Calculating Home Heat Losses for Window Replacement | 30% Tax Credit Ends Soon!

[Naty1]

I'm in the process of replacing 1970's double hung wood windows with modern high efficiency vinyl windows...this is awfully long but I thought others might be able to use the methodology for their own homes if considering new windows...the 30% government tax cedit, up to $1,500 runs out the end of this year (2010) ...together with a sale of 10% or 20% off and doing some window installations yourself, it's not a crazy investment.

Edit add on:
Heat is transferred by conduction,convection and radiation. My calculations below cover conduction (via U and R factors)...what I have been unable to account for are heat losses due to convection (air infilitration) and radiation.

I'd like to know the heat losses thru my windows...and the cost of that lost heat...the heating side is easy as I know my furnace size, efficiency and the cost of a therm of gas...

When I calculate approximate total heat losses for my home based on an R of about 3.3 (per sqft per degree Fahrenheit temp difference) for new windows, an R 13 for walls, an R 5 for floors over a basement and an R 30 for ceilings, my total heat loss in BTU's is too small. Door losses seem negligible but I guess I should estimate them with a R of about 2..I did not do that the first time around...it's small...

I assume I am not accounting for quite a bit of heat loss due to air infiltration...I seem to recall seeing some figures many years ago associated with 15 mph winds...can't find anything like that now...
Anybody have a reference online or a typical figure/factor for air infiltration I can use?? ...How is this calculation done?

My interest is in trying to estimate how much heat is lost via my windows versus walls,ceilings, and floor. I know I'll not get back my window replacement costs anytime soon, but I would like to know the heat loss via windows...With walls at R13 and double pane low E and argon glass R is about 3.3, likely window loss per sq ft is 13/3.3 or about four times as much per sqft as walls...but likely air infiltration would boost that to five maybe six times overall...?

My window area is about 325 sq ft (U is 0.3), walls w/o windows about 1030(U is 1/13), floor and ceiling about 1380 sqft each (U is 1/30, and 1/5) respectively so losses using this single factor are per degree temperature difference inside vs outside:

Heat loss per degree temp difference
325 x 0.3 is 98 BTU/Hr (windows)
1030/13 is 80 BTU/Hr (walls)
1380/30 is 46 BTU/Hr (ceiling)
1380/5 is 276 BTU/Hr (floor)

So on a 70 degree basis inside, say 10 degrees outside, 60 degree basement temp:

Total heat loss @ 60 degree inside/outside temp diff:

98 x 60 is 5880 BTU/Hr (windows)
80 x 60 is 4800 BTU/Hr (walls)
46 x 60 is 2400 BTU/Hr (ceiling)
276x 10 is 2760 BTU/Hr (floor)

Total: 15,840 BTU/Hr my guess is this should be about 30K to 40K BTU/Hr...

Also makes me wonder why I have 100,000 BTU furnace at 96% efficiency...seems like over kill even if 20 below zero...such a temp would boost my total heat loss to 90/60 x 15,840BTU or about 23,760 BTU...still awfully small...

 

[FlexGunship]

<post removed>

Apparently my post was useless. So sorry to waste your time. Recommend you follow your "hunch" and buy yourself a 24,000BTU furnace.

 

[Naty1]

you're missing a crucial point about your furnace

absurd. useless. you need to read about specific heat...

 

[Naty1]

Ok no answer yet...so I'll post findings and see what I can figure out:

(1) Up to 40 of a homes heat total loss is due to air infiltration...and electrical outlets in outer walls might account for up to 4% of that or maybe 2% of overall heat losses. I'm guessing this would be for a home like mine whithout exterior single piece wrap..like TIVEK...

(2)“Air infiltration can account for 30% or more of a home’s heating and cooling costs and contribute to problems with moisture, noise, dust, and entry of pollutants, insects and rodents. Nearly 45% of this uncontrolled air infiltrates through openings in ceilings, walls, and floors, as well as plumbing penetrations.”

(3)Large homes or commercial buildings with multiple floors and high ceilings - or situated on high elevations - can experience significant air infiltration due to stack and wind effects. Stack pressure can triple for every additional floor in a structure, which amplifies the suction of air from the bottom to the top of the building...This explains why caulking and weather stripping in mid-envelope tends to save less energy than careful attention to the bottom and top of the envelope, where these natural driving forces are greater.

(This makes me realize that my basement windows, although very small in number and area, need careful checking for good caulk.)

(4) Certain types of cellular-structured insulation, such as dense-packed cellulose and rigid or close-cell spray-on polyurethane foams, can be effective at reducing air flow as well as heat flow. However, the most common type of insulation—low-density spun batt or loose-blown fiberglass—does not stop air leakage. Dirty fiberglass insulation is a telltale sign of air movement

(5)Reflective insulation is most effective at reducing cooling bills in hot, sunny climates. However, in some special cases the product can help reduce heating bills as well.
http://www.ornl.gov/sci/roofs+walls/radiant/rb_01.html

(6)In houses with forced-air heating and cooling systems, ducts are used to distribute conditioned air throughout the house. In a typical house, however, about 20 percent of the air that moves through the duct system is lost due to leaks and poorly sealed connections.

(7) ..ACH as a measure of house tightness. Persily (now at the National Institute of Science and Technology) obtained a reasonably good estimate of average infiltration rates by dividing the air change rates at 50 Pascals by 20, that is:

average infiltration rate (ACH) = ACH50(1)
-----
20

In this formula, ACH50 denotes the hourly air change rate at a pressure difference of 50 Pascals between inside and outside. Thus, for a house with 15 ACH at 50 Pascals (ACH50 = 15), one would predict an average air change rate of (15/20 = ) 0.75 ACH...Nevertheless, a correlation such as ACH50/20 does not include any adjustment for windiness at the house's location...nor for a stack effect...the ACH50/20 rule of thumb could overestimate infiltration by a factor of two, or underestimate it by a factor of about three.

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/94/940111.html

 

[Naty1]

(8) No insulation plays a major role in blocking air infiltration through the walls of a home. Resistance to air flow through walls is primarily done by gypsum board (77%)1 and sheathing, siding or housewrap (12%)1. The rest comes from proper sealing of the building envelope and the numerous gaps and penetrations to the outside such as pipes, ducts, and flues.

(9) Air infiltration and thermal performance are two separate and distinct issues.

(10) ASHRAE says that an air infiltration rate (air change rate) of 0.35 provides adequate ventilation.

(11) Pressures might vary from a few tenths of a Pascal on a quiet windless day to over 20 Pascals on a cold windy day...Homes will be over ventilated on cold windy days and undervdentilated on quiet windless days.

http://envtraining.org/images/udf/file/EES/Ventilation_Article.pdf

 

[Naty1]

So it seems that air infiltration contributes maybe 30% to 40% of heat losses in a typical home...and that might equate to approximately 0.75 hourly air changes...

going to find how to determine the #BTU per hour required to heat 3/4 the volume of the air in my home...be back again...

 

[russ_watters]

Rule of thumb for CFM to BTU is CFM*1.08*dT=BTU

.75 is probably high, even for an older (but not really old) house. I might believe 0.5 if your house is really awful (maybe .25 after the window replacement). The EPA or someone did a study a while back about it where they measured infiltration for a bunch of houses, then sorted them by age. You may find it with a google.

I agree your calculated heat loss is too low, though that's the new heat loss. The old windows were probably R1 or worse. Also, your other insulation values may be overly optomistic. In particular, I'd be surprised if you've got R30 in the ceiling - that's like a foot of insulation. My parents' house, built in about 1980, doesn't have that, so I don't think it was required then. Of course, if you have a regular attic, maybe you already upgraded that yourself...

In any case, my townhouse is 1500 square feet with 8' ceilings. Assuming .25 ACH gives me 50 CFM. With the 60F delta-T (that's what Phillly is), that's 3,240 BTU. I'm guessing you have twice as much house as me, so if your house is 3x as bad (mine's only 5 years old), you may have 19,000 BTU of infiltration loss.

And a note on furnace sizing: To simplify their product lines, furnace manufacturers only have one furnace size for each rated airflow and the airflow is determined by the cooling load. Because the furnace delta-T is so high, you end up with several times more heat than you need unless you live in Maine or Canada. If the manufacturers doubled the size of their product lines so they could sell 80MBH furnaces and 40MBH furnaces with the same airflow, it wouldn't save much on equipment costs anyway, since the furnace heat exchanger and piping isn't that big a fraction of the price.

 

[Naty1]

Russ...good post, thanks...that rule of thumb is what I needed...I tried to do the calculations myself using Wikipedia table data but so far I am not getting a reasonable answer...I have to recheck my math and conversions from metric in the tables...

I added insulation in my attic...a second layer of 6" over the original 6" so I know it's a full foot..likely it's not so efficient because of air infiltration...but a lot better R than the original since I had several openings to the attic visible from above!

My old wood double hung windows have an exterior aluminum framed storm window..cheap but effective at keeping rain out...You'll get a kick out of this: I installed interior 3/16" plexiglass storm windows, wood edges, so the U factor with about 5" of air between the outer storm and inner plexiglass is likely pretty good...and the plexiglass is never cold like glass...but I know there is some air leakage that modern windows will reduce...along with caulk around all edges which was not used when the house was built.

 

[Naty1]

CFM*1.08*dT=BTU

As noted in my numbered notes above, air infiltration of 0.35 is apparently considered quite good...so I'm guessing about a 0.5 or so is appropriate for my house right now...after work I have done over the years to improve caulking...

my house is about 2400 sqft, or about 19,200 cubic feet x .5 means an hourly change of air of about 9600 cuft...hourly or about 150CFM per minute (Man that seems high!)


Using your factor, 150 x 1.08 x 60 (for 70 degrees inside and 10 outside) is about 9,720BTU/HR

So combining with my conductance calculation of 15,840 gives about 25,560 on a ten degree day outside with minimum wind...that just seems too low...is it?? even adding in an another additional amount for radiation losses...perhaps in the 5000 to 8000BTU/Hr range...just a guess...

Whoever originally sized my furnace ended up with about 96,000 BTU per hour and while I don't know their design criteria (maybe 20 below with 50 mph winds? for example) and how much they oversize, the figures seem too far apart...

Hypothetical test: If my furnace runs 1/5 the time: that would be about 19,000 BTU/Hr..
If my furnace runs 1/4 the time: 24,000
If my furnace runs 1/3 the time 32,000

I have never timed that actual cycle time, but I'm guessing 15- 20 minutes per hour at ten degress outside, no sun, is ballpark...

oops, so based on what I think are reasonable run times, maybe a 26,000 BTU hourly heat loss IS actually in the ballpark...holy cow there may be a reasonable set of figures here!

OK !

 

[Naty1]

I think I've gone about as far as a non pro can on the above...
one other item of possible interest...

Solar heat Gain

I'm wondering what the balance is between solar heat gain for a few hours on a winter day and radiation losses from inside for the majority of day and night. On the north side of my house I have ordered triple pane windows with a low SHGC..meaning a severe limitation on radiation transference...I'm less sure about how to treat south facing windows...Insummer the sun is directly overhead and gain via windows seems to be via conduction not radiation.

On a sunny winter day, I get sun in all my rear facing windows as they face south... Those windows have an area of about (12.5(5)+ 16.5+16.5 +6+6) or 108 sq ft.

There is enough solar energy to cut my furnace run time by well over 2/3 from about noon to around 4PM when the sun disappears behind trees...how much heat am I geeting from sol?

If my heat loss is in the 26000BTU/HR range...then the sun must be pumping about 66% of that or about 17,000 BTU/Hr into my house...thats almost 160BTU/Hr per square foot of glass...of course the outside rear of my house is warm to the touch, being dark cedar
shakes, and that also cuts heat losses...but that still seems remarkable...

I have seen solar heat gain often measured in watts...have to look that up and see how it compares with the above hypothetical...anybody know??

 

[Naty1]

From the Department of energy website:
http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13360

Heating-Dominated Climates

..to control solar heat gain along with different glazings usually selected for different sides of the house (exposures or orientations)...

"To be effective, south-facing windows usually must have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer.

Windows on east-, west-, and north-facing walls are reduced in heating climates, while still allowing for adequate daylight. East- and west-facing windows are limited because it is difficult to effectively control the heat and penetrating rays of the sun when it is low in the sky. These windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used just to provide useful lighting."

 

[Naty1]

http://www.energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13430

To keep the sun's heat out of the house (for hot climates, east and west-facing windows, and unshaded south-facing windows), the Low-E coating should be applied to the outside pane of glass. If the windows are designed to provide heat energy in the winter and keep heat inside the house (typical of cold climates), the Low-E coating should be applied to the inside pane of glass.

Window manufacturers apply Low-E coatings in either soft or hard coats. Soft Low-E coatings degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life. Therefore, manufacturers carefully apply them in insulated multiple-pane windows. Hard Low-E coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. The energy performance of hard-coat, Low-E films is slightly poorer than that of soft-coat films.

 

[Naty1]

had not realized coatings could be either inside or outside...

so I just e-mailed the local business that will be installing my new windows...and asked about whether I have the option of selecting low e coating on inside of some windows and outside of others...

My south facing windows get no summer sun, north windows hardly any, so I think e coating on the inside (to retain house heat) and admit sun is best...am unsure about east facing window (which is exposed to sun about five or six hours daily in summer) and two west windows (only exposed a few hours in summer due to nearby shade trees)...

this gets more interesting!

 

[Naty1]

The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412 kW/m² in early January to 1.321 kW/m² in early July) due to the Earth's varying distance from the Sun,

via wikipedia...can't find figures at Earth's surface...anybody know??

1.321 KW/sqm comes to about 444 BTU/sqft...much higher than my crude 150 and likely much too high as I assume sunlight energy is dissipated at ground level relative to satellite observations...

edit: I forgot to correct my windows for clear glass transferrence...I think a single pane is about 0.7...so the solar energy outside might be 150/0.7 about 214 and I guess for my double pane closer to 214/0.7 around 306?

 

[OmCheeto]

I would suggest collecting some meaningful data.

I did what you are doing when I bought my house about 20 years ago. My house is powered/heated exclusively with electricity so I was able to take hourly readings of kwh, inside and outside temperatures on a cold winter weekend. I determined my aggregate R value was 6.3, and the heat capacity of my house was 5000 Btu / 'F.

I discovered that the R value was so low because on inspection of the wall cavities, the builders had insulated the entire house with a single thickness of cardboard stapled to the exterior walls. Needless to say, I spent the next several years, a room at a time, tearing out the sheetrock, and installing proper R-13 insulation.

I just walked outside and saw that my neighbors natural gas meter has readings in cubic feet. I would suggest taking a measurement in the morning and evening, converting that into BTU's, then determine the actual R value of the house, based on the differential temperature.

Determining the heat capacity is of course optional. Though I suspect that it is the heat capacity that determines the size of your furnace. Coming home after being at work with the furnace set to 50'F, the furnace would need to provide 100,000 btu's to raise the temperature of my house(not just the air) to 70'F. Since your residence has 3 times the floor area, I would imagine the heat capacity would be quite a bit higher. 900+900+(30*4*8) vs 2400+2400+(50*4*8) ---> 1 : 2.3
So I'll interpolate and guess your residence has a heat capacity of around 11,500 btu / 'F.

Also, if you are monitoring cfm of your natural gas, be mindful of your hot water usage in your energy loss calculations.

 

[Naty1]

Om..good comments, thanks...I may go further and do some readings..
meantime I spoke with the local business owner installing my first ten windows..he referred me to this site:

http://www.efficientwindows.org/factsheets.cfm

It's pretty neat: you can select window option recommendations by state, that is by climate, and obtain recommendations for your particular climate...

 

[Naty1]

Open issue on Window Selection

Recommendation from the Department Of Energy:

"To be effective, south-facing windows usually must have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer.

Is this correct in a cold climate like NJ??

And from a post above the issue:

I'm wondering what the balance is between solar heat gain for a few hours on a winter day and radiation losses from inside for the majority of day and night. ..I'm less sure about how to treat south facing windows...In summer the sun is directly overhead and gain via windows seems to be via conduction not radiation.

DOE and local supplier disagree:

My local window supplier has confirmed that high SHGC means a higher U factor...so more heat gain from solar means more heat loss from interior radiant energy...They recommend higher U and lower SHGC even south facing...I do not have data to confirm or deny...

Any recommendations?

 

[Naty1]

found a wikipedia statement that about 51% of sun energy hitting the upper atmosphere reaches the Earth's surface...so /51 x 444 BTU/ft²

is about 226 BTU/ft²

http://en.wikipedia.org/wiki/Solar_energy

 

[russ_watters]

The sun sends a much higher average frequency of radiation to Earth than the Earth radiates back, so I'm pretty sure windows are typically capable of allowing solar heat in while blocking heat from radiating back out.

 

[Naty1]

I'm pretty sure windows are typically capable of allowing solar heat in while blocking heat from radiating back out.

not very well from what I see so far...
I don't yet know how to optimize for this situation...I will query a manufacturer or two and see what they say...
What I want is low U high SHGC...low U to keep heat in, say 0.3 or lower and high SHGC, say 0.7 or so or higher to let in lots of sun...

I have sent queris to OKNA and PELLA...will see what these manufacturers say/////

 

[Naty1]

Got a reply from OKNA: They don't make low U and high SHGC...

They said that's because the energy star tax credits apply for U of 0.3 and SHGC of 0.3...so that's what manufacturers follow..
.
Dopey government once again.


Also, here is an excellent discussion:
http://en.wikipedia.org/wiki/Insulating_glass

There are several comments I doubt are correct, but overall this gives anyone considering high efficiency windows a solid foundation of understanding...

edit: I found a Department of Energy website that allowed me to ask an "expert" there questions...so I asked about high SHGC/low U and where I can find a table of solar power in BTU/sqft for wintertime NJ...will report when I get anything.

 

[Naty1]

Solar Radiation data...I found what might be the data on solar radiation by state...

http://www.ncdc.noaa.gov/oa/documentlibrary/surface-doc.html

I'm asking for assistance in the computer/database forum and will explain the data if I get can get to it.

 

[Naty1]

Looking further down at the bottom of the page at the website noted in Post #16,
http://www.efficientwindows.org/factsheets.cfm

I now notice they have "annual heating costs" for different window designs by state.

In NJ the annual heating and cooling cost difference between high SHGC and lower SHGC is shown in case 4 compared with case 5...it's an insignificant difference for the typical house illustrated...so I can now go ahead and order windows with moderate SHGC and not worry...

 

[Naty1]

The sun sends a much higher average frequency of radiation to Earth than the Earth radiates back, so I'm pretty sure windows are typically capable of allowing solar heat in while blocking heat from radiating back out.

Turns out, unfortunately, such windows are not available.

I've finished installation of all 27 replacement windows in my house. I hired a local retail store to install 10 triple pane windows on the second floor and watched them. They also capped those exterior wood frames in vinyl coated aluminum and will be coming back to do the rest at $60 per window frame capping. (I don't have an aluminum bending machine else I'd do it myself.)

The next day I started my own installtion of double pane windows in the remaining 17 windows in my house. I replaced some from the inside and some from the outside.

I can already tell on warm days in March, when the sun is still fairly low, that the new windows block a LOT of the suns thermal energy...my house stays cooler than with the old windows and my heat runs a bit more. So as I worried, I'll surely lose some solar heat gain in the winter...

I'll have to monitor over the summer to see if less solar heat gain in the summer has any effect on reducing my air conditioning bill...with the sun largely overhead summertimes, I'm not expecting much reduction in air conditioning and will post again when I make a subjective determination...