System design for off-grid super-insulated home

[ericdd]

Hi folks, this is my first post. I'm building an off-grid super-insulated home this summer in Central Ontario and wanted to get your feedback on my system design. We are self-building most of this project on a fairly tight budget.

Site: The house is 2 storeys + basement, 800 sq ft/floor so 2400 square feet total. We have double-stud superinsulated walls so our heat loss calculation came out at 14,000 BTU/hr (17,000 if we don't run the HRV). Available wood on the property is typically Oak, Birch, and Elm.

Chimney: ~27' straight up. The house is oriented south with lots of glazing so we want to take advantage of the passive solar gain. For this reason we planned a masonry chimney with insulated SS liner for the basement and first floor. The brick would act as a thermal mass for storing ambient energy produced by the wood stove as well as passive solar gains, and ideally will "even out" the heating and cooling of the space. The chimney will run up through the middle of the house so going with brick and liner wouldn't significantly change the footprint. We have the luxury of pouring a footing now if we decide to go this route. On the second floor we want to transition to a standard double-wall prefab chimney pipe in drywall chase up through the roof. We spoke with someone who thought the thermal advantages of the masonry chimney may not be worth the expense (footing, brick, mason - or maybe we will DIY?). Any thermal mass / passive solar nerds with opinions on whether the masonry is worth it?

Stove: I'm considering either the Pacific Energy Super 27 or a Blaze King... undecided about whether the advantages of the long, low burn of the catalytic is worth the extra cost in our case. Our advisor also recommended an outdoor air supply, but I've read elsewhere that the advantages are questionable. This will be an airtight house with HRV and no major exhaust loads. We could always install a 4" sleeve and cap it off unless we have drafting issues. Does that make sense?

Since we're off-grid, we're trying to minimize electrical loads like fans to circulate heat. Any tips and tricks for non-electrical ways to move the heat from the basement up to the main and second floor? Main floor is mostly open concept, but second floor has lots of partitions. We will not be finishing the basement ceiling, and not insulating between floors 1 & 2 (I know we're sacrificing some acoustics for this).

Thanks a bunch in advance! These forums have already been very helpful.

 

[begreen]

Welcome, and kudos for good planning. By description, your home is not that dissimilar to the one that my sister and her husband built in mid-state NY. What is the primary heating system, wood or electricity? Will your local bldg dept. and bank allow wood only?
You are planning well. By description, I would skip the complexity and cost of the masonry and go class A chimney all the way. It will give off some heat. You could vent the chase with a small grille at the top and bottom of each chase on the 1st and 2nd floor. It will also heat up the sheetrock which has some mass.

As for stoves, for most of the heating season you won't need that much. What you choose will depend on how you want to run the stove(s). One option is to have a smaller stove on the main floor that covers swing season heating and milder winter days and the basement stove as supplemental. Another would be the reverse, with the primary heater in the basement and a supplemental heater on the 1st fl. In my BIL's house they built a custom, built-in wood-fired oven with a lot of interior thermal mass on the 1st floor. This is what they use for mild weather heating. In the basement they have a wood-oil-electric boiler that only gets used when temps are in the teens or lower.

 

[DickRussell]

.... The house is oriented south with lots of glazing so we want to take advantage of the passive solar gain. For this reason we planned a masonry chimney with insulated SS liner for the basement and first floor. The brick would act as a thermal mass for storing ambient energy produced by the wood stove as well as passive solar gains, and ideally will "even out" the heating and cooling of the space.....

Be careful of too much south-facing glazing in a superinsulated house. Similar designs have resulted in overheating at times, when the sun loads the house with heat that has no place to go easily.

For moving heat from the basement to the upper floors, you are talking either a fan and ductwork or just leaving stairway doors open. Our house in central NH (CZ 6) is superinsulated, two-level, 2000 sqft/level, and has a small woodstove in the lower level. In the 2010-2011, winter, that woodstove was the sole source of heat for the house, running perhaps 8-9 hours/day. The interior doors were not hung then, so air currents moved heat from the lower level to the upper. Thermometers on both levels, well away from the stove downstairs and the stairwell upstairs, showed only a degree difference when the stove was running.

 

[ericdd]

Thanks for the replies!

What is the primary heating system, wood or electricity? Will your local bldg dept. and bank allow wood only?

We will have electric baseboards installed and listed as primary for code-compliance. They might be just right for shoulder season heat, but there will be some trial and error. We are also planning to rough in tubing for a future hydronic system into the basement slab, but we don't have the budget up front for the full system.

By description, I would skip the complexity and cost of the masonry and go class A chimney all the way.

The masonry chimney is a really tough call. Even if we abandon that idea, I'm tempted to pour the footing, in case we decide to rejig things in the future.

Be careful of too much south-facing glazing in a superinsulated house. Similar designs have resulted in overheating at times, when the sun loads the house with heat that has no place to go easily.

Yes this is for sure a concern. I actually predict this house will be trickier to keep cool than warm. Although we might have enough watts in the summer months for a window-shaker A/C unit, the neighbours are experimenting with that now! In the meantime we have planned an overhang that will shade the south glazing in the summer and let the winter sun in, and heavy curtains can help a lot, too.

Happy to hear your house heats pretty evenly! Hope ours performs like that.

 

[Highbeam]

Skip any sort of masonry chimney fireplace. Do something else for thermal mass if you think it’s needed.

14000 is a small load. The catalytic bk can sit there and make that constantly or the noncat PE can pulse and glide to make that output on average with more temperature swings.

Really though, with just over one ton of demand at the design temperature, a little mini split is all you need if you had power. The wood becomes backup.

It’s really hard to heat a small home or one with a small heat load using a modern woodstove. Are you really off grid? Or will you have a big propane tank? That propane tank could run a gas stove that could make 14000 btu with no power needed.

 

[ericdd]

It’s really hard to heat a small home or one with a small heat load using a modern woodstove. Are you really off grid? Or will you have a big propane tank? That propane tank could run a gas stove that could make 14000 btu with no power needed.

We will have propane for our range and domestic hot water. We have wood on the property so like the idea of using it rather than paying for (more) propane. You're right that it wouldn't take much propane to keep our place warm if the house performs as expected, but I think we're just attracted to the wood heating lifestyle - ask me in a few years if I still think that, though!

 

[begreen]

We will have propane for our range and domestic hot water. We have wood on the property so like the idea of using it rather than paying for (more) propane. You're right that it wouldn't take much propane to keep our place warm if the house performs as expected, but I think we're just attracted to the wood heating lifestyle - ask me in a few years if I still think that, though!

Yes, at 45 this may be a workable plan, but at 75...?
If you go with a mid-sized PE stove, I would recommend the T5. The cast iron jacket will provide nice thermal mass for evening out temp swings.

 

[DickRussell]

.... We are also planning to rough in tubing for a future hydronic system into the basement slab, but we don't have the budget up front for the full system.
....

A slab hydronic system in a superinsulated house usually is considered a waste of money. People like the idea of having a warm floor underfoot, but a concrete slab won't feel at all warm until its temperature gets near 80 F. With no place for the heat given off from a whole slab to escape the building quickly, the whole floor, and likely those above, will get to near the same temperature, which most consider stifling, even in winter. So the temperature of the water circulating through the floor coils would have to be controlled to lower 70s, which won't feel warm underfoot. With ample insulation beneath the slab, all you need down there for comfort is low-pile carpeting. That's what we have, and the floor never feels cold.

 

[ericdd]

A slab hydronic system in a superinsulated house usually is considered a waste of money. People like the idea of having a warm floor underfoot, but a concrete slab won't feel at all warm until its temperature gets near 80 F. With no place for the heat given off from a whole slab to escape the building quickly, the whole floor, and likely those above, will get to near the same temperature, which most consider stifling, even in winter. So the temperature of the water circulating through the floor coils would have to be controlled to lower 70s, which won't feel warm underfoot. With ample insulation beneath the slab, all you need down there for comfort is low-pile carpeting. That's what we have, and the floor never feels cold.

This is a very good point! I should clarify, the main reason we want the hydronic system in the slab, is to have an efficient way to keep our pipes from freezing if we were to go away from an extended period of time, not wanting to count on electric baseboards which could drain our batteries in the winter. It won't be used other than that.

 

[begreen]

not wanting to count on electric baseboards which could drain our batteries in the winter.

Wait, this house will be off-grid? Will the bank and code go for electric baseboard heating in that circumstance? A high-efficiency propane heater might be a better plan. Will the basement be fully insulated and surrounded on all sides by earth? If so, it may stay above 40ºF all winter. To avoid a freeze-up, keep plumbing out of the perimeter walls.

 

[Simonkenton]

What kind of insulation do you have in the ceiling?

 

[ericdd]

For heat circulation we're leaving the ceilings between basement/1/2 uninsulated, and blowing in a pile of cellulose into the attic (nominal R75). Exterior walls are double-stud with Rockwool.

 

[mcdougy]

Have you looked into a wood furnace and or a outdoor boiler system? Using windows as a heat source on a 2 storey home is pretty tough...as in the summer heat gain.... I'm curious about the double stud.... As in double 2x4 or 2x6 offset with the rockwool in the cavity?

 

[ericdd]

Have you looked into a wood furnace and or a outdoor boiler system? Using windows as a heat source on a 2 storey home is pretty tough...as in the summer heat gain.... I'm curious about the double stud.... As in double 2x4 or 2x6 offset with the rockwool in the cavity?

Looked at wood boilers a little bit, but found the efficiencies of the outdoor ones were pretty low compared to the modern EPA stoves... I think because we're off-grid I'm trying to keep it simple and minimize reliance on fans and pumps as much as possible.

The summer heat gain will be partly mitigated by an overhang built to shade the windows in the summer, but let in the low sun in the winter... but it could still overheat and something to definitely plan for.

The double stud setup is exterior 2x6 24"OC with R22 rockwool and interior load-bearing 2x4 24"OC with R14 rockwool. The studs are staggered wherever possible (other than around doors and windows) to minimize "thermal bridging" (heat leaving your house through the lumber). The continuous layer of exterior rigid insulation also helps with that. It's a lot of cost up front in the hopes of very low monthly costs if the building envelope performs as expected.

 

[Simonkenton]

R75 in the attic. Good God that is some serious North Land insulation. Good job it is costly but well worth it.

 

[SpaceBus]

Since you are off grid have you considered a wood cook stove? We heated our R23 wall/R46 celing 1300 sqft salt box on a slab with south facing sliding patio doors last winter with just the wood cook stove, but it was a very mild winter. Seems with R37 walls and a 14k btu/hr heat loss a wood cooker would easily do the job. Downsides are shorter burn times with the small fireboxes which aren't designed for long burns anyway. If I stuff ours totally full of hardwoods and run it as low as possible I'll have coals in the morning, but I can get overnight coals much easier in our Morso 2b Classic using soft woods. What I'm saying is you would need two stoves, but you probably won't ever cook yourself out. I've had some close calls this summer getting the house up to 78 df running the wood cook stove, but I've gotten better at timing when to run it. We also have an electric glass top range oven, but I prefer the wood cook stove. Our wood cooker is also plumbed into my domestic hot water system and functions as a pre-heater for a tankless electric water heater.

 

[mcdougy]

Looked at wood boilers a little bit, but found the efficiencies of the outdoor ones were pretty low compared to the modern EPA stoves... I think because we're off-grid I'm trying to keep it simple and minimize reliance on fans and pumps as much as possible.

The summer heat gain will be partly mitigated by an overhang built to shade the windows in the summer, but let in the low sun in the winter... but it could still overheat and something to definitely plan for.

The double stud setup is exterior 2x6 24"OC with R22 rockwool and interior load-bearing 2x4 24"OC with R14 rockwool. The studs are staggered wherever possible (other than around doors and windows) to minimize "thermal bridging" (heat leaving your house through the lumber). The continuous layer of exterior rigid insulation also helps with that. It's a lot of cost up front in the hopes of very low monthly costs if the building envelope performs as expected.

How thick are the concrete walls? And how thick is your exterior rigid insulation? I'm asking because I'm interested on how your looking at the envelope, not to be a nay Sayer...we built a home 3 years ago and it is very cost effective to operate.

 

[ABMax24]

I think your heat calcs are pretty much spot on. My parents here have a similar house, 2600 sqft 2x6/2x4 offset stud walls with R60 blown in attic insulation, and a PE Super 27 in the basement in a very similar climate to Ontario.

The Super 27 is almost too much stove in their case, you can't really burn it 24/7, at least not without having it essentially smolder all day long, and just one full load for night. I'd recommend a cat stove in your case, I think it will deal with the low heat output much better. Thermal mass would help out but isn't necessary, they have a wood basement and even without the extra mass of concrete the house maintains a fairly constant temperature.

One thing I absolutely wouldn't do is electric heat. I also have a solar system in these northern climates and the output is dismal at best in the winter. My 3kw system only puts out about 50kwh in December/January, it can do that in 2.5 days in June for just over 400kwh average that month. Unless you can clean the snow from the panels in the winter and have an absolutely massive solar system electric heat is un-achievable without extensive use of a backup generator. And since a backup generator will throw away at least 60% of it's fuel as waste heat you're better off just to heat with that fuel in the first place. I'd recommend a condensing propane furnace for primary heat. You'll still need that backup generator to run the blower occasionally in the winter though.

 

[begreen]

It's still unclear whether the house is off-grid or just that the desire is to have that option.

 

[ABMax24]

It's still unclear whether the house is off-grid or just that the desire is to have that option.

Seems pretty clear that the house is off-grid.

I'm building an off-grid super-insulated home this summer in Central Ontario and wanted to get your feedback on my system design.

Since we're off-grid, we're trying to minimize electrical loads like fans to circulate heat.

Although we might have enough watts in the summer months for a window-shaker A/C unit, the neighbours are experimenting with that now!

not wanting to count on electric baseboards which could drain our batteries in the winter.

 

[Poindexter]

I think of thermal bridging as cold getting into the house through the lumber. I can feel that with my hand on the wall. Heat getting out I only see on the bill when the oil man comes intermittently.

I don't have much to contribute here, you are already above my experience and getting good advice within the limits of my experience.

I will say don't scrimp on the HRV system. I am in a 1980 build that qualified as tupperware for tightness when built, 40 years later I am in ongoing skirmishes with mold deposits. Air turnovers good, stagnant air bad. Humidity control good, wild seasonal humidity fluctuations bad. I'll be looking at HRV systems with washable reusable filter systems for our next house, I don't want to get married to disposable filters.

 

[Highbeam]

If you’re truly off grid in a deadly cold place then I would for sure install a woodstove low in the home. Due to the application I would go for a very durable, rugged stove even at the expense of efficiency and performance. Slam an oversized plate steel noncat in there.

You can burn furniture if you have to.

 

[Simonkenton]

What kind of firewood do y'all have up there in Ontario?

 

[begreen]

Seems pretty clear that the house is off-grid.

Electric resistance heating and off-grid are incongruous unless there is a major battery bank or generator. I am just not sure the financing institution and insurance companies would go for this. If truly off-grid, then I would not bother with resistance heating and instead have a large propane tank and an area heater in the basement to deal with heating while away or sick.

It's good that the heating load analysis has been done. A very realistic electric load and power management analysis and plan should also be obtained. This should extend to every possible load with a focus on long-running and high consumption loads like refrigeration, clothes washing, AC, HRV, lighting, etc.

I think of thermal bridging as cold getting into the house through the lumber. I can feel that with my hand on the wall. Heat getting out I only see on the bill when the oil man comes intermittently.

That's exactly right. When that issue is addressed, the difference is quite amazing. I thought my BIL was BSing me about their low wood consumption until I visited them in January. Temps outside were in the 20s and they were heating the place with the little wood-fired oven. When sunny they can daytime heat mostly with the R10 greenhouse that can vent into the house.

 

[SpaceBus]

Electric resistance heating and off-grid are incongruous unless there is a major battery bank or generator. I am just not sure the financing institution and insurance companies would go for this. If truly off-grid, then I would not bother with resistance heating and instead have a large propane tank and an area heater in the basement to deal with heating while away or sick.

It's good that the heating load analysis has been done. A very realistic electric load and power management analysis and plan should also be obtained. This should extend to every possible load with a focus on long-running and high consumption loads like refrigeration, clothes washing, AC, HRV, lighting, etc.

That's exactly right. When that issue is addressed, the difference is quite amazing. I thought my BIL was BSing me about their low wood consumption until I visited them in January. Temps outside were in the 20s and they were heating the place with the little wood-fired oven. When sunny they can daytime heat mostly with the R10 greenhouse that can vent into the house.

I don't think any bank will lend for a totally off grid home, but I've been wrong before.