Months fly by, and it is closer to Fall in New England.  Soon most local Plumbing, Heating, and Cooling companies will be consumed with Fall maintenance programs.  I remember spending countless hours a week dealing with humidifiers, whether it was the old Skuttle drum style, the Aprilaire bypass version, or even the steam models that have actually been improved over the years - particularly Honeywell's Truesteam.  The one common theme that always arose was: What do I set the humidistat for?  If you loyal readers remember, a while back I wrote a short blog about ASHRAE's Psychometric Comfort Zone.  Based on this time tested theory, you can set the Relative Humidity (RH) anywhere between 30-60%!  With scorched, or forced-air, systems it is desirable to keep the RH between 35-45% in order to prevent static buildup (ACCA Manual RS).  But, every home is different.  In order to properly sense the RH, most humidistats are located on the return duct.  This may represent the RH at some point in the house, but not everywhere.  So, what is the maximum RH that should be set?  I say it depends on the windows - as visible condensation on the windows can cause many problems.  There is a possibility of concealed condensation in walls, but if a vapor retarder is used and installed correctly, this should not happen in New England.  As a Service Technician, we can only worry about the visible condensation.  If concealed condensation is a concern, please contact a qualified Building Analyst!

Equation for Surface Temp.








Visible condensation occurs on surfaces that have a temperature below the dew point of the air.  This is why we get "dew" on the grass or my car most summer mornings.  Or why an evaporator actually condensates during operation in our climate.  The temperature of the coil (Evaporator Saturation Temp) is lower than the dew point of the return air.  In order to find out the maximum RH prior to visible condensation, you should use this equation and a Psychrometric chart.  You will need to know the indoor and outdoor design temperatures, and the u-value of the device calculating for.  Since the weak point in most walls is the window, you should find the lowest u-value window in the conditioned space. If you were solving for a wall or ceiling, remember that the U-Value = 1/R-value, or the inverse.

.49 U-Value Window in MA














  For example, lets say your design temperatures were 0F (typical local outdoor) and 70F (2009 IECC and building code compliant indoor), with a window that has a .49 U-Value ( typical double pane, wood frame).  Using the formula, you can determine the temperature of the window at design temperatures, in this case 48F.  This correlates to a dew-point of about 47F.  Then, use of your psychrometric chart (I am sure "there is an app for that") you can determine the max setting for your central humidifier is about 45% RH.

.36 U-Value Window in MA


















   Control of this is very important for most new homes as this math with a Low-E window having a .36 U-Value may not yield any visible condensation until the relative humidity hits close to 60%!  If technicians are using the old method of visible condensation, it may be too late for "insert issue here" (concealed condensation, mold, damage to paintings, pianos, etc) or I like to say the many possibilities of IAQ.  In fact, did you know that just as much (if not more) bacteria, viruses, fungi, and mites grow above 60%RH as they do below 30%RH?  (see ASHRAE Systems and Equipment Handbook, 2000) Not only does the static vanish between 35-45%, but so does the rest of the unwanted possibilities.  Ever wonder why condensation builds in attic duct systems and supply registers during the winter?  Check out the duct insulation and do the math...

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Comment by Robert Riversong on September 23, 2012 at 1:46pm

I would have to differ with you on window surface condensation being a late sign of moisture problems, as it is typically the first sign - it will become visible immediately rather than take weeks or months to develop, such as mold and rot.

The correct formula for inside surface temperature is:

Tsurf = Tin – (Rfilm/Rtotal) x (Tin – Tout)


Tsurf = inside surface temperature

Tin = inside air temperature

Tout = outside air temperature

Rfilm = R-value of inside air film

Rtotal = R-value of entire cross-section (including inside and outside air films)


ASHRAE standard R-values for air films:

inside vertical surface = 0.68

outside vertical surface = 0.17

So your formula is an approximation that ignores the combined 0.85 air film R-value and uses a slightly lower (0.65 instead of 0.68) interior air film value to compensate. But at smaller unit R-values, it becomes increasingly unreliable. With an R-3 window unit, the surface temperature is about 5% low (54.83° instead of 57.64°). With an R-2 window unit, it's almost 13% low (47.25° instead of 53.30°).

Comment by Christopher Morin on September 23, 2012 at 10:12am
Hi Robert! Thanks for reading my blog and providing some valuable insight! I agree, concealed condensation in our climate during the winter months will definitely be caused by internal humidification and exfiltration, requiring air sealing. The problem with this is that normally by the time there is visible surface condensation it is likely too late for inside that wall! Like a lot of poorly controlled "finished" basements in New England.

Unfortunately, I do not know the derivation of the "visible condensation" formula. It was taken from ACCA Manual RS, by P.E. Hank Rutkowski. Almost all of their manuals are ANSI accredited, but I did not see a stamp on this one yet as the latest copywrite is 1997. Are you aware of a more accurate formula? If so, I would please ask you to share so I may compare and possibly ask Hank for a clarification...
Comment by Robert Riversong on September 20, 2012 at 9:38am


Do you know the derivation of that inside surface temp formula? It appears to be a very rough approximation and I wonder where the 0.65 factor comes from.

Also, a correction: In a cold climate, a vapor diffusion retarder has little impact on what you call concealed (or interstitial) condensation. It is almost entirely air exfiltration that causes such envelope condensation problems, by a factor of 9:1.

Comment by Curt Kinder on September 16, 2012 at 6:50pm

Holding indoor humidity between 35% (winter) and 45% (summer) seems to me the holy grail.

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