Claculating Insulation Benefit

Dan_15

Im a homeowner, not very math-minded, but trying to quantify the benefit in real dollar terms of adding wall insulation to the house. I have fully insulated attic space, double-paned windows and well sealed entryways--I want to know how much more I would really benefit from adding wall insulation.

I can do two separate heat loss calculations with assumption of wall insulation for one, and assumption of zero wall insulation for the other. But then what do I do with that information--how do I turn it into fuel consumption?

I have a Buderus G215 with Riello downfired 10%, outdoor reset, indoor sensor, constant circ, indirect tank.
Thanks!

Mike T., Swampeast MO

Here's a reasonable way to guesstimate:

1) Calculate design heat loss both with and without wall insulation.

2) Determine the percentage reduction. BTUs with divided by BTUs without. Then 1 minus result, then multiply by 100. By the nature of heat loss, such percentage will be quite stable regardless of the actual load on the system. (Remember--heat loss is the DESIGN loss--not "typical".)

3) Do your best to determine your actual space heating cost over a season. In other words determine how much fuel you use JUST to heat your domestic hot water during the months when the space heating is off and subtract from an entire year. (Likely far easier with natural gas than oil unless you have and have regularly used an ability to reasonably estimate the oil level in your tank.) If at all possible repeat for other seasons and make an average.

4) Reduce your average annual fuel consumption for space heating alone by say 75% of the percentage you found in step 2. The difference will be a reasonable estimate of how much fuel will be saved by adding the wall insulation.

In general, unless the walls are already open for some other purpose, wall insulation has the greatest payback period of any insulation/weatherization. I suggested only 75% in step 4 because it's EXTREMELY difficult to insulate closed walls anywhere near the perfection assumed by heat loss calculations.

Dan_15

Thanks for that information--I will try it! You confirmed my suspicion that wall insulation probably will not add much marginal benefit, but I am always in search of scientific support.

realolman

I am understanding not so good

Mike T. In general, unless the walls are already open for some other purpose, wall insulation has the greatest payback period of any insulation/weatherization. I suggested only 75% in step 4 because it's EXTREMELY difficult to insulate closed walls anywhere near the perfection assumed by heat loss calculations.

Dan: Thanks for that information--I will try it! You confirmed my suspicion that wall insulation probably will not add much marginal benefit, but I am always in search of scientific support.

Is the wall insulation worth it or not?

Gordy

Mike is saying, unless you can easily access the walls to do a detailed insulation job...knooks, and crannys via remodel or gutting. Wall insulation injected or blown in blindly will have a greater period to regain the cost of doing it because it tends to be a sloppy job. That is why he is knocking off the 25%.


I tend to agree some what, unless your out in the country living on a windy knoll with no wind breaks.


Gordy

Tom Hopkins

conductive heat loss calc

you could try this. It will give an idea. As the other guys said there is a lot of questions as to the R-value of you new wall.

Conductive Heat Loss:
Q = U x A x HDD x 24 x .75

• Q= BTU’s transferred through the component during the heating season

• U= The composite U-value of the component

• A= Net area of the component (For all components use gross area minus windows/doors)

• HDD= Annual Heating Degree Days for the location of the house

• 24 = Hours per day

• .75 = A correction factor to ensure that savings predictions are not overstated.

Then divide your BTUs by your fuel of choice, then price per gallon and you have your savings.

If it doesn't work out, sorry I didn't make the formula :-)

Dave_49

conductive heat loss calc.

Should U be the difference between the two u-values for the components being compared.

Tom Hopkins

heat loss calc

I've always done two separate calculations. U value of existing and proposed upgrade, but the way you point out will work.

Ken Field

Blown-in

doesn't have to be sloppy. Only if a sloppy contractor does the work. You will control infiltration much better with dense packed cellulose in the wall than you will with the best batt job you could ever do. If it is installed correctly and checked with an IR camera, I would take a job like that over what I see in new construction anyday. It will cost more if done correctly but it is no different than a boiler job. Many prices, many contractors, many levels of expertise.

Ken

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Gordy

If done correctly

Thats the problem Ken. Mike trys to convey this. How many insulation contractors use IR technology. It all looks good in the videos with a plexi glass wall, and very little obstructive assemblies in the cavity. Real world not the same.

I don't believe its being said to not do the walls. Its just not going to be as good of a job as an open wall situation.

Gordy