Is Negative Pressure Causing a Problem in Your Home?

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Negative pressure in homes can bring outdoor air in through the chimney

Ever have the smell of your fireplace inside your home?  It's not uncommon.  Here's a question I got from a reader named Rob last month:

We always have a smoke smell from our fireplace, more on some days than others.  I had the chimney cleaned really well and the damper closed and it was fine.  Then we just built our first fire since cleaning this weekend and the smokey smell is back.  It has a metal damper in the firebox. I assume somehow negative pressure is forcing air back down the chimney especially when we run bathroom fans or the dryer I suspect.

What's going on here?

Do you have negative pressure?

Rob suspects negative pressure is behind the fireplace smells in his home.  First, when we talk about negative or positive pressure, we're talking about the pressure difference between two places, usually indoors and outdoors.  A negative pressure in the house means the indoor pressure is lower than the outdoor pressure, which can cause air to move from outdoors to indoors.

But that infiltrating air needs a pathway to do so.  For air to move across the building enclosure, you need two things:  a pressure difference (positive or negative) and a pathway.

It's quite easy to find out if a house is indeed running a negative pressure.  All you need is a manometer, and the nice ones cost over $1,000.  You could get one with lower precision and spend less than $100.  You also could use a magnehelic gauge for less than $100.

Or you could do it the easy way.  With the house operating at what you suspect is a negative pressure, go to a door and open it just a bit.  If you stand inside and put your face near the crack in the door, you'll feel the air blowing on you if there's a negative pressure.  You can feel it with your fingers, too, and it works even better if you lick them (part of what Joe Lstiburek calls the "look, lick, and squirt test").  If you still can't feel any air movement through that opening after you lick your fingers, you don't have enough negative pressure to worry about, at least not right there.

If you have positive pressure, the air will be moving toward the outdoors.

The 3 causes of pressure differences in buildings

Three factors are responsible for pressure differences in buildings:

Wind, of course, can create pressure differences between inside and outside.  Hurricanes and tornadoes can induce pressure differences big enough to blow the roof off of a house.  That's why building codes require hurricane straps in high-wind areas.

Stack effect is a density-of-air phenomenon.  Warm air is less dense and thus rises when surrounded by cold air, like an air bubble in water.  That can create some pretty big pressure differences, as you can see in the video below.  The two factors that affect the magnitude of stack effect are height and temperature difference.  That's why it's so strong in that video taken in a 44 story building on a cold day in Denver.

And then there's the wild card of mechanical systems.  Exhaust fans in your bathrooms and kitchen, of course, will induce some negative pressure.  The more airtight the house, the bigger that negative pressure will be.  Let's put some numbers to that idea. 

My house currently has a blower door test result of 5,200 cubic feet per minute at 50 Pascals of pressure difference (5,200 cfm50).  That's pretty high, but it's a 3,800 square foot house built in 1961, and it was close to 7,000 when I bought it last May.  But let's get back to the main point.  With an air leakage rate of 5,200 cfm50, my house won't show much of a negative pressure if I turn on a 50 cfm bath fan.  It takes 5,200 cfm to induce a 50 Pa pressure difference, so 50 cfm will be lost in the noise.

At the other extreme, some Passive House projects have air leakage rates of 100 cfm50 or even lower.  Two 50 cfm bath fans would induce a 50 Pa pressure difference across such a house.  That's a lot of negative pressure for a house, which is why Passive House designers and builders worry so much about the mechanical systems they use.

Other mechanical systems that can create pressure differences are clothes dryers, heating and air conditioning ducts, powered attic ventilators, portable air conditioners, and supply-only ventilation systems.  Atmospheric combustion appliances draw in air from the space around the appliance and send it up the flue, but that's generally small compared to the pressure differences created by the others I listed.

Finding the source of negative pressure

If you suspect negative pressure is causing problems, the first thing to do is figure out which of the three causes of pressure difference is most likely to blame.  Does the fireplace smell (or whatever symptom you notice) occur only when it's windy outside?  You can't stop the wind, but you can see about eliminating the pathways for air to move by doing some air sealing and closing the fireplace damper.

Does it happen only when it's cold?  If so, the stack effect may be to blame because it increases with temperature difference and most places have bigger temperature differences in winter.  But you also run the heat when it's cold, so it could be unbalanced duct leakage.

To determine if mechanical systems are inducing a significant negative pressure, turn on the one(s) you suspect and do the cracked-door test again.  Then turn them off and see if there's a difference.  That also works for pressures induced by unbalanced duct leakage.  Do the door test with the heating system on and then again with it off.

If you have the fireplace smell in your home like Rob does, finding the source of the negative pressure and reducing it should solve the problem.  You don't want to stop using bath fans, range hoods, and clothes dryers, though, so keeping the damper closed when the fireplace isn't being used should help.

Remember:  You need both a pressure difference and a pathway to have air movement, so eliminate one or the other or both and you solve the problem.

 

Related Articles

The 3 Rules of Air Leakage

Heat Rises...and Falls — Stack Effect, Air Movement, & Heat Flow

The Surprising Building Science History Behind the Revolving Door

The Sucking and the Blowing — A Lesson in Duct Leakage

 

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Comments

This is so interesting and timely. I have a furnace exhaust going between my house and my neighbour (picture here: https://www.treehugger.com/energy-efficiency/we-dont-have-energy-problem... ) and just got a Plume air quality monitor. This morning at 4 AM the NO2 spiked way up. Bathroom exhaust outlet is just a few feet away from furnace outlet, so if there is any negative pressure in my house that's a nice pathway right back in for the furnace exhaust. Something is going on.

Allison
Bailes

Lloyd, your exhaust fan wall cap may well be the entry port for the furnace exhaust.  If your home is operating under a negative pressure, there's probably a pathway through that exhaust fan duct when the fan isn't running.  If you've ever looked at the damper in a typical exhaust fan, you can see that they aren't meant to stop all backdrafting.

Lloyd, you should consider converting to an electric boiler soon. Apart from flue gas potentially entering your home, it looks like some flue gas may be recirculated through the boiler via the concentric termination causing efficiency loss and corrosion in the combustion chamber over time. Also, the flue is contacting a brick wall, maybe the neighbours house. Sooner or later those bricks will start to spall and I'm thinking you'll be on the hook for it.

I have a client with this exact same issue we've been trying to fix for over a year without luck yet. Large 1.5 story house on conditioned crawl/bsmt. Great room/kitchen is wide open with tall vaulted ceilings, but the designer put the wood burning fireplace in the corner of the room and the flue only goes up maybe 5-7 ft before the pipe penetrates the foamed roof line and the majority of the flue pipe is exposed above the roof. Client always has smoke smell when they open the glass doors on the fireplace...no issue when they're closed. There were lots of air leaks from the 2nd story I had them foam up to reduce stack effect and they fixed some issues in the crawlspace too, but didn't solve it. I don't think the combustion air intake into the fireplace is anywhere close to big enough either, and the flue pipe being exposed mostly above the roof line is contributing to the flue draft issue. Any other thoughts??

Allison
Bailes

Eric, I think they may be fighting that cold flue as long as the fireplace stays in that configuration.  With a lot of the flue outside in the cold, it's hard to get air moving up into it and out the top.  Sealing the leaks at the top of the house definitely should have helped, though.  I doubt the combustion air intake has anything to do with the problem.  They're mostly ineffective anyway, from what I understand.

Dr. Bailes,
Could you please address the stack effect in Coastal areas that experience 'reverse' stack effect due to hot outside and cold inside. There are some studys from Thailand but most simply address warm building and cold climate outside. My findings have been that there is definite air movement downward in low rise condos ( 5-7 stories) that is exasperated by penetrations from plumbing (especially sewer chase), electrical and any bone headed renovator punching holes over the years. Having some serious issues on Siesta Key, FL. Any suggestions about how to prove it exists, remediate and finally will a blower door accurately diagnose this movement of air during baseline operation prior to testing?

Thanks Karl Peer, Energy Rater, FSEC QA

Allison
Bailes

Karl, reverse stack effect is definitely a thing.  I wrote about it briefly in my stack effect article linked to in the Related Articles section above.  You can prove it exists using the cracked-door method I described in this article or using the technique Mac Sheldon described in his comment below but using an attic hatch in the ceiling instead of the fireplace.  Air sealing helps a lot with remediation because you can't get more hot air leaking in from the attic than is able to leak out at the bottom of the house. 

The baseline measurement during a blower door test may help a little, but it will depend on the house.  It'll work better in a 2 or 3 story house than in a single story house on a slab.  If the baseline is very positive on a calm day with all the mechanical systems off, it may be because of reverse stack effect.

This is one of the many reasons that I don't like fireplaces--of any kind. I have only had them because the previous owner had them. Oh well, to each his own.

Allison
Bailes

Roy, I'm with you on that one.  Fires should be built outside in the firepit!

OK first of all, who is still enduring wood burning fireplaces? :)

Convert to a proper gas retrofit, and get on with life! Eliminate the smell of smoke but be sure to include a CO detector to the project.

And second of all (then I'll go away :)), Allison, where did you get that photo of the home atop your always informative article?

Don't believe I've ever seen a home with such a lovely patina of weathered OSB, on the outside! And on the chimney, too 8-).

Allison
Bailes

Steve, yeah, I'm with you, especially if it's not a sealed combustion unit.  As I said to RoyC, fires should be built in the fire pit in the backyard.

I hadn't noticed that similarity before, but yes, it does lokk kinda like weathered OSB, doesn't it?  It's actually brick, though, and I took the photo in Pittsburgh back in 2011 when I visited a friend there.  I took a few photos of this house because I've never seen brick laid that way anywhere else.

I have seen (smelled?) what I consider to be a bigger issue with negative pressure in houses: Sewer odors. I was involved with a very tight research house that often had a bad sewer odor and I finally figured out that it was due to two factors. First, there was a very efficient and quiet bathroom exhaust fan that would often get left on because it was so quiet that you couldn't tell it was running. I could easily measure the negative pressure when it was running, a couple tenths of an inch of water. So the other obvious factor was a dry drain trap somewhere. I kept pouring water in drain traps, but it didn't help. I finally figured out that it was coming from a floor drain in a mechanical closet that did not have a trap. I contacted the plumber who did this installation and he admitted that he forgot to put in a trap. How do you forget to put in a trap? It turns out that he was smarter than me. For floor drains, he uses a dry trap which is just a rubber flapper valve that can be inserted in the drain after construction is complete. It worked great and you don't have to worry about it going dry. No more sewer smell. This was cheaper than installing a fireplace to cover up the smell ;-)

Allison
Bailes

Roy, you anticipated a future article there.  I've got another one planned about negative pressure and sewer gases with a nice little video from a LinkedIn friend. 

I didn't know about the flapper trap either, but that's a pretty cool idea.  And what a relief that you didn't have to install a fireplace!  (That was hilarious, by the way!)

Allison, in your next article on negative pressure or any other future articles, are you going to discuss how to keep back-draft dampers on kitchen, bathroom, and dryer exhaust fans from rattling when the wind blows? I live in windy Oklahoma and this is a terrible problem for me. I hate to admit that I have a new home of unknown tightness with no mechanical ventilation, so I wonder if a continuous exhaust would solve this problem.

Now that I think about it, I probably should have my house tested for tightness. Anyone know of anyone with a blower door in the Oklahoma City area?

I had to laugh this morning as I was sitting in the space that has a rattling damper while reading this question. Tamarack has series of non-mechanical fabric dampers that is a perfect solution for your problem. They are made to slide inside of metal hard-pipe of various sizes. Take out the plastic rattler and insert some pipe and this damper if you can get to the flex anywhere in the run.

Roy -- No Mechanical Ventilation?

Thanks Thomas, I will look into Tamarack. I didn't realize that they sold this type of product. Most of my rattling problems are with the dryer and range hood exhaust. Can these dampers work reliably under these conditions? Would it really be necessary to take out the existing flappers (rattlers)?

Yea, John, I confess. My house is not 62.2 compliant. When I moved to Oklahoma, I looked for the "best" new house that I could find in terms of construction practices. The one I bought is the only that I could find with the range hood and gas log vented to the outdoors. The real selling point for me was the conditioned attic (spray foam roof deck). But it has no central mechanical ventilation and no HERS score. I really would like to get a blower door test, but I can't find anyone who can do it in this state yet.

The builder also advertised that it had energy-efficient lighting. It turns out that he put some cheap LED fixtures in the garage and a CFL in the tornado shelter. The rest are incandescent. He advertised high-efficiency air conditioning. It turned out to be 14.5 SEER. It has been an interesting experience.

Roy, If you think about what air movement is causing the dampers to rattle, you will find adding a quiet fabric damper in series with the same direction of air flow will not change the dynamics. Low pressure on the outside from wind away from the house and high pressure from wind entering the house somewhere else will cause the dampers to move in and out (rattle). That same air flow will occur with the additional damper installed. In my opinion, you will have to remove the offending dampers regardless.

Thomas, you are probably right about the need to take out the rattlers, but I think that I will try leaving them in first to see if the problem still persists with the Tamarack dampers. However, I am still a bit concerned about using "fabric" back-draft preventers on the hot linty outlet of a dryer or the dirty outlet of a range hood.

Roy,
Try using a small piece of mans best friend, Duct Tape.

THis is such a perfectly timed article (was planning to google your site for this topic). I got a Airthings radon detector for Christmas and I'm bouncing over and under the safe limit. I have a couple of whispergreen bath fans that run on low all the time in very airtight "pretty good house" and it seems to be pulling air up from my basement. I have caulked and sealed everything I can down in the basement, so I think I am going to have to switch ventilation strategies.

Allison
Bailes

Clay, is the air in your basement worse than the air above?  If not, why would it make the IAQ worse?  If so, what are the problems you need to fix in the basement?  Or is the problem with the outdoor air being pulled in through the basement?  So many questions.

Thanks for the reply, Allison. Yes, the basement has poor air quality. It is unfinished, has a dusty (not musty, but like chaulky drywall) smell, and the radon levels are pretty high (~14pCi). 1st floor IAQ gets worse when we run the range hood, bath fans, etc. I put weather stripping on the door down to the basement, which helped the dusty smell, but the radon levels still increase. So, I am assuming that the negative pressure strategy I have now is pulling outside air in through the basement, and up into the living space. Granted, with it being colder outside, I have more stack effect right now than at other times, but I still wonder if I need to make changes.

Or maybe I just need to look into a basement exhaust system and/or a radon mitigation system.

Radon gas comes from the soil or groundwater and generally seeps into your house through basement cracks, drains, sump tiles, etc. That is why radon mitigation systems use a fan to depressurize the soil under the slab. So make sure your mitigation system results in a lower pressure under the slab than in your house.

Clay -- Yes you need to pull air out of the basement and exhaust it to the outside. The basement should be negative with respect to the floor level pressure above.

A fun project for a rainy Saturday is to mask your fireplace or a door then cut about an 8" diameter hole in the plastic. Tape the open end of a lightweight plastic bag to the opening. One from the produce section works well. Run your HVAC and watch the bag. Is it sucking into the house or out? Start your dryer and kitchen hood and watch what happens. With no mechanical equipment running you can observe stack effect too.

Pretty fun and a great science fair project for your kids!

Allison
Bailes

Mac, that's a very clever idea!  As I said in a reply to Karl Peer above, the same method would work with other openings, too, like the attic hatch.

Thanks Mac, you finally gave me something useful to do with a fireplace ;-)

This is a great topic and loaded with overlapping issues. First, I build very tight hurricane resistant homes in humid coastal NC. I always aim for less than .5ACH at 50 Pascal. I have learned that a dedicated (non-HVAC associated) dehumidifier is a requirement here, even during the summer months as my HVAC has been sized for low sensible loads that reduces my latent load capability as well. We maintain 45% or less pretty much year round including the 4 swing months when little heating or air is needed. Now on to the make-up air issue. Leaks in a building's envelope are only an issue is there is a pressure differential. Negative equals air infiltration and positive equals air exfiltration. I NEVER want my houses to be negative as I have no firm idea where that air might be coming in, along with whatever else it wants to drag along. Letting the house decide where it wants to breath is like having you nose placed near your lower exhaust port. God and nature intentionally placed every creature's nose at the opposing end of their body for a good reason. That being stated, I provide a 6 inch duct to the outside with a one-way damper that only allows air to pass if the house goes negative, thereby giving the house a "nose" where I want it and can properly deal with it. But that does nothing for air quality and the need for .35ACH makeup air. Enter a new product that I just found, the Air King QuFresh 120cfm Air Exchanger with temperature control. I simply connected it's 6 inch duct to my outside air duct and Voila, I can set the cfm needed, the temp and humidity ranges allowed, completely independent of my HVAC system just like my dehumidifier is. By the way, I fully condition all of my attic space with 3" closed cell foam under the roof-deck, so I simply add the makeup air to that volume so it can be mixed and conditioned as necessary prior to introducing it into the living spaces through vents between attic and the living spaces. The The Air King only operates when outside conditions are suitable per my settings. I have now introduced a pressure device that provides the necessary air movement.

In addition, if any are wondering where that makeup air gets back out, the bath fan dampers are always there as pressure release if needed. Which helps with air distribution as well in moving stale bathroom air out.

Great stuff. Thanks.

Can I create negative pressure in my entire home if I circulate air from one room to another? Basically, it would be a closed system with a partition between the two rooms. I want to exhaust on one end of the partition and intake on the other end of the partition.

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