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Daniel Boone’s Dew Point Problem


In Missouri for the ASHRAE conference, I took some time to visit with my Cajun baby sister the other day. As we were making our way to one of the local wineries outside of St. Louis, we saw a sign for Daniel Boone’s house. Now who can resist that? So we took the short detour and spent a few minutes there. (Yes, minutes. There was wine at the wineries after all.) And here’s what I saw there.

Window condensation is usually an indoor humidity problem. Most people think of it as a winter problem when the windows get are cold because it’s so cold outside that the water vapor in the indoor air rushes to those glass surfaces. But dew point works both ways.

For humid climates in the summer, the humidity is outdoors. When you air condition the indoors, as Daniel Boone was wont (but unable) to do when he built this house back in 1810, the cold surface and the water vapor are on the outside of the window. In this case, the outdoor temperature was about 95° F and the dew point was about 75° F. What does that tell you about the window?


The lesson here is that if you’re a frontiersman in a humid climate, as Boone was, you need to do one of two things. Either you need to forego the air conditioner, or you need to install better windows. Those cool, single pane windows are a water magnet, as you can see above.

Also interesting is that the condensation doesn’t completely cover the panes of glass in the window. Perhaps the cold air from the vent indoors was hitting only part of the glass. Or perhaps something outdoors was warming one part of the glass more than the rest.

In any case, if you see condensation on the outside of your windows at 1 o’clock in the afternoon, be aware that you, like Daniel Boone, have a dew point problem. Condensation in a humid climate is a good thing…if it’s on the coil of an air conditioner or dehumidifier. If it’s on the windows in summertime, you’re probably paying too much for air conditioning.

Window replacements most often aren’t cost effective, but that doesn’t mean you can’t improve them. One product I like for this application is the Indow Window insert. They’re interior storm windows you hardly notice.

The good news, though, is that Daniel Boone didn’t really have a dew point problem because he didn’t really have air conditioning back when he lived in this house (1810-1820). No, it’s just another urban legend that the pioneers had to lug air conditioners with them when they walked uphill and backwards all the way to Oregon.


Related Articles

Dew Point — A More Meaningful Measure of Humidity?

The Problem with Relative Humidity

A Line in the Sand — The Dew Point Duct Duel


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This Post Has 13 Comments

  1. A dew point problem? Why is
    A dew point problem? Why is exterior condensation a problem? I mean, aren’t windows are designed to withstand rain? The only problem I see here is here is extra heat transfer one would expect through single pane glass. But that would be true whether you have AC or not. Or am I missing something?

  2. Yes, David, it’s an
    Yes, David, it’s an indication of extra heat transfer. And that means extra energy use, extra cost for the electricity bills, extra pollution, and extra carbon emissions (depending on the source of electricity).

  3. The problem with excess
    The problem with excess condensation is potential window rot down the line….especially with these windows that have unglazed muntins…but the bigger question is what is condensation doing elsewhere in the home? From an energy standpoint, windows comprise less than 10% of the energy footprint of a structure. Most energy is lost upwards or through those holes in the home you’ve already made your audience aware of (and i have learned from!) Weatherizing old windows for air-infiltration may only increase the condensation but might save some dimes. Condensation can have a myriad of sources (indoor humidity problems, heavy window treatments, indoor cooling and heating preferences, hvac problems, improper retrofits, etc.) but then the next question has to be: is this normal condensation or is it excess? And does it really affect my quality of life? To say it’s a window problem is simplistic – though not innacurate. Even double paned windows or single paned with inserts/storms can develop condensation if the variables are right. Even Daniel Boone likely saw condensation on his windows in the winter. Thanks for the blog, condensation is a favorite topic of mine.

  4. Allison – a good fifteen
    Allison – a good fifteen years ago I supplied a humidity control that my company made for the crawl space of this house. The sensor was remote from the control so they could operate it without crawling. I didn’t think that Daniel’s house was so close to St. Louis or I would have visited myself. I’m curious if it is still working.

  5. I like Nick’s answer. The
    I like Nick’s answer. The condensation may be an indicator, a canary.

    I (and I suspect Robert Bean also) would like to know indoor temperature, Rh, and blower door.

    This seems a borderline issue as only part of the window is wet. If the indoor temperature can be run warmer without comfort complaints – possibly through better RH management – maybe they can save some Canaries and some miners…

  6. The first question that came
    The first question that came to my mind, is something not even our regular experts have said:

    What is the outside dew point temperature?

  7. Very neat! Our factory in
    Very neat! Our factory in Mocksville NC is only about a mile from where Daniel Boone’s parents are buried in the community cemetery. I know he did a great deal of his childhood/teenage years growing up in the Mocksville, NC area.

    Question regarding the “Indow” product – doesn’t that simply move the condensation – that would be on the outside of the old window as in your story – now to the inside window framing with condensate accumulating between the outer surface of the Indow product and the interior surface of the old single pane windows? And if so, wouldn’t that create a new problem where there wasn’t much of one before?

  8. Allison, by framing this as
    Allison, by framing this as ‘dew point problem’ you’ve only confused your audience, as evidenced by other comments herein.

    Although there may be many other issues at play in this home, exterior condensation has zero value as a canary). It only tells us what any practitioner already knows — that single pane windows have a high u-value.

    The physics behind this is simple — the outside dew point is higher than the glass surface temperature, likely because fifty-something degree supply air is directed at the glass. (Although not impossible, it’s unlikely that room air would be cold enough to cause this.) The only thing I would do based on this observation is redirect supply air away from the glass.

    BTW, some HVAC contractors purposely direct supply air toward glass. Winter or summer, this is bad practice as it greatly increases heat transfer through the glass.

  9. Sorry about that, Mark. I
    Sorry about that, Mark. I meant to include that in the article but somehow missed it. I’ve added it now but so you don’t have to go looking for it, the outdoor dew point was ~75° F and the outdoor temperature (dry bulb) was ~95° F. Thanks for raising the question!

  10. Sorry about that, Mark. I
    Sorry about that, Mark. I meant to include that in the article but somehow missed it. I’ve added it now but so you don’t have to go looking for it, the outdoor dew point was ~75° F and the outdoor temperature (dry bulb) was ~95° F. Thanks for raising the question!

  11. Oh, stop being such a
    Oh, stop being such a fussbucket, David. If this were your article, you could write it however you want. It’s my article, and I think it’s perfectly fine to talk about dew point and U-value.

  12. Good article and comments.
    Good article and comments. Controlling moisture and “relative humidity” (or dew point) is an important design parameter.
    For water to condense on a surface, the temperature of the surface must be at or below the dew point. Solving this condition would eliminate the condensation. As mentioned, the ducts could be directed at the windows. Redirecting the air-flow could possibly eliminate the condition. Also, if the indoor air temperature is below the dew point outside, moisture could be condensing, not only on the windows, but wherever temperatures are at or below the dew point. This means possibly in the middle of an outside wall, at some point in the attic insulation that is at the dew point, etc. This is one very important reason to ventilate these places. If the indoor temperature could be set up a few degrees during these high outdoor dew point conditions (look at windows to “monitor”), condensation could be controlled or even eliminated. If the relative humidity inside the building is at 50% or preferably lower, an indoor air temperature of around 79 degrees will feel comfortable compared to the outside temperature. Raising the indoor temperature will have the added benefit of lowering the cooling costs significantly. Maintenance of the windows and potential for mold to grow in walls, attics, etc. can also be reduced.

  13. I know. This response is a
    I know. This response is a bit late in coming. But this article brings to mind an issue I come across each year around Labor Day. Let me explain. I am a builder on an island here in Casco Bay, Maine and I regularly hear the question, from summer folk leaving their vacation homes for the “off season” (by the way it’s the best season), “…should I turn off my heat or leave it on low?” Besides making sure their domestic water systems (supply and waste) are drained, air pressure cleared and treated with non-toxic antifreeze and that any hydronic heating systems have an antifreeze additive, an answer to whether the heating staying on or being off has eluded me and has never been appropriately addressed. At least not from a building science perspective.

    What does this issue have to do with Allison’s Daniel Boone Dew Point article? Well, on certain occasions during wintertime particularly around January thaw (2nd week of January) or late March, when outdoor daytime temps can rise significantly beyond the seasonal norm and above what the indoor unconditioned space had adjusted down to, the “Daniel Boone House” conditions occur and with it, condensation. A lot of it. And not just on the windows. The interior surfaces appear at these times to be sweating. Makes me wonder what the insides of the building’s enclosures are looking like?

    This leads back to the heat on/off question. Would love to hear other’s (well, to be honest, Allison’s) perspective on this. On? Off? I know there probably isn’t a simple answer but this simple mind would appreciate some feedback.

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