U-value of double pane glass with failed seal

MN Renovator · 02-14-12, 09:22 AM

I was walking around the house and measuring all of the windows because I want to install white single-cell shades for my windows. The weather has been mild and I made a pizza, french fries, and brownies in the gas oven early morning yesterday, so the humidity in the house was high. I found that multiple windows in the house had condensation between the panes from the indoor humidity. I'm not sure if it is just a leak from the inside of the house to the air gap or if there is a separate leak on the outside too that might be leaving an air bypass.

I realize that windows are the worst part of the envelope as far as convective heat loss/gain since their insulation value is so low. I've been doing a bunch of retrofits to my house such as air sealing, capping a vertical knee wall(rigid foam sprayed foamed in), adding insulation to that same knee wall, and will be pumping more cellulose onto the ceiling facing the attic as soon as it is warm enough to do that. The heat loss in the house was dramatically lower last year and this year with the mild winter my January 2012 heating bill is 1/3 of what it was in 2010. So far my cost to insulate and air seal (roughly $300) has seemed to already paid for itself during the heating seasons.

The windows in question include the following:
4 foot wide by 4.5 tall picture window(non-
2 foot wide by 4.5 tall casement(swing out) windows to the sides of the picture window
The two octagon windows in the master bedrooms, 2 feet I think, didn't measure those though.

The rest of the windows in my house are sliders and none of those seem to be leaking into their air gap. Whether these are losing more heat or not, I'm not sure if it is worth replacing them as they aren't rotting out yet(27 years old but seem to have remained well painted so far). My house is able to remain passive on sunny days where the inside and the outside temperatures are about 15 degrees difference for the high temperature of the day and about 30 degrees between setpoint of the overnight low temperature. So if the setpoint is 65 degrees outside and the nightly low is 35 and the day high is 50, the furnace wouldn't run at all and the inside temperature will be 70 with the sun and drop 5 degrees overnight. I keep my house on the cold side and wear a hoodie inside and on I work 10 hour shift and usually I get home and don't bother to turn up the deep setback, so even if its colder the furnace often doesn't run. I can comfortably sleep at very low temperatures under the covers. December and January heating bills were under $75(combined) this winter(about 15 therms so far February), last winter the highest methane gas bill was a little under $70. 2200 sq ft quad-split.

I'm trying to do a load test(on paper) and compare it with my actual load tests and be able to set up hypothetical situations on how much fuel would be saved and do more cost predictions on things like heat pumps, or a decision of going for an 80% or a 90%+ 2-pipe condensing setup. It's a little odd to think about but at this point hanging on to an old 76% atmospherically vented gas furnace and I'm having a hard to justify replacing for a 90+ unless this ones fails. Usually a conventional double-pane is considered 0.5U. My question is, what sort of U-value should I expect now that the windows seem to be breathing the indoor air?

lucerne96 · 05-13-12, 08:08 PM

May I suggest you think of windows two ways. The U-value of the assembly and the air tightness of the window. Look for a modeling program which accounts for air tightness values of windows & doors. Generally, it is a function of gaskets; but also installation. I look for windows with .03 ACH or better, but understand it is part of an overall system. You can substitute less efficient windows & make up for it somewhere else in the design.

I also try to install windows so that the frame is buried IN THE INSULATION ENVELOPE of the building. To do this, you must properly design the rough opening; also design it to minimize thermal bridging.

I could include pix, but can't figure out how to upload. They are not on-line.

PS. install the shades ON THE EXTERIOR of the windows. Once the sunlight (heat) has crossed the energy envelope (window) it's too late!

MN Renovator · 05-14-12, 11:12 AM

I mostly asked the question because I wanted to know whether or not my current windows will make much of a difference if I ever am required to replace my furnace and A/C. This thread is old though and at this point, I already know heat loss is smaller than any standard residential furnace that is on the market today. It seems there are RV furnaces that match my heat load but those aren't to the same standard as one designed to a house and it would be against code to use one. The heat gain doesn't really matter much in this situation because that is more of a SHGC situation than whether or not air is getting between the panes of glass through a broken seal.

I like how you mentioned installing shades on the outside. I have actually thought about this a bit before and thought of using window darkening film on the outside but found the price was sky high and not worth it. I didn't want those large shades that roll down because they don't make them for a large bay window that I have on the side that needs shading. I've actually settled on the idea of building a discrete frame with either a very dense bug screen or multiple layers of bugscreen to block the light so I don't have anything dark inside. This will also be easy to remove unlike installing window film on the window because I really need that sunlight in the winter so removing them is a MUST or I will be required to turn the furnace on in the winter when we have 20+ degree sunny winter days. I'd rather not, I liked this winters 50 therm heating bill, dirt cheap.

lucerne96 · 05-14-12, 01:45 PM

Generally, I use louvered shutters on my windows to keep out unwanted heat.

At my office, I made a roll down external shade from this ClearDome Solar Thermal fabrics for cooling and heating over a big plate glass window which faces south. I Use it doubled, it hangs from an externally mounted bracket, where the open end of the folded material are fixed to a board, (attached to the bracket), mounted horizontally as a header over the window. I inserted a broom handle in the fold of the material to weight it & keep it from blowing up. It has been in operation for 12 years without problem. In the fall, when I want solar gain, I simply roll up & tie the material; no muss - no fuss. It seem impervious to the weather.

A pergola would work as well.

02-14-12, 09:22 AM	#1
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 940 Thanks: 41 Thanked 117 Times in 91 Posts	U-value of double pane glass with failed seal I was walking around the house and measuring all of the windows because I want to install white single-cell shades for my windows. The weather has been mild and I made a pizza, french fries, and brownies in the gas oven early morning yesterday, so the humidity in the house was high. I found that multiple windows in the house had condensation between the panes from the indoor humidity. I'm not sure if it is just a leak from the inside of the house to the air gap or if there is a separate leak on the outside too that might be leaving an air bypass. I realize that windows are the worst part of the envelope as far as convective heat loss/gain since their insulation value is so low. I've been doing a bunch of retrofits to my house such as air sealing, capping a vertical knee wall(rigid foam sprayed foamed in), adding insulation to that same knee wall, and will be pumping more cellulose onto the ceiling facing the attic as soon as it is warm enough to do that. The heat loss in the house was dramatically lower last year and this year with the mild winter my January 2012 heating bill is 1/3 of what it was in 2010. So far my cost to insulate and air seal (roughly $300) has seemed to already paid for itself during the heating seasons. The windows in question include the following: 4 foot wide by 4.5 tall picture window(non- 2 foot wide by 4.5 tall casement(swing out) windows to the sides of the picture window The two octagon windows in the master bedrooms, 2 feet I think, didn't measure those though. The rest of the windows in my house are sliders and none of those seem to be leaking into their air gap. Whether these are losing more heat or not, I'm not sure if it is worth replacing them as they aren't rotting out yet(27 years old but seem to have remained well painted so far). My house is able to remain passive on sunny days where the inside and the outside temperatures are about 15 degrees difference for the high temperature of the day and about 30 degrees between setpoint of the overnight low temperature. So if the setpoint is 65 degrees outside and the nightly low is 35 and the day high is 50, the furnace wouldn't run at all and the inside temperature will be 70 with the sun and drop 5 degrees overnight. I keep my house on the cold side and wear a hoodie inside and on I work 10 hour shift and usually I get home and don't bother to turn up the deep setback, so even if its colder the furnace often doesn't run. I can comfortably sleep at very low temperatures under the covers. December and January heating bills were under $75(combined) this winter(about 15 therms so far February), last winter the highest methane gas bill was a little under $70. 2200 sq ft quad-split. I'm trying to do a load test(on paper) and compare it with my actual load tests and be able to set up hypothetical situations on how much fuel would be saved and do more cost predictions on things like heat pumps, or a decision of going for an 80% or a 90%+ 2-pipe condensing setup. It's a little odd to think about but at this point hanging on to an old 76% atmospherically vented gas furnace and I'm having a hard to justify replacing for a 90+ unless this ones fails. Usually a conventional double-pane is considered 0.5U. My question is, what sort of U-value should I expect now that the windows seem to be breathing the indoor air?

05-14-12, 01:45 PM	#4
lucerne96 Helper EcoRenovator Join Date: May 2012 Location: house Posts: 50 Thanks: 3 Thanked 5 Times in 3 Posts	Generally, I use louvered shutters on my windows to keep out unwanted heat. At my office, I made a roll down external shade from this ClearDome Solar Thermal fabrics for cooling and heating over a big plate glass window which faces south. I Use it doubled, it hangs from an externally mounted bracket, where the open end of the folded material are fixed to a board, (attached to the bracket), mounted horizontally as a header over the window. I inserted a broom handle in the fold of the material to weight it & keep it from blowing up. It has been in operation for 12 years without problem. In the fall, when I want solar gain, I simply roll up & tie the material; no muss - no fuss. It seem impervious to the weather. A pergola would work as well. Last edited by lucerne96; 05-14-12 at 01:52 PM..

05-13-12, 08:08 PM	#2
lucerne96 Helper EcoRenovator Join Date: May 2012 Location: house Posts: 50 Thanks: 3 Thanked 5 Times in 3 Posts	May I suggest you think of windows two ways. The U-value of the assembly and the air tightness of the window. Look for a modeling program which accounts for air tightness values of windows & doors. Generally, it is a function of gaskets; but also installation. I look for windows with .03 ACH or better, but understand it is part of an overall system. You can substitute less efficient windows & make up for it somewhere else in the design. I also try to install windows so that the frame is buried IN THE INSULATION ENVELOPE of the building. To do this, you must properly design the rough opening; also design it to minimize thermal bridging. I could include pix, but can't figure out how to upload. They are not on-line. PS. install the shades ON THE EXTERIOR of the windows. Once the sunlight (heat) has crossed the energy envelope (window) it's too late!

05-14-12, 11:12 AM	#3
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 940 Thanks: 41 Thanked 117 Times in 91 Posts	I mostly asked the question because I wanted to know whether or not my current windows will make much of a difference if I ever am required to replace my furnace and A/C. This thread is old though and at this point, I already know heat loss is smaller than any standard residential furnace that is on the market today. It seems there are RV furnaces that match my heat load but those aren't to the same standard as one designed to a house and it would be against code to use one. The heat gain doesn't really matter much in this situation because that is more of a SHGC situation than whether or not air is getting between the panes of glass through a broken seal. I like how you mentioned installing shades on the outside. I have actually thought about this a bit before and thought of using window darkening film on the outside but found the price was sky high and not worth it. I didn't want those large shades that roll down because they don't make them for a large bay window that I have on the side that needs shading. I've actually settled on the idea of building a discrete frame with either a very dense bug screen or multiple layers of bugscreen to block the light so I don't have anything dark inside. This will also be easy to remove unlike installing window film on the window because I really need that sunlight in the winter so removing them is a MUST or I will be required to turn the furnace on in the winter when we have 20+ degree sunny winter days. I'd rather not, I liked this winters 50 therm heating bill, dirt cheap.