Should I insulate and/or heat my unfinished basement?
I have a 100 year old house in Georgia with an unfinished basement. We have no intentions of finishing the basement but we do want to use it for storage and a place for our dogs to sleep when we are at work. The floor is a concrete slab with a vapor barrier under the slab. The foundation walls are mostly above grade and are a mix of brick piers with brick and cmu infill. There are currently no water issues. The walls have several foundation vents that we close in winter. There is old deteriorated batt insulation between the floor joists and at the rim joist (that we are going to remove soon). In the basement, there is a 12 year old atmospheric gas-fired furnace with ducts and an electric water heater. Overall, the entire house is leaky and uncomfortable (mostly in winter) and we want to improve the comfort level. We are installing storm windows to our old double hung windows and adding insulation in the attic. My questions are about the basement.
Should I spray closed cell foam on the basement walls (and cover/remove the vents)?
If yes, should I replace my atmospheric furnace?
Should I spray closed cell foam at the rim joist? Or cut and seal pieces of foam board?
Should I heat the basement? If yes, I am assuming that I should not replace my batt insulation at the floor joists? If no, should I replace my batt insulation at the floor joists?
I am wanting to heat the basement (maybe just temper it) for the dogs and we have a super leaky8 original pocket door that intersects the basement door (a complicated detail that is impossible to get airtight).
Budget is a concern but not the top priority. We plan to live in this house for a long time and we are more concerned with comfort. Also a concern is the evironemental impact of the plan.
Thanks!
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Replies
J. Greene,
The answer to your questions depends on your goals and your budget.
If you want to improve your home's comfort, and you can afford improvements, the first step is to perform air sealing work. This article explains what you need to do: Air-Sealing a Basement.
The next step, if you can afford it, is to insulate your basement walls and rim joists. (In a new house, basement insulation is required by building codes.) For more information on this work, see these two articles:
How to Insulate a Basement Wall
Insulating rim joists
There should be no need to provide heat to your unfinished basement. Performing air sealing work and insulation improvements will make the space more comfortable.
Rigid EPS or polyiso are greener and cheaper than closed cell polyurethane if the brick & CMU wall is flat enough. Closed cell foam is usually the best choice for the band joist and foundation sill, but that too can be done with foam board, despite the labor-intensity. HFC blown closed cell foam runs about a buck a square foot per inch of thickness (called "board-foot"), but is very environmentally unfriendly. HFO blown closed cell foam is higher R/inch, and a LOT friendlier, but usually north of $1.25 per board-foot in my area. Either does a pretty good job of air-sealing, without the need for a lot of detailing the way you would with rigid foam, but would also require a thermal barrier against ignition. Most foam-board would need it too, but there are fire-rated polyisocyanurate board that doesn't (eg Dow Thermax), and could be the cheapest way to go. Foil facers are easy to air-seal with aluminum tape.
The basement vents should be closed up permanently, whether or not you insulate the foundation. Vented crawlspaces and basements in a GA climate is just a big air leak bringing more humidity into the house in summer than it ever purges. Unless it's a very tight house it's probably not necessary for combustion air on an atmospheric drafted furnace too. On the other hand, it's likely that the gas-burner is ridiculously oversized for the load, and a right-sized heat pump might be both cheaper to operate and more comfortable. To get a handle on your heat load and oversize factor on the furnace, run a fuel use heat load calculation (wintertime only fuel use please):
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
With the foundation seals and the ducts in the basement, the basement is going to run a LOT warmer in winter than it has been, and may not need any sort of tempering to keep it above 65F in winter. My basement in MA is reasonably sealed and insulated to code min, and stays around 65F in winter just from heat radiating the ~ 68F first floor above, and some of the distribution losses off the hydronic heating zones & water heater, etc. It's coolest down there in mid-spring, when the heating system isn't running very often, dropping slowly to the low 60s before summertime weather starts to warm it up a bit. You may still want to do something about the leaky pocket door though. Air leaks into the basement are more important than leaks of similar size at mid-level, since it's the bottom of the "stack effect" stack, and is a major 24/7 infiltration point in winter.
Where in GA? Georgia spans US climate zone 2A in the south, to climate zone 4A in the north, with a big stripe of 3A in the middle. In zone 3A a continuous R5 ( inch of foil faced polyiso or 2lb spray foam, or 1.5" of EPS) would meet current IRC code min, but in zone 4A it would take R10 ( 2" of polyiso or 2lb foam, or 2.5" of EPS.).
Since you're adding storm windows, it's well worth the upcharge to order low-E storms. Despite the higher upfront cost of the hard-coat low-E glazing it pays back quicker on energy savings than clear-glass storms. The payback of a tight low-E storm window on comfort is immediate, winter or summer. The low-E Larson storm windows sold through box stores are pretty good, but are tighter & have better hardware if you bump up a grade above their lowest cost line.
Thanks for the info Martin and Dana.
The house is in Atlanta so it's zone 3A, I think.
The foundation walls are very, very irregular so I was thinking spray foam would be the way to go but I am concerned about the environmental impact and the permanence from a historic preservation point of view. The rim joist is somewhat irregular as well but I think foam board could be done (I was just thinking if we spray foam the walls, I would spray foam the rim joist at the same time). I will look into the HFO option.
We can't airseal the pocket door since the recess for the pocket door is in the unconditioned basement stairwell. We've tried a few different gaskets/weatherstripping details but the pocket door has panels and has been impossible for us to seal. Cold air rushes past the door.. I could install a weather stripped door at the base of the basement stairs and heat and cool the stairwell. It would be relatively easy to add a supply duct to this area from my existing furnace.
If I could get the basement to 65 in the winter, it would be great for storage and the dogs, etc. I would love to not heat it (in terms of the expense). I was thinking I would install a dehumidifier (there is a floor drain).
So, if money is not a deal breaker and comfort is the goal, I should foam the walls and rim joist and figure out a way to either seal the pocket door or get the recess in the conditioned space? And since the house generally leaks like a sieve, I can keep my atmoshperic furnace until it dies (and at that point, do a recalc and maybe swap it for a heat pump)?
Thank you!
Some sort of secondary door/air lock/mudroom approach may work for creating a more continuous air barrier and keeping copious air leakage from occurring.
Atlanta is the cool edge of zone 3, so a continuous shot of R7-R10 closed cell foam from the slab up & over the top of the foundation & sill all the way up to the subfloor wouldn't be insane.
A coworker of mine recently insulated his basement this way with HFO blown foam. For a thermal barrier he did a 1.5" deep steel-stud studwall in front of the foundation, setting up the studs the prescribed distance from the foundation to use as a depth gauge. It's hard (impossible?) to do 1" spray foam accurately, but as long as you have at LEAST and inch it'll do quite well. With steel furring/studs the channel plates at the top & bottom give you a couple inches of slop to work with, which means the studs can all be pre-cut in batches to work over a wide variation in studwall height. The labor savings made that approach cheaper than a 2x4 studwall, and it doesn't take up as much interior floor area (if that matters to you.)
As you tighten up and insulate the house the heat load will drop, (and from the description it'll drop by quite a bit if you get serious about air sealing). While fixing up the house to lower the load is the biggest part of gaining comfort, it's worth doing pre & post- improvements fuel-use load calculations to track the progress. At some point when the furnace is 5x+ oversized for the actual load the low duty cycle even during cold weather becomes a comfort issue, since it'll be cycling between the scorched air hot blast followed by the long slow chill, often cooling off in some rooms much sooner than others. A right sized furnace or heat pump would run almost continuously during cold weather, which is a lot more comfortable- especially if the house is still leaky/drafty.
But for now finding and fixing as much of the air leakage as you can is job-1, starting with the basement walls & attic floor. Any plumbing or flue chases that run from the basement to the attic are major infiltration drivers dragging humidity into the basement in summer, and cold air in the winter, but any air that gets through the attic floor/upper floor ceiling are large stack-effect infiltration drivers too. Fixing all the big leaks comes first (like permanently closing the basement vents). At some point after you've fixed all of the obvious stuff it may be worth a round of blower door & infra-red imaging directed air sealing on the rest.
Thanks Dana.
If I don't heat the basement (just seal it and insulate the rim/walls) should I install new batt insulation between the floor joists? Should I install a dehumidifier?
And what do you think about running the furnace fan all the time in the winter (instead of auto) to stop the cycling? Anything to minimize the long slow chill would be good (until the furnace is replaced).
J.,
Q. "If I don't heat the basement (just seal it and insulate the rim/walls) should I install new batt insulation between the floor joists?"
A. No. You want to insulate the basement walls, not the basement ceiling.
Q. "Should I install a dehumidifier?"
A. Perhaps. Is your basement damp? You might want to read this article: Fixing a Wet Basement.
Q. "What do you think about running the furnace fan all the time in the winter (instead of auto) to stop the cycling? Anything to minimize the long slow chill would be good (until the furnace is replaced)."
A. Running the furnace fan all the time is simply a waste of energy. (Most furnace fans are energy hogs.) If your basement is chilly, follow the advice we've given. Focus on air sealing and insulation.
What Martin said!
Regarding the furnace fan: In addition to being a waste of energy, running the air handler without the burners going delivers a wind-chill draft, which is the opposite of comfort!
Thanks Dana and Martin. This is super helpful.