Mechanical Systems in Cold Climate
Hi all,
My question is primarily in regards to heating our future house. We are finalizing the design of our roughly 1,400 sq foot home (+ an unfinished basement) and plan to begin building this fall. The house is a long skinny rectangle with south facing windows and a roof overhang that will block the summer sun and allow the winter sun. Hoping for lots of sunny winter days! We live in Iowa (climate zone 5a) and that’s not always the case :). The windows will likely be vinyl and we haven’t decided about double or triple pane. The current plan is to have 2×6 walls with rigid spray foam insulation.
I have read through a handful of these Q and A posts and feel I should preface this post with the disclaimer that I am not as knowledgeable on building lingo and technology as most seem to be, but hopefully I can provide adequate information in my post!
Our general hope for our build to balance – simplicity, cost effectiveness, comfort, quality materials and being thoughtful and conscientious with our decisions. We’d like to have an all electric home and would also like to install PV panels.
I’m rather stumped on what to do in regards to heating our home. While I’d love to go without central heating and AC, it seems having central AC is wise if for no other reason than to control humidity. Plus, it does get rather hot in the summer and I don’t want to get extra hot because I’m mad that I didn’t install any AC 🙂 Since we will have AC, it seems like we will have central heating as well (our builder is under the impression that you can’t really find an AC only system here in Iowa but he was going to check on that).
However, I don’t plan to use the forced air heat. I have never liked forced air heat and my husband and I find that we are really only comfortable in cold weather with radiant heat…and I want to be comfortable. In floor radiant seems expensive — especially the more efficient hydronic systems. I’m also concerned with the challenges in fixing the system if it ever did break. Plus, we’ve been told that an electric boiler to heat the fluid is not very efficient, so we’d need to do propane and I’d like to avoid that. I’ve read we could also do geothermal to heat the fluid, but that seems costly (though I haven’t checked).
We would also like a wood burning stove. I know they are not the best for indoor air quality and also not the best for keeping the house super insulated, but regardless of all of that, I have always loved the idea of having the stove going on a cold snowy winter day.
So, that leaves us with a forced air system, a wood burning stove and some other radiant heat system (I don’t want to use the stove as the primary heat source). That all sounds expensive and complicated. Plus our house should be fairly well insulated. I am thinking of going with a few simple baseboard heaters but still annoyed about having a forced air heating system that we hope to not have to use.
Any thoughts or ideas are appreciated! And please correct me if any of my assumptions seems incorrect.
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Replies
Hi Claire!
A lot to balance, but an exciting process. Some thoughts:
1. This is not a large home and it seems like it'll be well built with a low heat loss. These attributes really make hydronic systems difficult to justify, especially if you want it to be all-electric. It also makes geothermal difficult to justify too.
2. In-floor heating also struggles with small houses and low heat losses. The equipment/labor costs are high and the floor temperature would be barely above room temperature.
3. These attributes also make a wood burning stove difficult to justify too, as it's difficult to prevent overheating in a space with a low heat loss. Plus you add in the indoor air issues, etc. That said, it adds ambiance and is a reliable backup heat source. If you install one, make sure it has an reasonably small output.
4. Central AC is really the best way to provide cooling. Establishing this, you'll be installing ductwork throughout the house. All other methods of heat distribution will be redundant, which can be both good and bad. If you wanted radiant heating, an air-to-water heat pump can provide that and cooling through the ductwork. It would not be cheap or simple as these are extremely uncommon in the US.
5. I'd challenge the assumption that a well installed forced air heating is uncomfortable - a central ducted, inverter driven heat pump can be extremely comfortable. I have the ducted Mitsubishi hyper heat and it's great - it's silent and can adjust the heat output so temperatures are extremely consistent, but there are plenty of quality brands. Since you'll be installing ductwork, this is the cost-effective, electric, and comfortable option in my opinion. It'll also cost about 1/3rd as much to use compared to electric baseboards. However, installing electric baseboards as a supplemental heat source is a fine idea too - they're easy to zone, redundant, and cheap. If the heat pump turns out to be not for you, they can handle the heating. The cost difference is so small between an AC unit and a heat pump of equivalent quality that it's worth trying - if you like it, it'll probably pay for itself vs. the electric baseboard within a few winter months.
Paul,
Thanks so much for the succinct and clear information. It's very helpful to hear your opinion all the heating options I mentioned and also very helpful to have a solid recommendation.
I appreciate your time in responding and for your ideas.
Claire
Not an answer to your question, but if you want to be conscientious you'll ditch the spray foam insulation. You could not choose a less green insulation.
Hi Trevor,
Do you have any recommendations for alternatives? Robert has great information in his reply but curious if you have anything to add. We likely won't do double wall construction unless it turns out to really be the best option.
Claire
Agree with Trevor and Paul.
Spray foam is about the highest insulation R-value per inch, and can be used to air-seal stud cavities, but otherwise it comes with a host of serious negatives. High cost and high risk. Trevor noted the global warming/sustainability problem with site applied closed cell foam. Installers wear protective gear with an air supply because spray foam emits poisonous gas when installed. Hopefully the off gassing stops being a serious problem within days. But site applied spray foam isn't foolproof. There are many instances documented on this site that off gassing and undesirable odors persist long after the foam should have fully cured. And removing the foam is almost impossible without removing much of the surrounding building materials. Personally I feel sorry for those who experience this disastrous outcome, and believe its not worth taking the risk. GBA should warn about this problem more often, IMHO. In addition, it makes little sense to have such an expensive, high R-value material in stud bays, when the studs, wall bottom plate, top plates, and window and door framing allow far more heat to pass through them than whatever insulation you install between studs. (Wood framing is R=1.25 per inch, or R-6.75 total for 2x6 studs and plates in walls. Wood framing "factor" is about 25% of the typical platform framed wall and floor system that faces the cold exterior). So the real insulation value of the wall is much lower than the R-30 or whatever the spray foam contractor quotes. As an alternative, "Continuous insulation" can be provided by exterior insulation foam board, polyiso, or mineral wool board; or by double wall construction with continuous insulation between the two stud walls. These eliminate most of the thermal bridging that occurs in simple wood stud walls.
For new construction, there are many more affordable, safer and more sustainable insulation choices than site applied closed cell spray foam. Roxul ComfortBatt mineral wool insulation is a preferred choice of many high performance home designers and builders. Fireproof, allows water vapor to pass, easy to install, R-23 for 2x6 construction. Most high performance homebuilders like to add either exterior insulation (polyiso R-6/inch same as closed cell spray foam, or EPS or GPS foam boards, all of which are factory produced to avoid site-applied curing risks). Other builders and designers prefer double-wall construction (two parallel 2x walls, with a space between the parallel walls for continuous insulation without any thermal bridging of wood studs). Both of these are cost-effective choices.
They also include a rain screen on the exterior, to drain any water that gets past the cladding, for both types of walls, or for plain stud walls without continuous insulation anywhere.
Most professionals on this site would suggest you use mini split heat pumps, as Paul suggested. Typically ducted for bedrooms and other small spaces. You might post a separate question, showing your floor plans, to get more good (and free!) detailed advice on this one topic.
Everyone has their own preferences for something special for their home, like your desire for a wood stove. Good for backup heat and cooking if power goes out. If you have adequate ventilation in your home provided by an Energy Recovery Ventilator (ERV), and are using the wood stove only occasionally, indoor air quality is less affected. Again you might post a separate question to get more ideas for smaller, efficient wood stoves if you don't already have a good source of information for making your choice.
Finally, if you plan on building a home primarily heated by passive solar heat gain from south-facing windows; or a solar tempered house often heated during daytime by solar heat gain, but using another heating system for overnight and overcast periods, see this article for detailed information:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
Or this article on a classic passive solar home that was well engineered (lots of calculations of solar heat gains vs. winter heat losses for various conditions):
https://www.greenbuildingadvisor.com/green-homes/a-passive-solar-home-from-the-1980s
Short summary: You need to have an airtight and highly insulated building, before the solar gains will equal the heat losses mid-winter. You have to look at your climate and compute gains and losses for sunny coldest days, and overcast winter days (which won't be the coldest). Nowadays it makes more sense to use less south-facing glazing and more insulation, and paying careful attention to creating one or more detailed air barriers in your walls and roof/ceiling. For south-facing windows, you might select a double-pane window with solar heat gain coefficient of 0.5 to 0.7. For other directions, a more highly insulated glazing or triple glazing. You don't want much solar gains on the west side during summer, and not much on the east either. North won't get direct solar gains all times of the year, so maybe smaller windows on that side. Depends upon your room layout and views.
See weatherspark.com for great charts for climate data. Attached is Iowa, but if you put in your city, you will get nicer examples. Other references are in the article cited above.
Regardless of which of all these choices you make, the feedback you will get here on GBA is that airtightness is VERY important. You need to have an air barrier (or two or three) imbedded somewhere in your walls (and roof/ceiling). That might be taped sheathing, airtight drywall, or other approaches. At least one continuous air barrier, and two or more wouldn't hurt. Easy and relatively inexpensive to do during new construction. Can test and improve by doing a blower door test at least once during construction, typically before drywall is installed, so you can fix leaks you find. Leaky buildings defeat insulation by cold outdoor air bypassing your insulation, and conditioned indoor air escaping out of your building.
Lots of useful information on this site on air sealing, blower door tests, and mini splits.
Best of luck with your home plans and construction!
Trying not to get to into the weeds here, but while I have a ducted minisplit, that’s less important than getting a heat pump (“minisplit” or otherwise). Minisplit is tricky terminology because it was somewhat specific before but now it isn’t.
Robert,
Thanks for all this specific information! Lots of good things to consider. We will likely alter our insulation plan. I'm happy to have this info on insulation...we weren't too keen on the spray foam but hadn't been informed of any alternatives that we were sold on.
Also happy to have that info on double glazing on the south windows with possibly triple on east and west windows and smaller windows on the north walls.
All in all, these replies have been very helpful! I will consider another post with our design for more specific questions.
Enjoyed reading the two articles you sent! Lots of info for me to digest. Thanks for sending (and writing them!).
Another vote against spray foam here. I would use mineral wool in the walls, put the savings (from not using spray foam) into exterior continuous insulation like polyiso. Seal the polyiso to the frame and that will give you a good air barrier. I'd then detail the interior side drywall airtight as a secondary air barrier.
I would use blown cellulose in the attic, assuming you can use a vented attic. It's hard to beat cellulose for both low installation cost and high performance. Air seal the attic floor the old fashioned way, with careful details and some caulk and canned foam. You don't need spray foam here either.
Be sure to insulate your basement walls. There are some advantages to insulating them on the exterior. I would use XPS here, since I prefer XPS for underground use over EPS due to lower water absorption, but either can work. Under slab insulation is also a good idea.
Good insulation will help to avoid any cold spots in your home.
Bill
Thanks for all the specifics! Going to need to sift through all these great ideas and decide what is best for our build given our goals.
Have a great weekend!
Lots of good comments here, but I want to endorse Bill's comment above. He said basically everything I wanted to say about insulation, except one detail: If you go with XPS below grade, and you want to be green, be sure to specify a low-global-warming potential version of XPS. Owens Corning's "NGX" is one example--it's probably a special-order item but should be available from Home Depot, for example, if you order from the pro desk, even if it's not otherwise shown as available at your local store. Regular XPS has off-the-chart high global warming impact because of the gas they use to blow the foam.
Wonderful, thank you for that specific info
Hi neighbor, I'm west of Iowa City.. Why long and skinny, as that will require more exterior wall compared to interior space? "Golden Ratio.. "
I plan on a hybrid heat-pump system that uses propane as secondary heat. And if I were building a home on a basement that was going to be used for storage and mechanical's, I wouldn't insulate the concrete.. I know some wont agree with that, but I'm sitting in my basement now and its 71 degrees down here when its over 90 outside.. In the winter, I'll move back upstairs. And you could always insulate the 1st floor joist space if you were concerned with heat lose.. I'd use dense packed cellulose everywhere and get energy heel trusses.. The main take away I have from this site is the importance of air-sealing and a blower door test to verify and find leaks.. Oh, and there's a outfit in Iowa called trenchlesssolutionsiowa.com who can install geothermal wells for about 2K per ton if your interested in that.
Well hi Iowa neighbor! Yes, long and skinny isn't ideal in a lot of ways but I think it'll be best for us...I like the skinny aspect (I think the idea came from the book A Pattern Language) and my husband and I didn't want to do a second story ... so long and skinny it is!
Anyway, good thought on the basement. I do bot think we will insulate the basement floor.
I understand you are invested in this plan but remember that it is just a pile of papers at this point. I hate to say it but you may want to start over with a blank sheet of paper if you chose to make the house long and skinny in an effort to gain south facing glass thinking it would help heat your house.
The way I see it passive solar is a quant idea that that has proven to be a failure in the real world. You get too much heat when you don’t want heat and not enough when you do need heat.
I built a long skinny house because I have a great view and I wanted most of the rooms to have the view.
FYI my 4 foot overhangs do very little to block the summer sun.
As for heating I think you need to reconsider forced air. Yes, poorly designed and installed systems will be uncomfortable but don’t reject the idea of forced air out of hand.
Hydronic system are budget breakers.
The price volatility of propane kept it out of consideration for me.
I could not make the pay back work for a ground source system.
In the end only a air source heat pump made sense for me.
A wood stove is a poor fit in a high performance home in that it is very air tight and well insulated. For the wood stove to work large amounts of air must flow up and out the flu pipe where will this air come from in a tight house? Wood stove often produce well over a hundred thousand BTUs in a house that only needs 20-30% of that on the coldest day of the year. You end up with an open window so you house doesn’t fill with smoke and overheat.
Walta
I too have had to come to terms with specifying a new HVAC heat pump system instead of radiant floors, radiators and a wood stove for our major renovation. I have heated my other home for over 40 years with a wood stove with an oil fired radiator back up system and we are all quite comfortable with that concept. But it's old, old school.
As hard as I try, I'm still in an old home doing a major renovation, so the best I can hope for is a "pretty good home". It's much easier and cheaper to go old school renovation, but I'm insisting we get the tightest envelope possible. Once spec'd and actually accomplished, the HVAC needs go down. Once the drafts go away and the insulation is in place, heat pumps make sense. Fireplaces and wood stoves don't. Romance shmomance.
Still, I have a fireplace that is the centerpiece of our existing LR. What am I going to do? One of two things: either seal it up and go propane fireplace with outside air supply, or have an operable chimney cap and tight FP doors for that once a year XMAS fire. I'll open a window.
You can't have your cake and eat it too. If we were building new, there would be zero ignition in our new home.
Walta,
The long skinny idea is partially for sun and partially for views (south/southeast are the good views on our property as well...and also facing away from a noisy highway). I don't think we will get a ton out of the passive solar aspect of the design but probably some.
After reading through these posts, I am definitely starting to reconsider a wood stove and have nixed the idea of hydronic in floor heating. Maybe we'll opt for a nice fire pit outside instead :)
Happy to have guidance from everyone in all of these matters. Air source heat pumps seem like the way to go.
"Long and skinny" does lead to a larger building envelope surface area for a given floor square footage (or lower total floor area for a given perimeter). But up to a ratio of 2:1 long vs. short sides, it makes little difference. Do the math. Unless you are trying to meet Passivhaus/PHIUS certification, its not a deal breaker. To compensate in more extreme cases, you need more $ for insulation.
Successful passive solar or solar tempered buildings require:
- careful engineering of solar heat gains and the home building envelope losses, including an airtight, super insulated building for a low heating load.
- an adequate amount of solar thermal mass, such as an insulated concrete slab floor, insulated interior brick or masonry or water, to absorb excess heat and radiate that heat back during overnight cooler interior temps
- a cold, mostly sunny winter climate
- careful placement of south-facing windows to capture winter solar heat gain, and minimal or shaded west and east-facing windows to avoid summertime overheating
- south-facing solar access (building not shaded much 9AM to 3PM during mid-winter)
Poorly designed or poorly located passive solar buildings are the ones that don't work well. Poorly designed buildings don't demonstrate that well designed buildings can't be built.
Passive solar is hard to get right. I have a bit over 10% south facing windows and can easily hit 80F on a sunny winter day with no heat running at all.
According to the passive solar folks, the magic bullet should be heat capacity with a something like concrete slab which sounds good but doesn't work in reality. To put heat into the slab the temperature must go up, for any reasonable amount of concrete you are looking at close to 10F, this means the house will still overheat during the day. The slab buys does buy you some night time heat but I'm not sure if it is worth the cost and environmental footprint.
I would pick reasonably sized south facing windows, properly shaded they do help reduce your yearly heating bill and it is great to get extra toasty room in the middle of winter. Make sure to model where the light comes in and furniture placement as you can get a lot glare.
Wood heat in a low load house can be made to work but does require some careful design and tweaking. Installing a standard wood stove designed for a house with 50000BTU heat loss is asking for trouble. I would read through Scott's blog:
https://www.greenbuildingadvisor.com/article/flatrock-passive-firing-up-the-heating-system
The only time hydronic ever makes any financial sense is if you don't need cooling. As soon as you need cooling, as others have said, plan for ducts. If you have ducts, might as well use that for heat and cooling. A well designed force air system in a low load home is surprisingly comfortable.
Thanks for your reply!