What is the value of using a pressure pan to test outlets?

I had this question recently and my response was:

If the blower door takes the home up to 50 Pa, and the pressure pan reading across the outlet is 49 Pa for example, that says that total of the rest of the leakage path to outdoors is about 7 times larger than the outlet which means the outlet is acting like the primary air containment barrier. Similarly, if the pressure is only 1 Pa, it says that the air barrier actually lies somewhere else and the remaining path to outdoors is 7 times smaller than the tiny hole in the outlet.

 

It is possible to make some conclusions about what this means but they are obscured by the fact that the major leaks in the house that affect energy are probably 100 to 1000 times larger and therefore 100 to 1000 times more significant than the outlet. It is important to keep hole sizes in perspective to avoid getting distracted by holes that are easy to locate. A large pressure across the outlet says the outlet is not a problem and in all likelihood, the wall is connected to the attic by 20+ feet of cracks between the sheet rock and the top plates. Sealing these top plates will prevent cold air from falling into the walls, cooling them down and affecting comfort even though the reduction in air leakage will be unmeasurable because it exists as a few small holes in the living space such as outlets that are not in themselves the problem nor will fixing them cure much if the wall is connected to the attic as it usually is.

 

Colin Genge

 

 

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Hi Colin,

A CFM50 tells us how leaky the house is, but it fails to tell us much about the outside shell.  I have run into a few old homes sheathed with board siding and finished inside with plaster and lathe.  The plaster inside with 80 years of paint and more paint can be surprisingly well sealed, however, the board sheathing can have 1/4" gaps and thus the house leaks like a sieve outside to outside.  Where fg insulation has been added, thses wall cavities being open to the outside can render that fg totally useless.

For me, checking outlets is helpful in improving my understanding of what is behind those walls.

As for sealing the outlets, it has a payback, because the effort is so low, any savings are worth while.  I always suggest this for the DIY homeowners, even though I qualify it as a small fix.

Bud

Colin

Very interesting. Just out of curiosity where do you get the times seven figure. I am not doubting you just trying to learn a trick or two

Where would you suggest a pressure pan is effective?

flow is equal to the square root of the pressure drop. Square root of 49 is 7 and square root of 1 is 1, so ration of the flows is 7 to 1.

Air flow is proportional, not equal, to the square root of delta-P. Otherwise, you're correct.

Pressure is not flow, so even though you might read a 49 at the receptacle, how much air is actually there?  It can be instructive (although not terribly accurate) to use a flow hood, or an anemometer with a hood, to check the CFM50 at various leaks.  A pressure pan might give me 1pa at a duct grill and the flow hood will measure 8 CFM(50), and a 25 pa difference at a recessed light might give a 40 CFM(50) with the flow hood, but I might find under 2-3 CFM(50) at a receptacle with a 49 pa difference.

We did a project for the Air Force in 1984 where we air sealed base housing.  The base engineer insisted that we install receptacle gaskets which we had proven to our own satisfaction did not mean diddly.  So we spent 30 minutes installing gaskets on switches and receptacles in a 3 bedroom officers unit and showed him ZERO change in the blower door reading.  He no longer required the gaskets.  As Bud says, the problems are in the wall, so gasketing the receptacles just pushes the same leak elsewhere.

Did you ever notice that the receptacles leak air on the first floor interior walls of a 2 story house?

I built a house for myself 4 years ago and it is at .8 ACH50 and I was dissappointed to feel air at most receptacles and switches.

>> the problems are in the wall, so gasketing the receptacles just pushes the same leak elsewhere

 

If you reduce the hole size without reducing the delta P, you reduce air flow, you do not "push the leak elsewhere".

If a window is open 12" and you close it 1/2", have you reduced the amount of air going through it as much as you would think?  Or does closing the window a bit change the delta P?

If the delta P is held constant--such as with a blower door in cruise mode, or (presumably) with natural pressures, reducing the hole size reduces air flow. If the delta P is not held constant, such as with a blower door controlled only by the speed control, the flow will be reduced by a smaller hole size, but not nearly as much. Maybe I'm not understanding your point.

Part of the problem with electrical box gaskets is that some of the air leakage comes through the receptacle or switch body itself, not around it. 

Of course you are right, but what I am saying (obviously ineffectively) is that the network of leakage paths through the exterior and interior walls, and through the joist cavities, caused by the drywall not being fastened tightly agains the studs and by the various wire and utility holes is so large that the very small hole caused by the hole in the drywall only partially filled by the receptacle or switch is doing nothing to stop it when the gasket is installed.

I would urge you to try installing a large number of gaskets with a pre- and pst- blower door test to see for yourself.

I've thought of doing that. I have a building that's perfect for it, and I actually already have a very large supply of the gaskets, which were given to me . However, what I've observed it that air comes right through the receptacle or switch itself, so my guess is that I could only shave a few CFM50 off the entire building by installing the gaskets, not enough to pay back. I've done enough air sealing to know that you want to find larger holes than that, hopefully some massive ones.

In spite of this, I remodeled my house this year and installed those damn things at every hole, thinking "what if they make a difference??" and besides, I still have hundreds left.

I don't have a flow hood, but it would be interesting to have one long enough to do some testing on various common holes. We do use the pressure pan on can lights and other common holes sometimes.

Colin-

I just have to chime in here - the dP across the pressure pan has nothing to do with how leaky the outlet is. When covered by the pressure pan the outlet leak has no flow - it has become a pressure tap into the wall cavity. The pressure is related to the ratio of the size of leakage paths to the inside vs. outside, excluding the outlet. Also, I would not estimate this ratio as 7 ( the square root of the ratio of the dPs) but as closer to 12 (49^0.65=12.5) since 0.65 is a more typical flow exponent for building leaks. I would also not take this number too literally given how sensitive it would be to small measurement errors.

Michael, I'm trying to follow you here and wonder if you could comment on the following example. We recently did work on a house with a large number of can lights, some with floor above and others with open attic above. Lots of pressure pan readings in the 40pa range at these lights, with the house at -50. I can't tell from your description whether these pressures can be turned into any sort of CFM50 number. It sounds like you're saying no to that.

There is of course a lot of perceptible air leakage, especially around the hole cut in the drywall. We are easily able to seal these down into the 10-15pa range. This seems to translate into a reduction of maybe 2 CFM50 per can, based on very crude analysis. As stated above I don't have a tool for measuring this. 

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