Adding insulation to floor vs adding insulation to walls
Hi, we have a basement that is partially below grade on two sides and on grade for the rest. The walls are cinder block, and the foundation is a concrete slab with a layer of gravel under it. There are 5 windows that are 35×37, double-pane, with air leaks around them. There is a propane furnace that ducts heated air to ceiling vents in basement rooms and floor registers on the main floor. Three of the rooms in the basement have electric baseboard heaters. The basement and main floor are both 1500 square feet. The only floor covering in the basement are then stick-on vinyl squares. Two rooms have bare floors and nothing for the walls other than cinder block. The rest of the rooms have battens with very thin fiberglass between them and either sheet rock or paneling. The house is at the foot of the Blue Ridge mountains in Virginia.
You probably won’t be surprised that the entire basement is uncomfortably cool all winter despite us having a huge propane bill. We are planning to spend some major bucks to try to make the basement comfortable and lower our propane bill. Our strategy is to 1. Replace the basement windows with the lowest U-factor windows we can find and seal properly around the openings, 2. Add hydronic radiant heat flooring to the basement, and 3. Add 2″ foam board and sheet rock to all exterior walls, over whatever is there now.
Here are my questions:
1. Does this strategy seem appropriate? Any suggestions to improve it?
2. If we add to the walls and add to the floor, which should be done first? In other words, should we build up the floor with the radiant heating system and then add to the walls, so the wall additions are on top of the new floor, or should we add to the walls first and run the new floor up to the new walls?
Thanks for any advice!
John
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Replies
John,
You didn't mention air sealing work. Air sealing should always be Job #1. For more information on this issue, see Air-Sealing a Basement.
My second piece of advice: It's more important to insulate the basement floor than it is to add radiant (in-floor hydronic) heat to the floor. If you have worries about your ceiling height, it's better to add an extra inch of rigid foam to your floor than it is to add room for PEX tubing. (You can always heat your basement some other way.)
For more information on insulating a basement slab, click on this link -- Fixing a Wet Basement -- and scroll down to the section that begins "Insulating an existing basement slab." The section comes at the very end of the article.
John,
The title of your post asks a different question than the actual post. Concerning the implied question in your title -- "What's more important, adding insulation to the floor or adding insulation to the walls?" - the answer is "Adding insulation to the walls" (because there is significantly more heat loss through the walls than through the floor).
For more information on this topic, see How to Insulate a Basement Wall.
Firse, run a room by room heat load calculation on the existing construction, and after insulating the basement walls.
In 99 cases out of 100 replacing U0.50 -U-0.35 clear glass double-panes with U-0.20 will be uneconomic on a fuel savings basis, and in your area it's not cold enough to rationalize U-0.20 windows on a comfort or window condensation basis. But fixing the weatherstripping and properly air sealing & insulating around the window & rough framing with low expansion foam (as it should have been done originally) is almost always cost effective.
Insulating the foundation walls to at least IRC 2015 code min for your climate zone ( 4A ) is cost effective on a lifecycle basis and has a dramatic up tick in mid-winter comfort, raising the interior facing side of the above grade portion of wall more than 5F at the winter's lowest temperatures. For zone 4A you can get t here with would R10 continuous insulation. One possible soloution would be 2" of foil faced polyiso (typically labeled R9- R9.5) with seams sealed with foil tape, held to the wall with 1x4 furring through-screwed to the foundation, mounting drywall on the furring. The thermal benefit of the foil facer + air gap established by the .provides the additional R1 to bring it up to code min. If you're building an insulted studwall, put at least 1/2" of foam between the studwall & foundation, and between the bottom plate of the studs & slab, and use unfaced fiberglass or rock wool batts, with NO interior side vapor barriers.
What is your electricity cost, delivered? (Take the whole bill, divide the $ by the kwh.)
What is your current price of propane per gallon?
What was the average cost of propane over the past 5 years?
In your location/climate, appropriately sized ductless heat pumps for will have an operating cost less than1/3 that of your electric baseboards (more than 2/3 cheaper to operate) and in most electricity & propane markets is substantially cheaper than heating with a condensing propane furnace (in my neighborhood it's less than half.) That would likely be cheaper to install than outright replacing the windows & adding an insulated radiant floor, and cheaper to operate as well. And, it would also provide high efficiency air conditioning.
John,
Dana's right about the windows. For more information on this issue, see What Should I Do With My Old Windows?
I apologize for a misleading title. I wrote the title first and then worked on the situation and questions and did not realize how much I changed it, and I didn't realize the title needed changing. I don't see a way to edit it.
Thank you very much for the answers and the article links. You've given me a lot of good information and a lot of reading to do.
By the way, we have 8' ceilings in the basement and don't mind losing a few inches, but if I understand your advise (without having read all the articles yet), I should 1. add adequate insulation to all exterior walls first, 2. get someone to do heat loss calculations (is there a GBA article on how I can do that myself?) to decide if replacing windows would be worthwhile, and 3. then determine what and how much to add to the floor.
John,
You might want to read the 3-part GBA series on heat loss calculations and cooling load calculations. Here are the links:
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
The losses to the slab are to dirt that is well above your wintertime outdoor temperature averages. The wall losses above grade are much higher due to the bigger temperature difference, and over the winter the losses to below grade walls creep up as the thermal mass of the near-grade soil cools off. That make the return on investment of slab insulation much lower than walls.
The ranked priority in nearly all cases is:
1> air sealing
2>insulating the walls
3>upgrading windows
4> insulating the slab
With any heating system changes a heat load calculation (for the "after" picture, if building upgrades are planned) is an important first step.
Doing a preliminary load and fuel use evaluation to see if/when windows upgrades or retrofitting the slab make financial sense on fuel use basis makes sense, if reducing the fuel bills is a driver. There are other reasons for insulating a slab or upgrading windows can make sense, eg:
Slab insulation, even when not economic: The deep subsoil temperatures in your area are in the 55-57F range, (see: http://www.earthrivergeo.com/img/geothermal-article/geothermal-subterrainean-temperature-contour-map.jpg ), whereas your mid summer outdoor dew points are in the mid-60sF. During the summer the top of the slab is going to near or below the dew point of your ventilation air, which means that rugs or cardboard boxes resting on that slab will be prone to mold growth. An inch of EPS over the top of the floor with a standard subfloor above it would have surface temps above the dew point of the ventilation air, and low risk. For the slab insulation use EPS, not polyiso, since in the event of a damp slab or minor flooding polyiso can take on moisture and take forever to dry through aluminum facers & subfloor, whereas EPS will not take on enough to matter
Windows, when not economic: In colder climates than yours lower-performance windows can have constant &/or excessive window condensation, obscuring the view and puddling on the window sills. That is extremely unlikely in your location, even with the lowest performance double-panes. The other issue is wintertime comfort when standing next to the window, which is still something to consider if you're actually using it as living space and will be sanding next to the windows when it's below 25F outside. An upgrade that is far cheaper than replacing the windows would be to add low-E storm windows on the exterior, but if those windows receive a lot of direct summer sun there is a risk of overheating and ruining the seal on the double-pane glazing- it isn't a generic "works in all situations" type upgrade. A low-E storm window on a clear glass double pane will usually bring the net performance down to the U0.30-0.32 range, and over a U0.35-ish window it'll be under U0.30- call it U0.28 if analyzing it for fuel-use reasons.
If you're planning to heat with heat pumps you'll need to run the energy use/cost numbers for both propane & heat pump against the building upgrades too.
Using October 2016 statewide average pricing for propane, it looks like you'd be paying about $2.75/gallon this week (which is below the recent 5 years average)
https://www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=W_EPLLPA_PRS_SVA_DPG&f=W
At 91600 BTU/gallon, burned in a 95% efficiency gas furnace that delivers about 87,000 BTU/gallon into the house, so per million BTUs (MMBTU) of heating that's about 11.5 gallons, which at $2.75/ gallon is $31.62 / MMBTU . On top of that will be the power for running the air handler, and a slightly elevated load due to air-handler driven outdoor air infilatration (which varies, but is never zero.)
Electricity in VA is averaging 11.71 cents/kwh. Used in an electric baseboard each kwh delivers 3412 BTU/kwh into the house (the simple metric to British units conversion, assuming 100% efficiency, which it is.) So per MMBTU that's 293 kwh/MMBTU, which at 11.71 cents/kwh costs $34.31/MMBTU. That's slightly higher than propane until you factor in the power use and increased heat loss, and in fact it's pretty comparable.
If you right-size a ductless heat pump and get a seasonal coefficient of performance of 3.0 (any heat pump with a tested HSPF of 10.0-10.3 could hit or beat that.), the cost would be about $34.31/3.0= $11.43 /MMBTU, which is a heluva lot cheaper than propane!
You may want to consider replacing the propane burner with an old school ducted heat pump. Most will not quite hit the same efficiency a ductless solution, but it'll be cheaper installed price, and you'll get at least an average COP of 2 in your area. If going that route you could calculate the size needed based on the fuel use of the propane system rather than a room-by-room load calculation, using these methods:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
Wow, you guys are great! I can't thank you enough!!