation out there for roof venting. Lot of hand wavy arguments.
We can actually try to put some numbers to this.
Lets take a typical 3000sqft house. Most likely it will be leaky somewhere around 10ACH50. If older it might have R19 batts above the ceiling for insulation. I'm going to ignore for now that there is a good chance that ducting and air handler is in the attic as well.
So that 3000sqft 10ACH @ 50 PA house leaks around 225CFM. If the attic is 160 that means your are gaining:
225*1.08*(160-75)=20655 BTU of heat through air leaks
3000*(160-75)/R19=13421 BTU through the insulation
Brining the attic down to 120F would drop that a fair bit to 10935+7100 BTU so definitely a big difference.
Instead of venting, you air seal the place and get it down to 3ACH plus insulate the attic to R50. You natural air leakage drops down to around 67CFM so your losses with a 160F attic is:
67*1.08*(160-75)=6150 BTU of heat through air leaks
3000*(160-75)/R50=5100 BTU through the insulation
Which is 60% less than by improving venting, requires no extra power or equipment and saves you money all year not just in the summer. This also means that if there is another winter ice storm your place will stay much warmer longer even if the heat goes out.
Air sealing is really the best way to save energy in any climate. Not easy to do but well worth the effort.
Spray foaming to the roof to reduce attic temps is a giant waste of money. The only time it makes any sense is if you have your mechanicals up there and there is no way to move them.
Awesome details on every point, and I love the numbers. You are correct, both of our ACs ( 3 ton for bedroom and 5 ton for living area, 3600 sq ft total ) are suspended in the attic. Both ACs were replaced in the last 3 years ( compliments of the 2019 Dallas tornado ) with Rheem 18 seer units which also made a huge difference in energy cost and cooling abilities.
Oh boy.
8 tons and barely keeping up for that size code min house means at least half your AC is cooling the great outdoors.
With the air handlers in the attic getting somebody with a duct blaster to seal your ducting is a very short ROI.
Fortunately the main air loss is on the 5-ton ( living area ) side. I've pressure tested the 3-ton ( bedroom ) and it's much less. There are about a dozen old non sealed canned recessed lights that leak and I have a few outlets/switches that are leaking.
It's definitely work in progress but I've been watching teh smart meter usage and I'm happy to see the daily kilowatt usage go down even as the outside temperature has been going up. So it's progress!
In reply to your post #15, you won't find too many people on GBA who will fault you for spending extra money on energy saving things like insulation -- most of us are "go beyond what code requires" people, so we understand :-)
What IS important is to make sure you make good use of that extra money. An example would be using actual radiant barrier material for a radiant barrier and not spray foam, since the radiant barrier is much cheaper. You can use the savings towards something that will gain you more performance that way.
Akos pretty much covered what I would have said, but I'll add that in addition to air sealing, simply upping the insulation on the attic floor offers a lot of bang for the buck, and is much simpler to do in many cases than fancy vent systems. The type of insulation you use matters too -- loose fill is better than batts, and cellulose is better than fiberglass if your main concern is radiant heat. I would try the air sealing Akos and myself have suggested, and then maybe try for R60 or so of blown cellulose on the attic floor. I think you'll find you get at least, and probably much more, performance from air sealing and insulating the attic floor than you will for all the other things you've been considering. Maximize your bang per buck. Your vent system is helping you by reducing the thermal differential between the attic air and the living spaces. Insulating and air sealing reduces the thermal transfer between those two spaces, so you accomplish the same goal of improving the comfort of your home, but you do it in a different way, and the insulating and air sealing is saving energy all around, and in all seasons, which is why it's the preferred method to solve the issue you're discussing.
Bill
I’m currently working on replacing the last few light fixtures that are canned non sealed, non insulate contact with canless wafers. And we have an upper beam in the family room that recently developed an air gap after repairing the foundation.
My next steps as far as a repairs and upgrades are below.
* Replace the last few canned lights with canless wafers and perform an air pressure check of the whole house to make sure they’re not leaking.
* We recently repaired the foundation which shifted the family room ceiling and created an air leak. We’re going to pull the plywood appearance boards used to contain the wiring for the beam mounted fans and ensure they’re sealed entirely before replacing them.
* Every inch of the AV plenum, coil, etc have been checked and sealed. Next is going to every duct opening in the attic, pulling the insulation away and sealing them properly. I did this step 8 years ago but we’ve done foundation work i know a few are leaking due to the Sheetrock shifting.
* We’re already at R39 insulation in “most” of the areas. The few areas that are not R39 are the areas where work was done on light and fan boxes and the workers didn’t spread the insulation back properly.
The last step which is what this entire experiment has been for is to calculate exactly how much intake I can create ( by the use of fans initially and calculate CFM ) before the pressure if higher than the ridge vent can exhaust. Our roofer is coming by today so we can perform the measurements and review the ridge vent specs installed in 2019 when we replaced the entire roof.
Once we come up with those numbers and verify it with multiple manometers we’ll begin installing continuous ridge vents ( also recommended by Bill above ).
I’ll keep comparing foam insulation to radiant barrier until we get to that point. But I will say in the past 8 years we’ve had 2 tornados and 5 batches of straight line winds. Multiple houses in our neighborhood have been either demolished or damaged due to the roof be compromised . The closed cell foam insulation is appealing because is adds to the structural integrity of the roof. One of the new builds built prior to the tornado was built with closed cell foam in every exterior wall and rooftop under the trusses. It was the only house that was not scratched during the tornado.
I just signed up for the membership for this site since it clearly has a wealth of information and a massive set of brains like you guys behind it. I’ll spin up a new post with some data around our house, sq footage, intake/exhaust and the temperature history I’ve collected on the house recently.
As Akos mentioned above, you probably have significant air leakage. I think you're targeting the wrong thing here: solar gain and a hot attic doesn't help cooling performance, but air leaks are a much bigger problem. I'd attack the air leaks first, since you have more to gain there, then consider doing more in the attic. I really do think that a thorough air sealing job and some loose fill (blown) cellulose on the attic floor will do wonders for improving your air conditioning performance here, and those are much cheaper ways to go than spray foam or anything fancy.
I'd consider rigging up a DIY style blower door, then using some smoke sticks to pinpoint the worst leaks and fix those first. Canned foam, a foam gun, and some of the plastic tips for the gun are your friends here. Some polyurethane sealant is also helpful. Go to town on the air leaks you find, then see how your situation improves. I think you'll be pretty surprised at the results.
Just as an example, when I replaced my T+G ceiling in my 'sun room', I spray foamed the roof (not because I'm a huge fan of spray foam, but because this was a very low slope, unvented, roof with no other practical options). The old assembly had the usual leaky T+G with nothing behind it, and it was connected to the rest of the vented attic areas in the house. The following winter, I approximately halved my heating costs after doing just that one project. Nothing else I've done has resulted in such a major energy savings as that one insulating and air sealing project, and it was primarily the air sealing part of the project where the major performance improvements came from. These projects have the added benefit of improving both heating AND cooling performance too.
Bill
100% agree. This 1970 house has ALOT of air leaks. I've spent the past few months foaming intake vents that were plumbed through the walls from the ceiling to the floor but never sealed. I found those by using the smoke gun and pressurizing the house. Between that and the old non-sealed/non-insulate contact cans I've eliminated a ton of air gaps this year. I've been watching the smart meter usage and I'm happy to say the usage is actively going down as I watch the outside temperature go up! ( we hit 110 for over a week this month ).
Even the electrical outlets and light switches in the exterior and some interior walls have air drafts. Those are turning into a mother to fix.
I'll spin up a new thread later on with some before and current progress details. It's turning into an adult science fair project at this point :D
I recommend red silicone high temperature caulk (NOT intumescent fire stop caulk) for sealing holes in electrical boxes. Smear it into the screw holes and cable clamp areas. I usually use canned foam around the outside perimeter of the boxes. You don't want to have the foam expand INSIDE the box.
It always feels good when you can actually MEASURE the improvements resulting from your efforts, so it's good to see changes on your meters!
Bill