Spray-foam and furnace return on investment

[index]

I posted this in another thread, but I have seen some questions regarding the ROI on insulation and/or new HVAC. I was doing some work sizing a new furnace and I got interested when I was digging into the data.

For background, we had 6 inches of spray foam (R21) installed on the underside of our roof deck in an uninsulated attic. The contractor appears  to have actually sprayed 7-8 inches of foam in the attic. We also got 2.5 inches of closed cell foam (R15) sprayed in a small 110 sf open crawl space and encapsulated the space.

Our home is two stories,1700 sf, 110 years old with no wall insulation, and low-e storms over original single pane wood windows. We had 4 inches of cellulose in the attic under T&G pine floors. I air sealed 8 years ago with about 20 cans of great stuff and added R-30 batts over the floor. There was a 2 inch open space beneath the floor and top of the old cellulose I couldn't get to without tearing up the floor. I disposed of the batt insulation just before the foam was installed.

Energy usage:     

2020 - November - May 2021: 127 heating degree days (HDD) - 974 CCF used - 7.66 CCF/HDD
2021 - November - May 2022: 122 heating degree days (HDD) - 632 CCF used - 5.18 CCF/HDD

So the spray foam saved about 32% YoY gas usage. On 12/2021, gas was $1.1/CCF (supply distribution charge); right now it is $1.4/CCF

So at 2021 rates it is saving $350/yr and at 2022 rates it is saving $440/yr - the insulation was $2700 so that is a return of 13-16%.

A new furnace would be 96% efficient versus 80% efficient of my current model using last winter's gas usage -
632 ccf x 80% efficiency = 506 ccf went into heating the house / 96% = 527 ccf in anticipated usage.
This would save 105 ccf which is  $147/yr at the current rate - the furnace will cost around $2k for a DIY install so a return of 7%. A contractor installation would be $5.5k (quoted 2 years ago) for a return of 2.7%.

Moral of the story is to insulate first!

I am kind of surprised the spray-foam performed so well being only R21 (below the code minimum). I think it performs so well because it air seals so much better than my attempt and the two inch gap between the old cellulose and batt insulation allowed for a lot of heat loss. I also suspect the air sealing on the roof deck and 3-14 foot now dead air space in my attic may significantly increase the effective R-value between the second floor ceiling and outdoors.

[Sibley]

Since you've done the research, I'll pick your brain.

My house is similar, not exact. Modern windows (purchased like that), air sealed the attic floor and 18 inches of blown in insulation. My biggest obvious air sealing problem is the floor over the crawl space. Dirt, semi-sealed up, no vapor barrier. I am not prepared to add ventilation from the HVAC. But that floor is cold in winter. Crawl does get some water periodically. Aside from adding a vapor barrier, what would you suggest for the underside of the floor? Entire floor plan impacted, so budget is a factor and so is long term moisture concerns.

[index]

@Sibley the Cadillac plan is going to be to solve your water issues and encapsulate the crawlspace. You really don't want excess moisture down there to begin with. I am fighting this at our vacation home. The order of attack is: make sure your gutters are working, seal and French drain along the perimeter of the crawl space, and a last resort of a French drain inside the crawl space. Vapor barrier then closed cell foam or flash and batt on the walls. Keep track of the humidity and add a register or two in the crawl space if your humidity gets over 70% routinely.

The cheaper and not as good option is to seal the rim joist with two layers of 2-inch rigid foam and use great stuff spray foam to seal the small gaps. Then cover the entire floor area with 2-inch rigid foam and tape the seams.

I tried everything in my little crawl space and had pipes freeze every year until I got it foamed. My advice is to save up and do it right. It is really hard to air seal well while crawling around. You get great stuff everywhere including your hair :-(.

Start by cleaning your gutters, put some gutter guards on, and use wide mouth downspouts. If you can figure out where the water is coming in, put an exterior French drain there. 

[Sibley]

My water issues are due to my neighbor cutting down two mature trees on my property. I didn't really have problems before that, except in cases of extreme weather (3 ft of snow and ice melting over a couple days due to heavy rain causes a problem, but I didn't actually flood, just got damp. The rest of the town did flood.).

To solve the water problem, I have to pull out 4 year old concrete sidewalk, install some sort of drainage and dry well that will involve tearing out a heck of a lot more concrete, and then put new concrete down and put my yard back together. It will be a massive project. The property line is too close and I have no where to drain the water to. There are 2 downspouts, one is (illegally) draining into the sewer, but the other dumps 4 feet from the house and there's nowhere else I can put it - that's where the water is stemming from. Previously, the trees sucked up all the water so it wasn't a problem.

Gutters are functional and get cleaned as needed. There is some rim joist sealing, but it's been disturbed so needs to be redone. Which is on the list to do. No pipe issues however, I've got that going for me.

I guess I'll have to encapsulate, I just can't tap the current HVAC to bring it into the conditioned envelope. HVAC is old and I don't believe it can handle the crawl.

[index]

The hvac beeds are pretty minimal. 1 cfm per 50 sf.Mike? You need 3% of the capacity of a 2 to 3 ton unit for a 1500sf crawl space.

[Paper Chaser]

There is some thinking that a standalone dehumidifier is more appropriate for an encapsulated crawl than modifying the HVAC.
The dehu will run as needed independently of the HVAC, which might potentially be shut off for days or weeks at a time in temperate weather.
The crawl will take on the temp/humidity level of the living space above.