FBBP says: "Based on the manufactures spec’s we see that double hung windows have a much larger infiltration rate of outside air than awning windows do. And both have a much larger infiltration rate than fixed in place glazing. "

Before we can accept your fictitious report for serious consideration, you need to apply at least a hint of science and engineering into your report to indicate that you have given the problem at least a little thought. To begin with, compute the actual heat loss and energy cost associated with a window that leaks air at the rate 0.3 cfm/ft^2 of window area, compared to a window that leaks air at 0.05 cfm/ft^2 of window area. Then compute the actual cost difference per year based on natural gas heating at 98% efficiency with gas costing $0.72 per CCF. If you have trouble with this calculation, please refer to previous posts by jonr and by me that covers this area. (You probably will not be able to get any help from Lbear as he has failed to produce such a calculation himself after talking ad nauseam about leaky windows.) I will remind you that your own abstract stated, "To determine if outside air infiltration affects the temperature of room air next to a double hung window and if possible determine the area of maximum infiltration." You need to quantitatively address your own abstract.

FBBP states: "Windows are often cited as the source of the single most heat loss in a building envelope." A good house design in this sunny area makes windows a net source of heat GAIN, not heat loss. Your report is short-sighted if it does not include the potential to use windows as a source of heat, with a good discussion of SHGC. Ever heard of passive solar heating (what some call tempering) to displace some of the need for conventional heating?

Imagine the problems that would be encountered if you designed a house to be cooled in the summer by trapping the cool air at night using windows that are closed in the morning, but you replaced the operable windows with fixed windows! As the early designers discovered, cooling a house during the summer evening hours using double hung windows allowed the hot air out of the top of the open windows and the cool air in at the bottom of the open windows. Simple physics. Are you able to cool your own house without air conditioning using fixed windows throughout the house? Please tell us and include that in your report.

So tell us FBBP, what type of windows do you use in your net-zero source energy home?

Well Lee, I thought by titling it "fictitious", it would be clear I didn't mean it to be a great work of science. If that wasn't clear to you, my apologies.

Now lets see if I can get this right. My house has 35 windows for a total of 836.05 square feet of windows (not including doors or fixed (picture) windows.) At .3, I believe that would be 15,048 cu. ft. per hour. At .05 it would be 2,508 for a difference of 12,540 cu. ft. per hour.
Now the basement of my house has 1,288 sq. ft. of floor at 9 feet high, the main has the same and the second has 1,958 sq. ft. at 8 feet high. I think that comes to 38,848 cu. ft. 12,540 cu.ft. per hour of window infiltration above the normal rate of window infiltration is what? About a third of an ACH on this house? If I were anal about getting the house under 1.0 ACH do you think it would make a difference?

You know, sometimes an item can be both the best and the worst of things. Since we were discussing saving energy by covering a window and not how much heat we could get through a window, I'm wasn't sure if it would be appropriate to introduce solar gain. Likewise I don't believe anyone mentioned cooling a house did they? But yes a couple of Velux venting skylite do go a long way in a two storey to cool a house. 'Course with ICF to the rafters, not much cooling is required.

Actually in this house I used some very expensive metal clad triple sealed unit casement windows. They simple were not worth the price. I would not use wood sash again for the reasons I mentioned in my "report".

FBBP-

Excellent. We have some actual numbers to work with. I agree with your calculations, except that you have not yet adjusted between the measurement conditions, which are 1.57 lb/sq ft (75 Pa), and infiltration that occurs under what are called "natural" conditions. The 1.57 lb/sq ft (75 Pa) of pressure corresponds to a steady 25 mph wind speed. When leakage rates are measured for a house with a blower door test, they are typically conducted at a pressure differential of 50 Pa. To convert to leakage under natural conditions, the leakage measured at 50 Pa is divided by a number that can depend on geographic area and local sheltering (see Table 5 at http://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf). However, the rule-of-thumb is to divide by 20 to convert from 50 Pa measurements to natural infiltration rates. Let us assume that the dependence is roughly linear, so to convert from window measurements made at 75 Pa to natural infiltration rates that occur over the year, we will divide by a factor of 30.

Therefore, the difference in leakage rates for the windows that you computed to be 12540 ft^3/hr should be corrected to 418 ft^3/hr, or 11.8 m^3/hr. For a sea level air density of about 1.18 kg/m^3, a heat capacity of 1.006 kJ/kg C, 7000 heating degF days or 168,000 degF hr or 93333 degC hr, I compute the infiltration energy to be 1312494 kJ/yr or 1.24 MillionBtu/yr. With an energy content of 1020 Btu/ft^3, and burning at 98% thermal efficiency, it would take 12.4 CCF of natural gas to reheat this air. At a cost of $0.72 per CCF, that would correspond to $8.96 to reheat the extra air due to infiltration by the windows rated at 0.3 cfm/ft^2 (@75 Pa) instead of the windows rated at 0.05 cfm/ft^2 (@75 Pa). How much extra did you pay to get the lower leakage rate windows? What does the payback look like?

I will address your other comments about cooling and air conditioning separately.

Lee - I think you are running me around in circles.
The windows where tested at 75 Pa. A blower door test runs at 50 Pa so my calc's are out by 50/75th which would make my ACH cost of the windows a little under a quarter rather then the one-third I said. Still a very significant number when trying to get under one ACH.

Unless you can by Pella a lot cheaper then I can, the dollar figure doesn't matter. I bought these cheaper from a local manufacturer that I have dealt with since '78 then I could from my Pella dealer. If memory serves, you still pay a premium for a Pella DH as compared to a Pella casement or awning. If not, it would not be much difference so you could have bought a .05 window for virtually the same price as a .3 window.

While I sympathize with your requirement for having to use double hung 'cause some architect doesn't know better, it doesn't change the fact that they are leaky, will lead to higher condensation rates and still throw off your measurements for the blinds.

FBBP-

You are labeling as "leaky" all of the windows in the North American market that are specified to have the industry standard leakage rate of 0.3 cfm/sq ft at a differential pressure of 75 Pa. I am showing you the hard numbers that say if you had windows with that leakage rate installed in your house, and that you put them through a 1-year service with a winter rated at 7000 heating degree (F) days, that the cost for all that "leakage" would be less than $10 more than your super-tight windows rated at 0.05 cfm/sq ft. That is not $10 per window, but rather $10 for the whole house. The leakage amount in windows that you are labeling as "leaky" is not financially significant.

It is like telling me that R-60 ceiling insulation leaks a lot of heat, and if I increased the insulation to R-100 I could save $9 per year. Big deal. It is simply not reasonable to label the R-60 insulation as "heat leaky," just like it is not reasonable to label a standard 0.3 cfm/sq ft window as leaky. Show me the numbers if you can that indicate that these "leaky" windows cost a significant amount of money to compensate for their leakage rate. If you cannot show the numbers, I suggest you change your nomenclature to "standard leakage" windows.

You may have some arbitrary specification for leakage rate at 50 Pa in your house that you want to meet, but you have not provided any financial analysis to say why that is a reasonable specification. In contrast, I have shown you with hard numbers that leakage rates through standard specification windows do not add significantly to heating bills in houses that have 7000 degree (F) day heating seasons. If you have 14,000 HDD(degF) instead of 7000 HDD, then the increase in your annual heating bill will be about $17.92 with the standard windows compared to the super-tight windows. If you pay nothing extra for super-tight windows, then go with them. If you do pay extra, put the money in solar PV instead (if the sun shines where the house is located) if the goal is to reduce your net energy consumption. If you have to go with fixed (non-operable) windows to achieve a low leakage specification that then require that you use an A/C for cooling instead of simply opening the windows, then you have suffered a huge energy penalty. How do you cool your house?

Lee
Maybe post prices in your area for leaky double hung and for much better casement or awning. As I said there is not a significant cost difference to the choice.

But since you like math, do it with a three storey building, with rural very windy conditions, 5300 HDD ºC. You might come up with not much money but I come up with intense discomfort.

The leaky windows still provide a discomfort in their near proximity.

The leaky windows still have a higher condensation rate and at the most critical point of the building envelope. (which is what this thread is about) Sometime cost come back and bite many years later.

Window suppliers are not allowed to sell any leaky windows in this area. All must meet the 0.3 cfm/ft^2 at 75 Pa requirement. I never see any condensation on my standard spec windows except when I seal them up with cellular shades with side seals at night. Even sealed up, I see no condensation unless the temperature drops below somewhere between 0 and 5 degF (-18 and -15 degC). No shades results in no condensation. Any condensation or frost that does form will evaporate within 30 minutes or so after the shades are raised. Or the shades can be left up a few inches and condensation/frost will not form. The condensation is predictable from the insulation value of the shades without resorting to any leakage calculations.

The cost savings for using lower than standard spec leakage windows in your home at your location would fall within the $17.92 annual savings noted above unless you use electric resistance heat.

So I guess it's not worth spending $100 + per window on insulating shades that will cause condensaion and rot my leaky double hung windows.

Would exterior storm windows add insulation value?

In my case, the problem is not insulated shades or double hung windows, it's indoor humidity.

Thanks, Chris

Posted By ChrisJ on 11 Dec 2013 04:28 PM

Would exterior storm windows add insulation value?

Yes, adding an external storm window would add roughly R-1 to your window package (http://www.coloradoenergy.org/procorner/stuff/r-values.htm), which would increase the temperature of your innermost glass surface (in cold weather), reducing the chances for condensation on that surface.

Lee -

Your home is a 1,600 square foot ranch and your Blower Door Test revealed 2.45 ACH which is just below 2012 code which is 3.00 ACH. Of course Passive House standards are 0.60 ACH. While your numbers are decent, it is not that airtight for a 1,600 square foot home. Did you conduct a smoke pencil test? I bet your windows are the main cause of your air leakage.

If one looks to the countless Passive Homes and other energy builds and you will find the absence of double hung and slider windows. Building science experits advise against installing single and double hung windows for that reason.

Per the science experts at Green Building Advisor:

"Generally, double-hung and horizontal sliding windows have more air leaks than casement windows where the sash closes tightly against a gasket."

Per David Paulus, PhD, P.E. - Building Science expert:

Does air leakage have a large influence on energy bills?
If a manufacturer elects to have its product tested, it cannot have more than 0.3 cfm/ft2 of air leakage. If this maximum is met, the energy loss through air leakage is likely considerably less than that through heat loss (ASHRAE 2005 Fundamentals, P. 31.55). Nonetheless, air infiltration has a major effect on perceived performance of the window. Even at the passing value of 0.3, disturbing drafts may be felt. These drafts are often the very reason a customer is replacing a window!

Are some window designs better than others for air infiltration?
Yes. Casement and awning windows offer excellent air infiltration performance because pressure from the wind tightens up the seals. Tilt-and-turn windows, with their dual compression seals and multiple locking points around the perimeter offer equal or better performance. Traditional double-hung windows (sometimes called “vertical sliders”) cannot do as well due to their sliding surfaces where compression seals are not possible. And, horizontal sliders generally have the worst air infiltration performance of all window types. Beyond the sliding surfaces of the double hung, these windows have frames designed for sash removal. This feature, along with necessary provisions for drainage, make it impossible to seal these windows as tightly as a double hung, much less a casement."

Posted By ChrisJ on 11 Dec 2013 04:28 PM
So I guess it's not worth spending $100 + per window on insulating shades that will cause condensaion and rot my leaky double hung windows.

Would exterior storm windows add insulation value?

In my case, the problem is not insulated shades or double hung windows, it's indoor humidity.

Thanks, Chris

But why the humidity? Do you have a cause that might be corrected?

Lbear-

Tell us about your house, including all utility bills for the last 3 years. With your great knowledge, we know that the house must be impressive, and the energy use quite remarkable.

Lee -

You are taking this too personally. We are just making the observation that double hung windows are not a good choice to go with when trying to build a tight home. I have never seen anyone recommend double hung windows for a passive home. As FBBP and other have pointed out, there are other better options like casements or tilt&turns that are sometimes even less expensive than a slider or double hung. The other point is that the better option would have been a triple pane window with lower U-Values and it would have worked better than the Pella series window. Not to mention it would have probably eliminated the need to install those blinds.

Nobody is perfect and we all learn from our mistakes and nobody knows it all. We discuss and strive to design and build better structures, that is the end goal. You freely put your homes information on a website for all to see so I don't understand why you are getting offended when someone questions or comments on the results. All I said was that 2.45 ACH for 1,600 square foot home is decent but there is some leakage going on and I would bet it is the windows.

"But why the humidity? Do you have a cause that might be corrected?"

House is pretty tight with no HRV/ERV. No blower door test but 2x6 walls with oc spray foam, poly vapor barrier over all walls and ceiling.

When I got the proposal from the HVAC guy it had $6500 on it for HRV system. He said "you mentioned you want a wood stove in the house". I said I don't need a wood stove that bad.

I should have asked if there was a cheaper option for ventilation.

At this point I am looking at retro fitting an HRV/ERV or a whole house dehumidifier with a fresh air intake.

We are in Rhode Island.

Chris

Chris,

What type of HVAC system? If it is hot air, it is easier to incorporate an HRV/ERV compared to ductless systems. The cost of my HRV was around $2000 installed in conjunction with a hot-air furnace, although that was a new house installation that might be easier than an existing house installation. I cannot recommend the HRV brand that I have any more than competing brands, although the ones with ECM motors (which mine has) certainly improve electrical efficiency. The $6500 sounds pretty steep, but perhaps your house is large.

Rhode Island will likely be a challenge in terms of humidity. You might ask locally what works, but it may include a dehumidifier.

HVAC is a combination GSHP water to air main floor, water to water radiant basement & drive-under garage in a 1920 sq ft ranch. He was probably quoting HRV as a seperate system of ducts.

This will be our 4th winter in the house. Last winter the lowest the humidity got to was 50%. Right now it's 54-58% I've been running the portable dehumid a few hours a day but it's not dropping the humidity very much.

I know I have to but I'm not looking forward to putting holes in the walls.

Chris

Fantastic conversation!! I am learning a lot from the distinguished members.

From what I have gathered, windows no matter how good/pricey they are represent a respectable energy loss. A friend is building a new home and is purchasing high performance windows that cost about $78/sq. ft. And that is with his architect son’s discount!! My son built a house in Iowa and priced double pane windows anywhere from $30 to $60/sq. ft. I guess the point is one can pay a lot of money for a low return investment. And with these new thick walled homes, and I’m being somewhat facetious, why not put in two sets of lower priced windows; one on the inside and one on the outside? Yes, more construction cost, headache to clean and maintain. However, they may theoretically perform better at a lower investment. Which takes us back to the concept of storm windows? Can one even purchase storm windows?

I have found from personal experience that if I lower relative room humidity as the temperature declines I can reduce condensation on our double pane windows. At -20 it's pretty hard to get the humidity level low enough to prevent condensation.

"Which takes us back to the concept of storm windows? Can one even purchase storm windows?"

Dana1 has posted links or names of companies that make them, so they are out there.

Chris

A couple of years ago some people hired me to install some Kolbe and Kolbe sliders with a factory storm window. Because of the storm the window label had the u value at .18 or somewhere in that impressive .1 range so these people thought they had some high performance windows.

We pulled the old wood windows, installed the new windows, sealed them in and they immediately iced up on the inner surface of the storm glass.

Naturally they blamed my installation, I had to give a lesson in humidity, a bit on air leakage and why sliders are a poor choice and they never hired me again. It was my wifes coworker so I know they went elsewhere for future work, and I know they still thought it was my fault.

I'm sure their is some sort of lipstick on a turd lesson for all in this story. Oh, and the storms had no seal to the window frame, it was held in by rotating metal tabs, another junk Kolbe product.

These were sliding windows (vs sliding doors?). Did the storm panel attach to each sash, or did it span both sash? Sounds like a poor seal of the interior windows - was that your understanding of the problem?