# Cost of Adding Insulation vs. Fuel Usage

| Posted in General Questions on

My first question on this forum! We have an 1800’s stone house in Pennsylvania (border of 4A and 5A zone) and are slowly remodeling, starting with our kitchen.

Obviously there is no insulation whatsoever, just 18″ thick stone walls, and a layer of plaster on the interior. Also, original windows. Lots of drafts and cold air. 1700sqft of heating.

Is there some way to calculate the difference in fuel usage vs increasing r-value? In our current remodel, we are adding a 2×4 wall to all exterior walls, to make it easier to mount cabinets and run wires, but also will add insulation. I want to weigh the option of eventually doing this to every room in the house so that all exterior walls have 4″ of insulation. We’ll also be installing new windows throughout. Attic was newly finished and has r19 insulation.

Does increasing the r-value of walls from 1.5 (current) to R17.5 (new) decrease fuel oil usage by X%? Hopefully this all makes sense. Thanks for any help!

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1. Expert Member
| | #1

In short, yes! Martin describes the process here: https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1. It's fairly simple once you understand the concepts but getting to that point can be a steep learning curve.

2. Expert Member
| | #2

You can calculate the amount of heat lost through a surface based on your local heating degree days.

Say you are at 5000 heating degree days and have a 1000sqft of about R3 (18" stone+air gap+plaster) wall area.

The heat loss through a season would be:

5000HDD * 24hours * 1000sqft / R3=400Therms

You bump that wall up to around R12:
5000HDD * 24hours * 1000sqft / R12=100 Therms

That is a pretty significant difference.

Make sure to read through the link bellow about insulating. Generally the safest is spray foam but you can also do foam free if you coat the inside of the store with a vapor permeable fluid applied WRB (ie Prosoco Liquid Applied WRB) as shown in Fig 11.

1. | | #3

Thanks so much for your reply, and for the information about insulation practices when doing something like this.

Forgive me for my simplicity - would that difference in heat loss directly correlate to fuel usage? So if heat loss decreases 25%, would that correlate to a 25% reduction in fuel usage?

1. Expert Member
| | #7

So for the above example, insulating the walls saves 300 therms.

1 gallon of heating oil burned in a 70% efficient boiler makes about 1 therm, so the insulation in that case saves 300 gallons of oil over a heating season.

That is real money saved.

The good news is that these savings are additive**. When you save 300 therms by insulating, even if your house is still leaky, you still save that much oil use.

Of course, if you can air seal the house, you'll save even more on top of that.

** This is true in most case, in same cases with very leaky balloon framed walls, the air leaks are so high through the wall cavity that the insulation doesn't work. In that case you need to air seal to get the insulation savings.

3. | | #4

So the answer is "yes," but with a lot of "buts".

Energy usage correlates directly to heat loss, which correlates directly to insulation level. But here come the buts:

First, houses lose heat through two main way: conduction directly through the walls, and air infiltration. The modern practice is to make houses as air-tight as possible, which minimizes air infiltration. Your house is probably very, very leaky by modern standards. Most insulation does little to stop air infiltration, you have to do air sealing as a separate step. The nature of air infiltration is that sealing one wall doesn't do much to change the overall air flow; think of a lean-to with three sides open vs. two sides open. So you really want to focus on sealing the entire structure.

After air infiltration there's conduction, and that's what insulation resists. And the answer there is that every surface in the house conducts independently of every other surface, so yes, doubling the insulation in a wall will half the heat flow through that wall, and will half the energy cost of heating that wall. But -- there's that word again -- houses are more than walls. There are roofs and basements and windows and doors. Roofs and walls are pretty straightforward to insulate, basements can be tricky and windows and doors you can't really upgrade without replacing outright.

If you want to be serious about the process, the way to approach it is to do an energy model. You estimate how leaky the house is and the insulating abilities of the existing materials. The first estimates are just guesses, but then you compare your existing energy usage against the model and keep tweaking the model until it matches up with your existing energy usage.

Then you can plug scenarios into the model: What would happen if I air sealed the entire house? Or improved every wall from R1.5 to R17? Or insulated the basement? Or the attic? Or replaced all the windows? Then you can price out those improvements and see which ones make financial sense.

Here's one final but -- you may be disappointed in the cost-effectiveness of a lot of the improvements. But, there's more to it than just the financial aspect. A tight, well-insulated house is more comfortable, less drafty, quieter, cleaner. It's just more pleasant and comfortable. It's hard to put a dollar figure on that.

1. | | #5

That makes a lot of sense, appreciate your reply. Is it safe to assume that a closed-cell spray foam would help with air infiltration if done correctly?

We remodel as funds allow, so right now it's basically half of the downstairs area, which will be kitchen/dining room. As we go, we plan to replace windows and add interior wall structure to allow for insulation. In the grand scheme of the remodel, adding insulation and new windows doesn't add much while we're already doing things. I like to put numbers to it to help justify it in my head, but seeing my wife/kids be warm and cozy has no price tag.

The house has cast iron radiators, with an oil boiler. Our first winter, we went through about a tank of oil a month (180 gallons)...that adds up quick! The payback period wouldn't be long, even if we had a 10% reduction in usage.

4. | | #6

Your 200 year old walls lasted this long because there is a balance between how much water is getting into the walls and how much moisture get out of the walls.

Things can get very ugly if you add modern insulation and air barrier and fail to add modern flashing and water barriers to your old walls. Lots of water could get in because there was lots of heat in the walls to evaporate the water and lots of air moving thru the wall to carry the moisture away.

Adding modern flashing and water barriers the old walls waterproof short of rebuilding the wall is no small feat.

Note modern standards require a one inch of separation between the masonry and your new water barrier then your new framed walls.

Walta

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