Building Science-Tight Homes and Negative Pressures, When Should We Be Worried?

This post originally appeared on the Green Building Advisor Website.

A couple years ago I was asked to perform a blower door test on a new home.  The home was small with a footprint of only 1130 square feet.  When the test was completed, the test report indicated an air leakage rate of 91 CFM at the test pressure of 50 Pascals, .33 ACH50.  Completely unexpected for this code-built house.  (I actually measured the volume of the home again and ran the test two more times before I was convinced I had an accurate test.)  The contractor contacted me some time later and indicated the homeowners were noticing air coming through the exterior light switches when the dryer was operating.  The dryer was producing close to the same negative pressure as my blower door every time it was being used, a negative pressure of about 50 Pascals.  Is this a problem?

Well, as with just about every building science question, it depends.  Let’s start with the risks.

Negative pressures in a home located in a hot and humid climate may experience comfort issues.  The air leaving the home may be replaced by hot and humid outside air that will require conditioning, increasing the required energy needed to maintain comfort.  Depending on where the air is entering the home, there may be the issue of condensation on exterior walls, air conditioning ducts, and even cold waterlines.  This can affect both the durability of the structure and indoor air quality.

Cold climate homes can also have issues related to negative pressures.  Comfort, cost, and extremely low indoor humidity levels during the winter months can all be the result.  I’ve even seen the formation of frost on the inside of exterior walls where air is leaking into a home.  Homeowners tend to get pretty excited when they see frost inside the home.

Homes in all climates may see back drafting of fossil fuel burning appliances if the home goes negative.  Another concern may be pulling air from attached structures, like attached garages.  There are a lot of things stored in a garage that we don’t typically store inside the home, like containers of gasoline and the lawn mower.  Dust, pollens, and other outdoor air pollution may also be drawn into the home, affecting the indoor air quality.  Lastly, there is also a risk of soil gasses such as radon entering a home that is under a negative pressure.

Sometimes we design systems so that outside air is intentionally brought into a home.

Combustion Air.  Combustion air is air supplied to the area of fossil fuel burning appliances so that air pressures directly adjacent to the combustion appliance are maintained close to neutral.  Most, but not all natural drafting appliances in newer homes will require some sort of dedicated combustion air supply vent in the room or vicinity of the fossil fuel appliance so that pressures near the appliance are equalized.  We also don’t want other exhausting equipment, such as a bath fan to affect the natural drafting of these appliances, back drafting can result in carbon monoxide and other exhaust biproducts being drawn back into the home rather than exhausting to the outdoors.  For these reasons, homes with atmospherically vented gas appliances should be less than -5 Pascals, some want to see no more than -3 Pascals.  A Combustion Appliance Zone (CAZ) test will determine the negative pressure of the area with the combustion appliances with the home placed under its worse negative pressure condition.  Powered vent fossil fuel systems are typically higher efficiency with sealed combustion, most take outside air from a dedicated vent to be used in the combustion cycle.  There is a lower risk,

but sealed combustion appliances still have some potential of back drafting.  I’ve heard a limit of -25 Pascals for sealed combustion appliances.

Make-up Air.  Makeup air is used to replace air that is exhausted out the building by way of mechanical ventilations systems.  Negative pressures in a home may be caused by kitchen and bathroom exhaust fans, dryers, or some sort of designed negative pressure ventilation strategies.  A good example of this are the large, often oversized kitchen range hoods.  Range hoods that operate at over 400 CFM are required by code to have makeup air systems to automatically replace the air into the home that has been exhausted.  Sometimes this air will need to be conditioned or dehumidified to maintain comfort.

So, how much negative pressure is too much?

I’ve been posing this question to as many smart people who I can find to talk to me.

I asked this question to Gary Nelson from the Energy Conservatory a couple years ago, he replied:

“I don’t have any problem with, when you turn the dryer on, the house goes negative 25 or even 50 (Pa).  It’s an intermittent thing.  When it’s below zero outside and you’re in a three-story house, the pressure at the bottom of that house could be -15 (Pa) because of the stack effect for weeks in a row.  At the top, it could be +15 (Pa).  Windows at the top of a house during real cold weather sometimes freeze shut.  There will be things like that.  If you have a big negative pressure for a long period of time, and you have some fairly large leaks where air is coming in through a wall and it’s hitting the back side of the sheetrock, making the sheetrock cold, you might have frost if you have big leaks.  But if the building is very tight, which it probably is because that’s why you’ve got the big pressures, and there aren’t any larger leaks, I’m really not too concerned about intermittent pressures up to about 50 Pa.  The reason for a 50 Pa limit is that at 50 Pa, it’s going to take about 10 lbs. of force to open an exterior door and there are safety standards that limit the force to open a door to 10 lbs.

But once you get a house that’s down under 100 CFM50 and you have exhaust appliances larger than 100 CFM, now you need make-up air.  There are make-up air units designed to be interlocked with exhaust appliances and that have heaters to heat the incoming air.  Electro Industries is a Minnesota company that makes one that I’ve heard good things about because all the required dampers, heater controls, and interlocks are included in a fully designed system.  I think there are possible solutions using an ERV or HRV but am not aware of anything on the market yet.  A solution for dryers is to use a heat pump dryer that doesn’t exhaust air from the house.  I almost put one in my house during a recent remodel but decided against it when I heard they were noisy.  I have recently heard that noise isn’t a big problem.

I probably should have mentioned that large negative pressures in a house will possibly significantly reduce the flow of an exhaust fan.  Also, if you have a place where a flow of outside air won’t cause a comfort problem, a duct to the outside with a barometric damper that is adjusted to start to open at a certain negative pressure might be a simple low-cost solution.  A properly sized supply fan that is interlocked to the exhaust fan causing a large negative pressure is another solution.”

Allison Bailes had this to say: “The negative pressure/makeup air thing is something that’s been bugging me for a while now.  We design makeup air for range hoods in our HVAC design work but haven’t yet done makeup air for someone worried about negative pressure from other fans in airtight homes.  I don’t know what the right number is, but my first answer would be, as usual, it depends.  For the type of house you’re talking about, [a tight one], they probably don’t have a nasty crawl space or attic that would send badly contaminated air into the house.  If they have an attached garage, that would be a factor in support of less negative pressure.”

Kohta Ueno replied: “I do *not* have an actual number that I would target as a maximum… I’m guessing that Gary is saying “nothing bad happens when we depressurize to 50 Pa for a while during blower door tests”—yep, absolutely true.  Agree with Alison that “what is it connected to” is huge.  Potential consequences of that depressurization—issues with door and window opening (doors blowing open/closed spectacularly), ghosting/sooting where the leaks are (e.g., plating out/filtering on carpets). Possibly whistling at window seals?  The back-and-forth argument I’m imagining is “the pressure doesn’t matter for bringing in humid air from outside in the summer, the CFM does.” Yep… absolutely true that the CFM is proportional to the moisture load.  But if you’re *concentrating* that moisture load in one or two small spots… yeah, that has the potential to be a problem, in terms of both interior humidity (“Why is this one room always so sticky?”) as well as possible condensation issues. I’ve dealt with a few relatively-airtight constantly-depressurized buildings where they reported these types of problems—e.g., the laundry room is always damp in summer… yep, the room that has the outside air louvers.”

I recently took some training at Construction Instruction in Denver, CO.  I asked the question to Gord Cooke and Justin Wilson.  Their thoughts were if the home has any natural drafting fossil fuel appliances, the pressure needs to be limited to -5 Pascals.  All other homes, pressures should be limited to -25 Pascals.

My takeaway

If there are combustion appliances of any type, negative pressures need to be limited, especially when there is a natural drafting gas appliance or wood burning fireplace.

Negative pressures need to be limited when an attached garage (or Allison’s nasty crawlspace or attic) is present.

Perform radon testing, negative pressures may increase radon risks.

I’m with Gary, houses that test less than 100 CFM50 should have some sort of make-up air system so that exhausting equipment operates at the design they were intended.

Again, climate would have a lot to do with the type of make-up air system, hot and humid may want dehumidification included, cold climates may require heating the incoming air stream to satisfy comfort needs.

The size and location of the holes in the air barrier do make a difference.  One or two large holes low or high in a building assembly can create bigger problems where several smaller holes may not be an issue.

We have the opinions of several building science experts, and for what it’s worth, my two cents.  What is your opinion?

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