"'m no sure exactly what the question is. What means "I don't really see another way to do the boiler unless there is a way to do it with a heat pump", exactly?

There are many air handlers designed to work with hydronic coils that can also work with a wood boiler. In heating mode they usually operate the blower via an aquastat on the air handler that senses when the hot water reaches the coil, turning off when the hydronic plumbing drops below temp. The same air handlers can be used to mix up the air in the house, distributing ventilation air when the HRV is running. But if it's a 500-1000 watt air handler- that's 10x the power draw of a dedicated HRV system.

The heat pumps Eric & I are talking about are essentially reversible air conditioners, that can both heat and cool the the place. I don't know of a high-efficiency heat pump that also accomodates a hydroinic coil. But there are many air handlers that have both a hydronic and refrigerant coils that can be adapted to old-school heat pump compressors if need be. It takes considerable design skill to get it to work though.

A good heating system design always start with a room by room heat load calculation. Without that it's hard to find the appropriate solution. When you have a high-R/low-load house most "standard" solutions are ridiculously oversized, but one well trodden path for low load homes is the ductless mini-splits, or the small variable speed air source heat pumps we've been discussing. Almost all wood boilers are way oversized for the heating loads, but that is managed by storing the heat in large buffer tanks. But the complexity of the hydronic heating system running a low load home off the wood boilers may end up being more expensive than heat pump solutions. Panel radiators are probably a simpler/better heating solution than ducted air for the non-radiant heating zoned running with the wood boiler, but without the room by room heat load numbers it's hard to say anything with certainty.

I am a little confused on what to do with the roof deck in our attic. I am going to follow what Dana said about the exterior rigid foam and go with the 2" of poly with sealed seams over zip sealed sheathing osb. Do I need to do anything with the roof deck though or can I just roll with the 24" of blown cellulose? I plan to hang 1/2" OSB from the bottom cord of the trusses and seal the seams so I do not forsee any seapage into the attic.

Another question I have, if I do that walls like I mentioed, and the ceiling/attic and the slab, and use good air tight gasket material on the door and window bucks am I going to be able to have enough heat demands for the boiler to work well with my set up? Seems to me that I am going to build one hell of a tight house....the boiler needs to have a draw in order to cycle and work properly....nto a HUGE draw but it will need something to call for heat and I ddon't think the basement slab with radiant and the heat exchanger for the hot water will be enough to make it cycle. I do not want to have to install ERVor HRV just to keep the house healthy...don't want it to be TOO tight. After all I am on a budget for this build and am trying to spend the money in the best places. GEOTHERMAL and HRV/ERV at first glance seem to be WAY over my budget and I honestly don't understand the ERV/HRV systems enough to say I want to employ one in my new house. Can you help me understand and really figure out if my boiler is going to be useless if I build this house tight? Would you say this house is super insulated?

The ratios matter- if you're really going to go with 24" of cellulose on the interior side snug up against the roof deck, you'll need about 3" or more of polyiso above the roof deck, unless the interior side vapor retardency is below 1 perm (a class-II vapor retarder). It's the ratio that determines the average temp at the roof deck, and it has to have a mid-winter, average above the dew point of the mid winter room air. (Typically ~37-40F.)

Gypsum with paint is too permeable, but if you stick with the half-inch OSB you should be fine, as long as it's air tight. has a vapor permenace of about 0.8-1 perms when dry, so with seams well sealed it should be fine.

Most wood boilers are way oversized for the heat loads of even code-min houses, which is why you fire them intermittently and pump the heat into a big buffer tank rather than heating the house directly. This is even more true of high-R homes where the heat loads are much lower, and why spending the money on mini-splits and solar PV is often a better financial choice. (There are many houses within an hour's drive of mine that have an average power bill of $0 despite heating with electricity, with PV arrays that fit comfortably on the roof.) The difference in efficiency between geothermal and mini-splits isn't enough to rationalize the huge cost-multiplier (even after subsidy), for small geo systems.

The only way to know what you heat load really is requires running the numbers- do a real heat load calculation.

That's also why it makes more sense to pick the heating/cooling systems AFTER the house design is fairly complete, with the heating/cooling numbers better known. If you take the R-4/8/16/32/64 approach and don't go overkill on the window area, your heat loads could easily be in the mini-split or 2-ton GreenSpeed range.

If you then traded the cost of the wood boiler/hydronic system and/or the difference in cost of geothermal to air-source heat pumps you too might come close to net-zero annual power bill, on an all-electric heating/cooling. system, but you'll have to forgo the joys of stoking the boiler and stacking/splitting wood.

Dana, I am basing all these numbers off the numbers you were helping me with before. the 4/8/16/32/64 so I was shooting for the R-64 in the attic at around 20-24 inches that will settle to 18 inches over 20 years. I guess the reason I am feeling lost is because every time I think I have something dialed in to the specs I want to use, another factor comes into play that I was not aware of or uncovered in my research.
Nobody had mentioned any kind of polyiso on the roof deck up until this point so that's why I seem a little confused. I may not be ready to take on this kind of task if I cannot follow this intro design phase. Seems like every time narrow something down and ask how it'll work there is another addition. That's frustrating for a beginner like myself. sorry to complain and I am very thankful for the help, I am just struggling to do what is best with my building budget.
We plan to use Standing Seam roofing if that matters at all.

Another question I have is what if I skip the exterior foam and just go with a 2x6 framed wall, with zip sheathing, 1" of spray foam in the stud voids inside to get it air tight and then wet blown cellulose over that. It would make it a lot less labor intensive and should still get the wall air tight.
Additioanlly if I do the very same thing with the attic....meaning, osb on the bottom cords of the trusses, sealed seams then drywall for the finished ceiling. In the attic 1" of spray foam to seal the osb to the trusses then 20 inches of blown cellulose over that. Is that going to require something special on the roof deck to keep the condensation down? I plan to do vented sofits and if needed a ridge vent of some kind in the top of the attic. Like I said before this will be standing seam roofing

Posted By easyrider470 on 03 Dec 2013 02:41 PM
Another question I have is what if I skip the exterior foam and just go with a 2x6 framed wall, with zip sheathing, 1" of spray foam in the stud voids inside to get it air tight and then wet blown cellulose over that. It would make it a lot less labor intensive and should still get the wall air tight.
Additioanlly if I do the very same thing with the attic....meaning, osb on the bottom cords of the trusses, sealed seams then drywall for the finished ceiling. In the attic 1" of spray foam to seal the osb to the trusses then 20 inches of blown cellulose over that. Is that going to require something special on the roof deck to keep the condensation down? I plan to do vented sofits and if needed a ridge vent of some kind in the top of the attic. Like I said before this will be standing seam roofing

Without the exterior foam you'd  get a whole-wall R of  only ~R15 with a flash'n'fill. That's at best R1 better than an R20 2x6 code min wall.

And it only air-seals the cavities- the leakage under the bottom plate and between doubled up top plates still need attention.

It's much cheaper to go high-R with a vented attic and insulating at the attic floor than insulating at the roof deck, and that's what makes the most sense on a new-build where you can design with deep "energy heel" trusses, and attend carefully to the air sealing prior to insulating.   With the vented attic it's most efficient to not use that space for duct work, air handlers, etc. just insulation.  With your supporting OSB air tight, and a soffit/ridge venting scheme, the wood in the attic will be fine, no condensation or rot issues to worry about (until you get a rain leak.)  Insulating at the roof deck has some rationale if you plan to build-out the attic for other uses, or if you have HVAC equipment/ducting in the attic, but it's WAY more expensive- you have to have a good rationale for going that route.

Thanks Dana....but I am still at a loss for all the extra things that keep coming up as I try to plan. Suppose I do the 2x4 wall, with 2" of foam on the exterior with sealed seams on the OSB sheathing and the foam sheathing. I intend to use the gasket material on the sill plates and wrap it up into the exterior foam to seal that area. If I use 1" of spray foam to make the interior of the wall cavities air tight with wet blown cellulose in the wall you were saying that the attic at R-65 would still need additional foam on the roof decking? I don't want to over insulate in one area and cause issues for another. Just really want an economic approach to keeping the house tight, but not over tight causing unnecessary need for ventilation or fresh air brought into the home...that seems like overkill to me. What in your opinion out of all the options here is the best way for me to keeop everything in lone with eachother and still make it work so the boiler will work enough to function correctly and the radiant floor in the basement won't cause issues with humidity. Lets do that!

If you insulate at the attic floor and vent the attic, you don't need any insulation at the roof deck. But you need to truly ventilate the attic, and air-seal the attic floor.

You don't really need to use a full-coating of 1" closed cell spray polyurethane on the attic floor. Sealing the seams of the OSB with duct-mastic and filling in around electrical & plumbing penetrations with can-foam or FrothPak is fine, and is nicer to the planet (most closed cell foam is blown with HFC245fa, which has a global warming potential greater than 1000x CO2, among the other not-so-green aspects of cc. foam.) Half inch OSB all by itself is a class-II vapor retarder at about 0.8 perms, comparable permeance to 1.5" of closed cell foam- you don't need or want any thing more vapor retardent than that in the stackup. Air sealing with minimalist amounts of spray foam and insulating with

With 2" of exterior foam on a 2x4 wall you don't need anything foam in the wall cavities either- seal the sheathing to the framing with acoustic sealant caulk, and any penetrations with can-foam or FrothPak.

All houses need ventilation- even leaky houses. There is nothing in a leaky house that guarantees the parasitic drafts somehow magically occur in the right place and proportion to where the ventilation is actually needed. It's a common fallacy that only tight houses need ventilation. But in tight houses you get to better control where that ventilation air is coming from and going to. If you make it intentionally leaky you give up some of the that control, and taken an energy use hit to boot.

I have a question, if I went with the 2x4 exterior wall, and decided to do the blown in closed cell foam to fill the whole cavity, along with the exterior Zip Sheathing and 2" of foam what type of R-value then? Seems that the foam has gained a lot of popularity in my area and the Amish crews can do a 2200 sq ft home exterior 2x4 walls for about 8,000 dollars. I am thinking that if I can get an R-21 wall with the spray foam, it might be cost effective in LABOR alone.
I am also concerned about the ERV/HRV being cost effective. I told myself that I wanted to employ SOME energy efficient construction methods to our new home. BUT i don't want to get away from things like wood burning fireplace, outside wood boiler heating with radiant floors, and windows. I really am concerned about negative pressure since the exhaust fans in bathrooms and the large kitchen hood fan we have. I think it may not be smart for me to spend so much time and money sealing up the house to just have to use an HRV or ERV

OK here is my take on it 2x4 walls with zip sheathing + 2” exterior iso is a good combination. OSB on the bottom side of the trusses and taped is a good idea. Lots of cellulose on top of that is a good idea, with a vented attic. Using closed cell spray foam on the walls is OK, but only a little better then using damp spray cellulose on the walls. You already have the air barrier with the zip sheathing , and 2x4 construction has a much higher framing factor, so the cc foam is less beneficial. Save your money for mechanical ventilation With what you are describing, you will have a house that needs mechanical ventilation, period. A HRV is the best way to accomplish this, but it is not the only way. Just because you don’t want a HRV does not mean you do not need or want mechanical ventilation. It can be as simple as a Panasonic wall mount fan supplying 40-60 cfm of air into a central location (supply only) Or it can be a Panasonic whisper green fan on continuously in each bathroom with passive air make up vents strategically located in the house. As far as the outdoor wood boiler goes, I still maintain that is a bad idea for many reasons which I won’t repeat UNLESS it is coupled with a large enough buffer tank so that the boiler can be lit and run to burnout at full throttle. At that point, you can use whatever you want to distribute the heat. Cheers, Eric

Thanks Eric. I would like to know what boiler you are using. You mentioned you use a boiler in your home. Do you use it for primary heat or just for a radiant floor system?

I have a Munchkin T-50 boiler. I elected to use standard baseboard in my house, mostly for cost reasons. I use the boiler as a backup to my wood stove and supplement to solar hot water heater. The wood stove is the primary heat source.
I think the one thing you need to keep in mind is that as you make the shell more efficient, you can make the heating systems simpler. My HVAC system is a boiler with baseboard, and an HRV, and a woodstove. At some point I will add a minisplit because I have no cooling in the house. I designed the house to work with a minisplit, but ran out of money before I put it in.
Cheers, Eric

So baseboard heat with a wood stove as the primary heat source for the entire home? I like the idea of a wood stove in my house but I am concerned that the stove will not cut it for heating my home. It's looking like the house will be about 4400 sq ft including the basement which will be finished in the future. Not sure if I could do two full levels in that house with a wood stove? I plan to do radiant in the mudroom and the downstairs bathrooms along with the basement slab. I also intended to use the outdoor wood boiler to heat domestic hot water. I was hoping that the radiant floor heat and the DHW would create enough draw to keep the boiler busy without water storage. Now that I am seeing that I can simplify my HVAC system, I am wanting to learn as much as possible now about the easiest most cost effective way to run the radiant floor heat and employ an ERV/HRV system.....looks like I will need to start a new thread!

The wood boiler is a better fit with radiant floor heat and a large house.

I built a 4800 sf home in north west Nebraska using an outdoor wood boiler (Central Boiler model 6048) and radiant floor heating. I works great and is, hands down, the most comfortable home I have lived in. The temps regularly dip well below Zero with winds of 60+ mph with wind chills of upwards of 80+ below zero. Evan in the nastiest storms the temps inside stay constant and seldom vary more than a couple degrees. I ran my pex tubing in the concrete of the basement floor and did a "staple up" on the main level under the subfloor. I did not use heat transfer plates, I insulated with r-19 fiberglass and radiant reflective fabric under the floor joist cavity. I made sure to space the tubing 1/4 inch off the subfloor to eliminate stripping.I also installed wood floor on the entire main level ( except for kitchen and bathroom which have tile) The only thing I have to say about that is use american cherry if you are going to use wood. It works best with radiant floor heating. The only drawbacks That I would say for radiant floors is, in the spring and fall when temps start cold and warm up in the day, you get some temp overruns at times, also you will need to think about cooling for summer as you will not have duct work and an air handler unless you install that system just for cooling purposes. I highly recommend the Central Boiler. It is very well made and the salesman I used has great service.My Boiler is the multifuel option which will burn wood, pellets, corn, Or LP. They also have the E-Classic for a more environment friendly option.
I have also installed wood boilers into conventional air handlers. works well too, but doesn't compare comfort wise to radiant floors.

Thanks BRC and welcome to the forum. Currently we use the air handler set up through the HVAC ductwork and we really like the way it works. However, its not the most efficient way to heat the house thats for sure. I am considering the radiant heat throughout the new house but since we do want to have Airconditioning I'm not sure how I will be able to do both....

You may want to look into a mini split system for cooling. They are quite efficient and cost is not too bad.

As an "old sport" 67 years, I can tell you that along with shoveling snow, cutting, splitting, stacking, toting wood gets OLD! Sounds like you are an independent young man with young children, but you cannot outrun age. And what happens if you are physically injured, will your little daughter stoke the boiler? Have you considered SIP walls/roof? They go up fast with minimum labor. The arguments for a hot water storage tank makes good sense to me. And I do think there will be continued stringent complaints about wood fired boilers. Vail Colorado passed a "no wood burn" ordinance back in the 70's, and there is "creep" related to this issue. The old saying "once the camel's nose gets under the tent" applies to the EPA.
Good luck Mate.

As an "old sport" 67 years, I can tell you that along with shoveling snow, cutting, splitting, stacking, toting wood gets OLD! Sounds like you are an independent young man with young children, but you cannot outrun age. And what happens if you are physically injured, will your little daughter stoke the boiler? Have you considered SIP walls/roof? They go up fast with minimum labor. The arguments for a hot water storage tank makes good sense to me. And I do think there will be continued stringent complaints about wood fired boilers. Vail Colorado passed a "no wood burn" ordinance back in the 70's, and there is "creep" related to this issue. The old saying "once the camel's nose gets under the tent" applies to the EPA.
Good luck Mate.

Thanks DAND, You are no the first person to encourage me to be aware that handling firewood and dealing with the labor associated with the boiler can get old. I agree that it might not be the best scenario for the long term. I am already considering GEOTHERMAL!