On the eve of 2018, what are the views of mixing a ‘pretty good house’ with a wood stove?
user-6969515
| Posted in Green Building Techniques on
Hi! Happy New Year. We will be building a new home in Maine in 2018 and are incorporating a high level of energy efficiency; but not bringing it to passive house level. Specifically, open cell foam insulation; triple pane windows; seal, seal; seal and high velocity heat pump; 2 by 6 walls.
We are considering either a wood stove or wood stove insert for the supplemental heat and enjoyment; however, not certain this is a good move; any thoughts or experiences would be greatly appreciated!
Thanks much in advance.
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Like any other heating system, take some care to size the wood stove's BTU ratings to the design heat load of the space or you'l end up turning the place into a sauna every time you have a fire. Oversizing by more than 2-3x the design heat load of house/space can lead to operation at a firing rate too low for it to meet it's EPA emissions ratings, taking a hit in combustion efficiency.
There are plenty of smaller cast-iron and soapstone stoves with max firing rates in the 25-35,000 BTU/hr range that can be reasonably fired at <15,000 BTU/hr. How you intend to use it also makes a difference. If it's occasional use to heat the place up quickly a cast iron version is just fine, but if it's to be used as a primary heating source a stove with some thermal mass to it (such as soapstone or ceramic) allows it to be fired intermittently at a high firing rate without huge temperature over/under shoots, but they take longer to warm up. At the extreme end of that would be high-mass masonry heaters such as Russian fireplaces, rocket stoves etc, where it's always fired a very high rate & high efficiency. These are often custom-built in place, though there are commercially available versions too (Tulikivi, et al.)
User-6969515,
First of all, can you tell us your name?
The most common challenge for using a wood stove in a tight home is providing adequate combustion air. For more on this issue, see All About Wood Stoves (especially the section that begins with the heading, "Can you put a wood stove in a Passivhaus?").
Also note that installing open cell foam against the roof sheathing is risky. See this article for more information: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
I agree with what the others have said, especially here in Maine where we have an abundance of firewood-quality hardwood. Most of the projects I design are Pretty Good to Passivhaus level, and often include a woodstove or fireplace insert for supplemental heat.
They can overheat the space easily, though, so I try to put them in a 3-season room that can be shut off from the rest of the house, though they usually end up being part of the fully conditioned space.
I recently saw a great tip from architect Steve Baczek--for the woodstove makeup air, he included a ball valve on the exterior, so it can be closed off for reduced air infiltration when the stove is not in use. Perhaps overkill for PGH homes, but perfect for Passivhaus.
We considered installing a wood stove in our pretty good house in Maine, but eventually decided we didn't need it. Putting a hole in the roof and a hole in the wall to bring in combustion air seemed inconsistent with the idea of a tight house.
We still don't "need" it. The minisplits keep the house as warm as we want. But during the last week of arctic weather, we've discussed how nice it can be to cozy up to a hot wood stove. If we had it to do again, we'd probably still opt out, but Mike's idea of installing one in a three season room has me thinking. Our South facing porch is insulated and has decent windows, but is unconditioned. A stove in there might be nice. Maybe next winter.
Have a couple of points I would like to make about stoves in near passive tight houses, but first, I am surprised that only one personso far has questioned your wall profile.
Based on several years of GBA reading and Joe Lstirburek articles I agree you are risking condensation issues. Especially in Maine as the summer season is generally humid enough to risk residual sheathing humidity levels to carry into the next cold cycle. As Steve Knapp notes, your wall is identical to the roof sheathing situation, except for orientation.
My own PGH is framed 2x6 with batts, sheathed in plywood and vapor wrapped with Henry BlueSkin, gapped exterior foam with stucco. I will skip all the drain plane details. The point to make is, you might be happier and have a more durable and higher R performing wall with the addition of appropriate outsulation instead of foam between 2x6s. The framing factor of 25% generally used for loss calculations proved low for my particular design. However, with the 6" of exterior foam my functional whole wall R is 30+ in windowless sections. The hole punched through are filled with Alpen windows which I love and are U-.15 or .19 if operable. Find Joe's book Builders Guide to Mixed Climates.
Back to stoves and my experiences so far. Practical experience with a soapstone wrapped wood stove has shown that the 55,000 BTU rating on ours seems a bit fanciful. We may be under charging the stove because we largely use it for "cozy" fires for my wife. It does take well over an hour before getting warm to touch. Locally I have heard stories about the big Tulikivi/Russian types of fireplaces being a nightmare unless they are run constantly (the way they were designed to be) as the chill to warm time can be several hours of constant feeding. Not the glass of wine in front of a cozy quick fire set up some expected. They also cost 10's of thousands of dollars and need their own foundation details just like old style chimneys.
Running our stove over Christmas for 8 hours on steady small logs did not make the room or house anywhere near overheated. The functional area it supports is about 1200 sf. The living room it is in got to 75 while the rest of the house stayed within 69-71. It does stay nice and cozy warm for 3-4 hours after we stop feeding it. Best thing is you can sit within 2-3 feet and not lose skin. Cats have to learn the hard way not to jump on the top. Small children you might reason with.
As for make up air, having an exterior feed is nice and having a valve of some sort would be even better. However, the options available for exterior air hook up varies a lot with stoves. I was not impressed with the brand we got on that front. It was expensive, ugly and awkward to hook up. National and local regulations should be looked at very closely to see if you can meet all the requirements and not have a really ugly install. The primary issue is getting through or around the fire rated surrounds gracefully. Secondarily, insect and rodent invasion protection on the outside.
Turning on the kitchen hood while trying to start a fire results in decisive back flow and smokey fun, but after a fire is established and the stove door is closed we don't have problems. I did deliberately size the hood for a max of 325 cfm which is rarely needed with our style of cuisine and induction cooking. An equal drain on house air is the electric dryer which we vent. The condensing types were new and very expensive when we were making decisions. Current reporting about them makes me glad we avoided them. Haven't done laundry at the same time as build a fire yet, but I suspect it would be like the range hood as dryers come in at 150-225 cfm according to postings here on GBA.
That said, I have not noted much commentary about the constant drain on the house envelope from the 6-8" hole one creates for the stove pipe. After putting the fire "to bed" the stove pipe is still sucking air out of the room just from rising warmth in the pipe and stove box. Soapstone may actually extend this effect longer than cast iron, but I have no way to check. Putting the damper on minimum slows it down, though the high winds we get here will still suck house air via the chimney pipe, wind cap or not.
I believe I have seen figures of 10cfm or less as the draw of most sealed door stoves, so I think it is just fine to have and enjoy the stove you like based on finish, style or cost. Detailing the pipe transit through the envelope and dealing with the fire rated surfaces required will require more of your attention.
In the time it took to write this two others have offered quicker advice that I agree with. Two cats, a dog, a wood stove are all a pain in the ...but we like them all. Worry about the stove pipe pass through details and where you put the barcalounger.
This discussion, and many like it, are why I return so often to GBA. Happy New Year!
Having a wood burning appliance in an efficient home is a lot like having a dog or cat. Time consuming, dirty, and not economically justifiable, but darn nice company.
Says the man who fires his masonry heater twice each day and has two cats. (Pix from early Christmas morning.)
PS The chimney penetration was one of the most complicated aspects of our installation. (Our home last measured at .9 ACH50)
Thank you! This is User 6969515, alias Catherine Trott
I am overwhelmed and appreciative of all the thoughtful responses. We are dog people, and have massively modified out home furnishings to accommodate our four legged children. So we are continuing to consider the stove and will incorporate the many insights provided on the installation.
I do also need to clarify the insulation; originally we were going to use the open cell foam; but we are also having the cellulose priced. In both cases we would be using the thickest zip wall system, which I believe is over 3 inches.
Thank you all!
A concern in three words: Indoor Air Quality.
Now that's true even if you bring separate outside combustion air in, which you may have to given the tight house plans.
For the ambiance of a wood stove, consider a propane or gas stove. The fake flames are getting pretty good.
If you're in an urban area, then consider: Outdoor Air Quality. Where I am winter night walks are tainted by air that feels unhealthy, due to all the fires going.
Bryce,
I completely agree with your concern over the effect of wood burning appliances on neighbours and outdoor air-quality. Thats why more and more municipalities are putting restrictions on them. But no current approved air-tight wood burning appliances should make any difference to the indoor air quality. If you can smell or measure that there is a wood stove in a house, then something has gone wrong.
Open cell foam wall insulation in Maine is just fine when combined with "vapor barrier latex" primer on the finish walls. A lot of builders would still install a polyethylene vapor barrier on a 2x6 wall. Vapor barrier latex runs ~0.5 perms making it a Class-II vapor retarder with some drying capacity toward the interior, whereas 6 mil polyethylene runs about 0.05 perms, a true vapor barrier.
A 2x6 wall with open cell foam cavity fill and no exterior insulating sheathing would not meet IRC 2015 code minimum performance for zone 6 (most of Maine), and zone 7 (northern Maine.) Code-min for both zones is R20 cavity fill + R5 continuous insulation, alternatively R13 cavity fill + R10 c.i., or U0.045 maximum, which is R22.2 "whole wall R" with all of the thermal bridging of the framing and the R-value of the wallboard, structural sheathing, interior & exterior air films etc calculated-in (which usually requires a qualified-certified party to run those numbers and sign off on it.)
But Maine's residential building code is currently based on IRC 2009. Under IRC 2009 in zone 6 a 2x6/R20 wall is code-minimum, in zone 7 2x6/R21 makes it. But that's still measurably below IRC 2015 performance.
Calling 2x6 with o.c. foam a "Pretty Good House" is a bit confusing to readers of this site, where that term usually means R40 (whole-wall-R) walls as part of the definition, whereas it's actually below IRC 2015 code minimums.
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house
https://www.greenbuildingadvisor.com/blogs/dept/musings/martin-s-pretty-good-house-manifesto
https://www.greenbuildingadvisor.com/articles/dept/building-science/ten-essential-steps-pretty-good-house
A 2x6/R20 open cell foam wall + ~4" of continuous rigid polyisocyanurate sheathing on the exterior would get you there, and with that much insulating sheathing it would NOT need vapor barrier latex or any other vapor retarder on the interior side.
Many (but not all) wood stoves have outdoor ducted combustion air options. Some of those kits are air-tight, others are mere proximity vents that still communicate freely with the room air. It's not always clear from the manufacturer's online documentation which type it is. As long as intake point the combustion air is below the firebox of the wood stove they tend to work pretty well. When the intake port at the side of the house is above the firebox it behaves as parasitic secondary chimney, competing with the draw up the actual chimney.
Bryce, are people in your area still using pre epa stoves, or are they burning unseasoned wood? Our air tube EPA stove emits no smoke and zero outside air after 5 or so minutes of proper operation.
On outside air(oak) many people told us they were unnecessary or harmful in NE Oklahoma. We decided to install one and it made a huge difference. I feel the stove operates better and we only need to put a water pot on the stove occasionally.
To the OP, if your decor can accommodate it I strongly suggest a freestanding stove. We had a pre EPA insert that protruded 1/3 into the room. It still relied on the blowers to extract heat. We purchased our current free standing stove and only run the blowers which came with to "get rid of" the coals for ash clean out. Also a freestanding will have a much cheaper total install cost due to minimal hearth pad versus a masonry hearth.
Dana, good points, but the OP noted in comment 9 that she would also be using Zip-R insulating sheathing. That brings up another round of concerns regarding moisture accumulation, but at least it addresses your heat loss concerns.
@ Michael Maines: Thanks- missed that!
The fattest ZIP-R (2.5", not 3") has sufficient R (R12.6) for dew point control on 5.5" of open cell foam or cellulose in climate zone 6A, but not for 7A.
So as long as Catherine's house is NOT in Aroostook County (the only zone 7A part of Maine) and not above 3000' in elevation it will be enough.
Michael,
Do you have any problems using Zip-R that thick from a shear perspective? Do you typically need to provide other measures, or is that just something we are sensitive to out here in the PNW because of our seismic concerns?
Dana, do you have any concerns about the reduction in effective R-value of Zip-R's polyiso as the temperature drops?
My main concern with 2 1/2" Zip-R is that it requires nails at least 4" long, which standard framing guns can't handle. The Bostich "Big Bertha" BRT130 and other jumbo nailers can do the job, they're just not standard issue.
Malcolm, I accept Huber's engineering numbers--215 plf for seismic shear, or 301 plf for wind shear, which they say will work (following their specs) for the seismic "B" zone (very low risk) where I work. They have a note that for higher-risk seismic zones it needs to be engineered.
I have not used Zip-R on any of my own projects, as I usually design double-stud or sometimes wrap-and-strap walls.
@ Michael Maines: When the polyiso is more than a third of the total R the average temp through the foam is now higher than when it's a thin layer on the exterior, and the derating is less. The half-inch OSB skin's R0.5 will actually go up a bit at colder temps. It only needs to be R11.25 outside the condensing surface for dew point control with air-permeable non-moisture buffering cavity fill. Cellulose would safely store quite a bit of moisture and still remain functional, open cell foam isn't air permeable, and is also a class-III vapor retarder on it's own at 5.5", so the amount of moisture accumulating during the coldest weeks would be minimal. Even 2lb density roofing polyiso (R5.7/inch at a mean temp of 75F) would still be delivering over R9. Lower density polyiso used in sheathing would be delivering more, closer to R10. If there's any concern at all, half-perm latex primer would be more than sufficient insurance.
And as seasonal temperatures moderate the performance picks up, so the "drying season" will start a bit earlier than if it were an R9-R10 performance across temperature.
In short, I'm not really very worried about moisture accumulation unless the home is kept at an unusually high humidity level.
The level of acceptance of wood burning on a 'green building' site is interesting. Isn't it a pretty big impact to both indoor and outdoor air quality? Worse than gas which has received quite a bit of derision here?
Brad (#20), a great question. Definitive and nuanced discussion here:
“Should Green Homes Burn Wood?”
Brad,
In certain regions the situation can be even more nuanced that Martin describes in that blog.
I live in a logging community. All the firewood used to heat houses here is harvested from the slash on cut-blocks. Slash that otherwise is gathered into piles and burned before the area is re-planted. You could argue that better use could be made of the slash, but given current logging practices wood burning appliances seem like a pretty efficient and green way of heating.
A lot has happened since Martin's excellent June 4, 2010 article: “Should Green Homes Burn Wood?”
For example:
* In 2015 the EPA released released updated emissions standards for new residential wood heaters.
https://www.epa.gov/residential-wood-heaters/fact-sheet-summary-requirements-woodstoves-and-pellet-stoves
* Significant swaths of the country are now encumbered with standing dead ash trees that should be removed for safety reasons.
https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/emerald-ash-borer
* Forest fire risk seems to have increased, perhaps due to climate change. Better management may be an answer.
https://www.usnews.com/news/best-states/washington/articles/2017-04-03/project-is-cutting-down-trees-to-reduce-forest-fire-risks
* There are now forest thinning programs that create habitat for endangered and threatened species like the Cerulean Warbler.
https://blog.nature.org/science/2017/08/15/recovery-farm-bill-provides-hope-for-the-cerulean-warbler/
Net net, I think the case for burning wood is stronger now than 8 years ago!