Comments on: Daniel Boone’s Dew Point Problem

By: Geoff Summa

Geoff Summa — Fri, 23 Dec 2016 00:46:44 +0000

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.

By: Marc Powers

Marc Powers — Wed, 13 Jul 2016 00:28:08 +0000

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.

By: Allison Bailes

Allison Bailes — Tue, 28 Jun 2016 02:51:48 +0000

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.

By: Allison Bailes

Allison Bailes — Tue, 28 Jun 2016 02:40:43 +0000

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!

By: Allison Bailes

Allison Bailes — Tue, 28 Jun 2016 02:38:58 +0000

By: David Butler

David Butler — Mon, 27 Jun 2016 21:44:49 +0000

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.

By: Charles Leahy

Charles Leahy — Mon, 27 Jun 2016 20:35:37 +0000

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?

By: Mark Johnson

Mark Johnson — Mon, 27 Jun 2016 18:09:20 +0000

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?

By: Ted Kidd

Ted Kidd — Mon, 27 Jun 2016 17:12:02 +0000

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…

By: Paul Raymer

Paul Raymer — Mon, 27 Jun 2016 16:59:55 +0000

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.