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More On Chimneys And Insulation In A Tight SIP Passive Solar Prefab Home


Sometimes I wonder if y'all think I'm a bit loopy.
I have a simple answer for those that do: I'm from Richmond.
Richmonders certainly have quirks, and a deep historic layer o' foibles.
Honeychiles, in this year's Christmas parade, we even killed Rudolph.

So, if you ever think I'm a tad touched... blame it on geography.

Passive Solar Prefab Off Grid Home
Under Construction

Anyhoo, as I've mentioned, the zero energy prefab house is back under construction with some new penetrations in our previously tight, energy efficient house kit made with SIP.  We have some temporary measures to make our winter more comfortable during this transition, but don't be fooled: Although crafty, we draw the line at a messy, gerryrigged lifestyle. The end result of this construction project will be sleek, modern, permanent off grid grace, albeit amidst I-thought-this-was-going-to-be-short term chaos. 

 We will get there. But face it: We're. Under. Construction.

Prefab House Kit With Passive Solar Design.
And, yes, #SheTakesAfterMe

In Virginia, we currently are enduring ARCTIC temperatures, with more snow on the way. Honestly, as much as I thrilled at our survival camping last year in the SIP house, I am not jumping at the chance to do it again, through another winter, with young children, especially with new penetrations and no radiant heat yet hooked up. (Remember when it was ELEVEN degrees and we were spending the night in the off grid prefab, still with no heat?!?)

So let's talk about energy efficiency, wood stoves, and penetrations in the SIP.
Here's our own zero energy prefab's initial energy audit post:

Wood cook stove pipe in a tight SIPs house.

Note how critical it is to insulate, insulate, insulate where there are not SIP - I wrote about putting insulation under and around your foundation two years ago, to note any thermal bridging and to address it, but also to critically eye insulating your crawl spaces, etc. (if you have them)... and here's a great article on putting foam under your footings, and how to get your local code officials to support it.

But this is a post about chimneys and wood stoves in a tight, energy efficient house kit.

Here is my technical explanation in a nutshell: I know it's impossible to completely seal this ole leaky, antique cook stove, I know that... but I'm gonna jigger the thingamajiggers & take it as far as I can.

The day I finally got mah cook stove...

As always, we walk the balance of energy efficient functionality with heartfelt touches, like the wood cook stove, that combines nostalgia with its own usefulness of off grid net zero cooking and heat, yet detracts from the prefab house kit efficiency when not in use: through its roof penetration.

Basically, the industry feels that to safely have wood stoves in *any* energy efficient home
1. The whole system (not just the wood stove but also the flue, etc.) must be airtight (ours will never quite be)
2. Have an ERV (energy recovery ventilator - external air supply)*
3. Make sure there's no back drafting (the combustion process of the sealed stove sucks in air from outside the envelope and exhausts it through the chimney without affecting the pressure in the house)

And don't even get started on asking for an open fireplace - which only reduces heat, sullies your air, and causes all sorts of problems.

*If you do not know what ERV / HRV are, which are critical in energy efficient homes, or want to understand more about designing good ventilation systems, read here

I decided to see how others are handling efficient wood stoves / chimney penetrations, knowing that in passive houses and other energy efficient tight homes you may not have an open fire place; however that may be, people are using sealed pellet and wood stoves with ERVs and I thought I'd put links to comments of others addressing their chimney penetration for your discourse.

Currently (now don't you forget we are UNDER CONSTRUCTION, no ERV, etc. yet) we have negative air pressure caused by the three penetrations and wood stove. ERVs DO NOT affect the pressure one way or the other, their sole purpose is to remove stale air from the house and replace it with the same amount off fresh air (same amount) while exchanging the heat from one air flow to the other.

Here is our own wood cook stove scenario:
Air escapes through the chimney.  The fire draws in new air, except in our case, since the house is was so tight, it depressurizes the house.  Now you could crack open a window, eh?
(Yes, architects, I just heard you gasp in disbelief. I'm breakin' it down here, be patient.)
People: You just let in frigid, frosty air into your cozy home.
Instead, what we're going to do is bring that external air supply as close as possible to the stove, so that the cold, external air can travel as directly and as close to the fire without mixing with our indoor air. We will be doing that via the gas line, which we plumbed for investment value but never intended to use for our own purposes.

Chimney: Of course we have an insulated chimney. However, the manufacturer states there must be a 2" gap!!!! Jiminy Crickets!
Our contractor, Jason Dorris, argued the manufacturer for mineral wool, refrax, other high temperature suggestions (on my own research I also saw mentions of flue caulk, fibre cement / muffler cement / muffler putty)... to no avail.  We shouldn't have something rigid, so that as the flue moves, contracts & expands with heat, it doesn't crack.  We will discuss that more, later, as I await a return call from the manufacturer before telling you what we ended up doing.

Insulation: In my research I found a nice illustrated explanation that you might find helpful on sealing leakage and insulation... just to give you some ideas on where to start.

Firestopping: (Not that I would recommend PVC, I was thinking of these products for use with other applications & materials. I'm really liking that SpecSeal® SSP putty at the bottom of the page...)

Whenever thinking about wood stoves and chimneys in energy efficient homes, I always think about Kat's great post, here.

Today, I reference this segment:

"Flue penetrations need to be detailed in an airtight manner as well as they need to be as thermal bridge free as possible. The connection between the fireplace and the outside should be via a class A insulated metal chimney, or if possible one should be using a zero clearance chimney. There are ceramic insulating materials that are rated for very high temperatures. A double wall flue could also be used to provide intake of combustion air at the same time while needing only one envelope penetration. "

Interesting Products:

  • The Absolut Chimney System :
    "Two block stones with thermal separation for where the flue passes through the ceiling of the cellar and the roof of the house effectively prevent heat from escaping along the flue, and a specially developed roof seal also helps to save energy.

    The thermo-airdraft is another innovation which helps to save energy. Similar to a heat exchanger, it uses the heat of the exhaust gases produced – which would otherwise be wasted – to pre-heat the inflowing air needed for combustion in the stove."

  • Wood stove for a passive house:

This is also a good time to bring up solar or thermal chimneys, which I think would be a great addition in our prefab house kits if you'd like to include them. I especially have our R1 Residential prefab house kit in mind for such a feature.  For our area of Virginia and our family needs, for cooling, we've found opening the clerestory windows in our casa ti house kit provides enough breeze and hot air escape for our comfort. (We are also installing fans for those still summer days throughout our own house kit....)

The R1 Residential prefab house kit from

Here's a nice overview of how passive solar / thermal chimneys work by Al Herbel, LEED AP.
He also utilizes earth tubes and fans to cool the air. Handsome Husband's comment: "This is passive in the true sense of the word - there are no moving pieces. The solar chimney creates a draft up and out, which is replaced by air sucked into the house through the earth tubes which carry relatively cooler (and fresh) air. It's brilliant!"

Cozy passive solar prefab where I can cook up a storm

But wait. WHY am I talking about cooling in December?!? Give me some heat, honeys!
I know I've addressed these issues before - see this post in particular.
But here's more useful information. Please read all latest information available, not just what I find, when trying to make an informed decision on your own home. (Oh, come on, do I really have to say it - "this is for informational purposes only, honeys, are you kidding me you'd actually take building advice from ME?!?")

From - wood stoves, masonry heaters:

From, [which also has an interesting discussion on what it means to be "passive" as well as more information I encourage you to read]:
"It is perfectly possible to have a chimney in a passive house. This one as it says is being built in conjunction with the PHI.

It has 2 chimneys. 1 inside the body, 1 outside.
Both were built using traditional masonry.
The internal one was built with an adapted detail using insulating blocks where it bridges the airtightness\insulation layer in the roof.
The flue also contains 2 valves even though a room sealed is specified. There is no such thing as a truly airtight stove apparently. 1 of the valves is on the air source pipe for the stove, the 2nd in the chimney itself close to the insulation layer.
This chimney is so well designed that it is not considered a cold bridge in the PHPP.
It is easier to detail the chimney (thermally) if it is outside the body of the house. This can introduce structural issues depending on your build method.
If the company that did your PHPP knows what they are doing they should be able to provide a detail."


From an Irish chimney company:
"Chimney insulation/capping - An insulated chimney makes the world of a difference in any build. A warm chimney insulated with vermiculite or rockwool sleeves, not only contains the heat in the chimny and property, but gives a better operation in the chimney overall. A cold flue presents problems such as damp, condensation, heat loss, and in more sever cases, lack of "pull" - the smoke failing to rise up the chimney and resulting in the smoke coming back into the room. Chimney insulation is one of the best investments you will make in your property(s). An unused chimney should be capped and sealed making the flue airtight (leaving a small vent for the chimney to breath) and keeping the air from your radiators, stove, cooker etc in the house and not evaporating into the atmosphere!"
And back to the previous thread,, an RWagner chimes in:

"While I have yet to design a house that meets the Passive House standards, I do regularly put a wood stove into the very low-energy houses I'm doing. I do this for several reasons. We have 9800 heating degree days here, and many days in a row without sunshine, so often our peak heating load cannot be met through the ventilation system only. The wood stove adds an element of "energy independence." If the power grid goes down and you cannot heat the house through the ventilation system (no electricity), or if there is a disruption to supplies of natural gas (potential used for another heat source), the wood stove is there to provide heat at any time. In our cold winters this is necessary. Some clients want to heat with wood to reduce their reliance on non-renewable fossil fuels, and because it is a clean, renewable source of energy. We only use stoves with a dedicated combustion air route but it does add another hole to the envelope and so it does weaken the envelope a bit. I've decided that the trade-off is worth it. Does a wood stove overheat the house? My experience is no, not if the stove is located in an open area to promote heat distribution. There are very small stoves available, and I still prefer this to a masonry heater because the regular wood stove acts like a small radiator and you can more easily control the amount of heat in a short time. I've found that the small wood stoves work beautifully in a passive solar home. Make a small fire when you first wake up, let it die down and the sun takes over. In the evening if it it gets very cold and the passive solar gain doesn't last into the night, make another small fire and that stabilizes the house through the night. Also, another reason I use a more standard wood stove is because in our area firewood is readily available, and many people are practicing good forest stewardship as they harvest and/or sell this local, renewable resource.

A final note based on two experiences: The "Skyline House" that is calculated to need 23 kWh/m2 annually has been staying comfortable all this winter almost exclusively with the use of the wood stove and the passive solar gain. Another house recently completed has higher heating load (not as thermally aggressive in the envelope, but still has a peak load of only about 8 Btu/sf) but is very very tight - tested at 0.04 cfm/sf@50 Pa (0.4 ACH @ 50) and the wood stove with dedicated combustion air route works nicely."

From the same thread:
"Jesper, I have installed four masonry heaters in four superinsulated houses. They work very well. These are Tulikivi soapstone stoves from Finland. They burn kindling once or twice a day, each fire lasting 30-40 minutes. They are closed up the rest of the time (flue and combustion air from outdoors) so the airtightness of the house isn't compromised much. Each stove is sized for the house it goes into. These masonry heaters can be combined with other heat sources, such as a coil at the HRV. One thing I like about them is that when the fire is going, it's going all out. It's never damped down for a slow burn, so it's exempt from EPA regulations because it doesn't pollute. One last thing: After your morning fire you close everything up and this pile of hot rocks just sits there radiating heat all day long while you're off at work with no worries about a fire burning in your absence.

Ted Nickell"

Last thoughts: Farewell To The Chimney?
Read the comments, you even get a Robert Frost poem on chimneys! AWESOME!
Chimney, solar, and Freak Dog
at the off grid passive solar house kit.

The last comment mirrors our thoughts and beginning experience - we build the fire, get it going, then shut down the flue - and it radiates. I personally think our cook stove is perfect, although we certainly will work on the seal, even with the ERV assurance.  The firebox is small, it heats quickly but not overheats. If we wrap the cook stove with thermal mass, we'll aid its longterm thermal mass output even further!

Well, speaking of cooking, here, I am starting to cook, again, in anticipation of the coming snowstorm.
That includes erecting a Gingerbread House, Take 2.
Last night, as we attempted Take 1, the dogs ran off with the walls.


I leave you with Bing Crosby and David Bowie.
Pah-rum-pah-pum-pum, honeychiles.
Little Drummer Boy - David Bowie and Bing Crosby

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At 2/8/12, 8:27 PM , Anonymous chimney pipe said...

Awesome post, I want to check out a couple of your other messages. Thank you!


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