how can I calculate "R" value from delta T ?

Old 01-27-09, 01:41 AM
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Red face how can I calculate "R" value from delta T ?

I have measured the temperature of the interior wall surfaces, and find about a 10 degree drop from the interior temperature of 70 degrees F.
The outside temp. is 5 degrees below zero with no wind .. so how do I calculate the effective "R" value??
(the walls are 2X4 with fibreglass insulation) ..... I am currently "treating" the windows to decrease the heat loss, and find that after treatment , the temperature of the window surface now is the same as the walls ... (?) I am guessing that the walls have a "R" value of 4 or 5 (since fiberglass is "DAMN" poor insulation!!!) I hope that the decrease in heat loss through the windows will cut my heat bill.
any constructive ideas will be helpful .
dddon (a new GREAT granpa!!)(#8)
Old 01-27-09, 03:11 AM
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Good morning Don, very good guess. With your delta Tís it falls right on the R=5 line. When you consider the inside sheetrock/plaster and outside sheathing/siding are adding r-value, that doesnít leave much for insulation. If you are using a laser temp device, you have to be sure you are in a cavity and not on a stud.

Old 01-27-09, 03:36 AM
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R-value alone does not provide an accurate assessment of wall performance. You have to take into consideration thermal mass, air tightness, and moisture tolerance. North and east walls will naturally be cooler. Wall performance will not be as high on windy days. R-value does not take into account windows, doors, and corners. R-value calculations are for insulated walls only.

Minimum R-values required vary depending on where you live. Colder climates require higher R-value. Here's a chart: The r value of house insulation

You can find Whole Wall Thermal Performance Calculator here: BERC - Whole-Wall Performance

An interesting study comparing plastic to batt insulation follows. The significance of the findings reveals input about the performance of batt insulation and the significant impact of wind on wall performance. "It was concluded the supposed performance values based on traditional R-value measurements and calculations are not a complete indicator of how well a wall system will resist the loss or gain of energy."

Green Building Solutions - Plastics Takes Improvement To The Wall

Welcome To Home Energy Magazine Online

Insulation Fact Sheet

Here is a U-value calculator for internal surfaces. The lower the U-value (coefficient of heat transmission), the better the insulation. U-value is The rate of heat transmission through 1 square foot of building envelope for 1 degree Fahrenheit difference in temperature between indoors and outdoors.


R-value of batt insulation in walls can vary depending upon thickness and density. R-values for standard batt insulation tend to vary "R-2.9 and R-3.8 per inch of thickness. High-performance (medium-density and high-density) fiberglass blankets and batts have R-values between R-3.7 and R-4.3 per inch of thickness." See Department of Energy chart: EERE Consumer's Guide: Blanket (Batt and Roll) Insulation

I can not comment on Delta T. The physics of thermodynamics is beyond my area of expertise. Thermodynamics - Overview of Thermodynamics

Some tips for reducing your heating bill:

1. Change your air filter and make sure the unit has been serviced and working efficiently.

2. Reverse direction on ceiling fans to circulate warm air throughout the home.

3. Run only full loads in dishwasher and clothes washer.

4. Change bulbs to compact fluorescent, which you 1/2 the energy of incandescent bulbs.

5. Turn off lights during the day and take advantage of natural light. Watch how the sun moves over the house. Open blinds and let the sun shine in to warm the room. When the sun moves, close the blinds to conserve the heat.

6. Programmable thermostats can reduce heat while sleeping or away from home.

7. Turn off it off if not in use. Lights, electronics, TV, etc.

8. Check what minimum required R-value for attic insulation in your area. Note, that's just minimum. Make sure you have adequate attic ventilation.

9. Monitor humidity levels in the home with hygrometer. Sold where thermometers are sold. Maintain humidity at 35-55%. More humid air heats more easily and makes rooms feel more comfortable. By monitoring humidity and running humidifier, it is possible to lower the thermostat a few degrees. Do not over humidify for fear of condensation and mold/mildew.

10. Most importantly seal all gaps around windows and doors. Use weatherstripping. If windows are old and leaky, cover windows with 3M plastic kits. You install double-sided tape around window and apply plastic and shrink with blow dryer. Plastic does wonders to save energy. Weather strip your doors. Make sure there is no draft coming under or around doors. Remove light switch and outlet covers and caulk to seal out cold air. Keep damper closed on fireplace when not in use. Seal gaps around pipes under kitchen sink and vanities. Remove molding and seal gaps beneath baseboards if you feel a draft. Think like you are a cold draft and want to come in. Where would you do it? That's where you seal.
Old 01-27-09, 12:55 PM
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WOW! I got more info than I can handle!! (short term)

Thanks for the great responses ... I have just finished all 29 windows with my "treatment" ...
the windows are 50 years old ..some are casement , and the rest are double hung, and now are almost as good (I believe) as the $25000 worth of replacement windows that salesmen claimed would "pay for themselves in 5 years!" ... I put a 10 mil plastic film between the inner glass and the storm window ....... whoa! I know it ain't perfect, but damn ... it took only a couple days of my time and a cash outlay of $300 .... And I believe the same is true of the cellulose I used to "bury" the fiberglas in the attic ...a couple days of time and an outlay of
$300 .... the house had horrible bypass from basement to attic ... the old insulation was stained black in many places ... that job paid back in 6 months!! ... is there a place where the humidity never gets above 5%?? if so , it is the ONLY place that fiberglass is any good as an insulator !!! last year I removed the fiberglass from the walls of my daughters home and blew in cellulose ..the difference is almost unbelievable .... AND the house is MUCH quieter too! .... the "cellulose" firms I have talked to have inferred that they are "scared " to silence by the fiberglass insulation manufacturers ... maybe it is time for a class action suit against them ... the cost to the world is staggering!! in the sun belt alone , i have talked to people that run the AC nearly 24/7 just to be comfortable ...they have fiberglass and 80% humidity .... or to put it another way ...80% humidity and NO insulation! Me? I got 30 below an "cabin fever" .... but I am working on making our home more comfortable and less costly...
please add comments that can help other "victims" of fiberglass ... (it also seems to help grow black mold?)
Again ...Beer 4U2 thanks!! dddon

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