# Insulation calulations

Posted By Bad Wolf, Oct 17, 2012 at 9:36 AM

1. #1

### Bad Wolf Minister of Fire 2. ```NULL ```

Jun 13, 2008
523
45
Loc:
Eastern CT
I’m putzing around with some ideas and need a sanity check.

I’d like to install some window insulation along the lines of an insulated panel that recesses into the wall like a pocket door. I was thinking 2”foam board nicely finished that would disappear into the wall when not in use and be able to pull it out at night.
Here’s what I’m having trouble with, so check me on it:

To simplify things I’m ignoring the studs and other factors, this is for concept only.
To do heat loss calculations I take the various R factors for the wall, add them up and multiply them by the square footage. (Example: 3.5” spray foam R-17, 2” foam board R-10 and 3.5” fiberglass R-14 equals R-41. Don't quote me on numbers).
Next I would take the window R factor (R-2) and multiply it by the area of the window. (Say 2’x3’=6 sq ft)
So far so good. If I slide the foam panel from the wall to the window I’ve removed that insulation from the wall, so now I have a 6 sq ft area that is only R-31 while the 6 sq ft of window goes up to R-12.
It seems like there is no gain, but intrinsically I feel there would be a large benefit since I wouldn’t have an R-2 hole sucking the heat out.

What am I missing??

2. #2

### jharkin Minister of Fire 2. ```NULL ```

Oct 21, 2009
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Loc:
Holliston, MA USA
I agree with your gut. Because R value is linear there is a diminishing return as you go up in value.

When you improve the window from R2 to R12 you cut its heat loss by a factor of 6! Conversely when you reduce the wall from 41 to 31 you only increased its heat loss by a factor of 0.25.

Plug the number for the 2 combos into this calculator:
(Note that for the wall you also get some r value from the siding, sheathing and drywall)

3. #3

### jharkin Minister of Fire 2. ```NULL ```

Oct 21, 2009
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Loc:
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For fun I ran the math for you in that calculator.

Using a 5,000 HDD winter and setting the heat source to natgas at 1.00 per them and 100% efficiency so we can just convert the dollar amout it spits out directly to BTUs...

Increasing the window from R2 to R12 saves 300,000 BTU
Reducing the wall from R41 to R31 loses 6,000 BTU

Your gut was right.

4. #4

### Bad Wolf Minister of Fire 2. ```NULL ```

Jun 13, 2008
523
45
Loc:
Eastern CT
Thanks, I know there had to be something more to it.
I'm trying to do the calculations on a house with normal 2x6 with fiberglass insulation, then again with various other scenarios. I know that the benefit drops off that why I’m looking at a hybrid design, going with cheaper fiberglass for the inner wall after I’ve gotten the most benefit out of the sprayed installation.
I’ll calculate the final wall with siding, foam sheathing, CDX, spray foam, sheet foam (to isolate the inner and outer studs), fiber glass and sheetrock.
After tightening up the envelope and super insulating the windows seem to be the next biggest heat loss.
I’m thinking for rooms that I don’t use very often, I could just leave them in place and for others I would close them for the night.

5. #5

### woodgeek Minister of Fire 2. ```NULL ```

Jan 27, 2008
3,678
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IN round numbers, one sq ft of R-1 single pane window, in a 4000 HDD climate will require one gallon of fuel oil per year. IOW, a 6 square ft of double pane at R-2, will require ~6sq ft * (1/2)(R-value) * 100,000 BTU = 300,000 BTU/yr.

Going to R-6 drops that by two thirds, saving you ~200 kBTU/yr. ~2 gallons of oil, or \$2-3/yr of gas depending on rates.

FYI....airsealing a typical older house can save 20-40 million BUT/yr. If your wall was conventionally framed and the top plate unsealed, your mod could increase air leakage and cost you more energy net.

6. #6

Jun 13, 2008
523
45
Loc:
Eastern CT
7. #7

### woodgeek Minister of Fire 2. ```NULL ```

Jan 27, 2008
3,678
639
Loc:
SE PA
yeah sorry, saw that later. Have fun with the superinsulation. Have you thought motorized panelon a timer?? Why not just buy R-4 or R-5 windows and call it done?

8. #8

### Bad Wolf Minister of Fire 2. ```NULL ```

Jun 13, 2008
523
45
Loc:
Eastern CT
This is a lot more fun, and anything worth doing is worth overdoing. By taking the windows to R-12 and super-insulating the rest of the house I should be able to heat it with a frelling light bulb.
Seriously, by lowering the thermal loss/gain, the subsequent heating and cooling systems can be that much smaller and cost much less to operate whatever the energy source.
From what I've seen triple pane windows are still only R-3 manufactures claims not withstanding and are pricey. I know a 2" piece of form will work. The simplest method would be to just make a panel that would fit snugly and take it out and set it on the floor when you don't want it. I thought the recessed panel would be cleaner and easier.
Actually I have considered a motorized unit, that’s going to be the Mark II version. Why stop there? I could wire it into a photocell so it wouldn't open till sunup and close at sundown, plus this is strictly a winter time thing. I think there are units that will open and close your drapes.
Dick Russell is my hero, he seems to have done everything that I want to do in my next/last house.

9. #9

### jharkin Minister of Fire 2. ```NULL ```

Oct 21, 2009
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Loc:
Holliston, MA USA
But you would then have no excuse to burn wood!

10. #10

### woodgeek Minister of Fire 2. ```NULL ```

Jan 27, 2008
3,678
639
Loc:
SE PA
IF he has a good HRV, he could heat it with candles.

11. #11

### GaryGary Feeling the Heat 2. ```NULL ```

Jul 12, 2010
291
18
Loc:
SW Montana
Hi,
There is a calculator for this kind of upgrade here: http://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htm
and a similar one for the whole house here: http://www.builditsolar.com/References/Calculators/HeatLossOld/HeatLoss.htm
These calculators just use R values, area, and Heating Degree Days for the calc just like WoodGeck's calc.

You can buy pretty good thermal shades that just pull up out of the way and give up to about R5 (the ones duofold type ones with internal reflective layers) -- we use these shades with the side track to reduce air flow around the shade: http://symphonyshades.com/comfortracks.html they have been very reliable and work easily. Also like the bottomup - topdown shades in that you can leave the shade on the bottom half of the window (for insulation) while still getting light and view out the top half.

More window treatment stuff here: http://www.builditsolar.com/Projects/Conservation/conservation.htm#WindowTreatments

For your in the wall scheme, making use of multiple reflective layers with air spaces between might allow a thinner sliding shade. Somewhat like these Astrofoil shades: http://www.builditsolar.com/Projects/Conservation/Astrofoil/AstrofoilShutters.htm So, use 2 or 3 layers with the layers spaced apart a quarter inch or so -- each reflective layer and airspace contributes additional R value.

There used to be a commercial shade that used half a dozen layers of reflective material to get up to R15.
Our old library in Bozeman had one, and it appeared to work well.
Seems like there could be a DIY design for this?

If you go with the sliding panel rigid polyiso insulation board, you might consider the Atlas R-Board product. It has nice looking face sheets on each side that take paint well. Looks like this: http://www.builditsolar.com/Projects/Conservation/Slider/SlidingDoor.htm
At our lumberyard, its the same price as the ones that have the very thin and easy to damage and ugly alum foil faces.

Gary

12. #12

### DickRussell Burning Hunk 2. ```NULL ```

Mar 1, 2011
204
84
Loc:
central NH
To evaluate options, you should be dividing areas by their R values, not multiplying. Here's the reasoning. The "heat transfer coefficient," usually designated "U," is the reciprocal of R: U=1/R, and in English units is in BTU/sq.ft/hr/degree F. The steady state rate of heat loss by conduction across an area is given
by Q = U*Area*dT, where dT is the hot-cold temperature difference. Substituting R=1/U gives Q=Area*dT/R. The total heat loss across a wall with windows is just the sum of Q values for plain wall and the windows. Obviously, a higher R for either wall or window reduces the total, as R is in the denominator.

One thing to be aware of is that if you add insulation in some form to the inside of a window, you shift the temperature profile across the whole assembly, and the inside glass surface temperature drops. If you don't have a tight fit of that added insulation layer against the window frame, then migration of inside air humidity around the edges (or through the insulation if it is porous) can result in considerable condensation on the glass surface, potentially causing deterioration of wood trim, etc. There is a good discussion of this going on over at: http://www.greenbuildingadvisor.com/blogs/dept/musings/rating-windows-condensation-resistance

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