Sound plan for no vapor barrier?

Hi all,
I am building a small 8x16 cabin in upstate NY, climate zone 6 (but on the border with 5). Listed in order from inside to out, the wall assembly will be: 2x4 rough cut hemlock frame (left exposed on the inside), 1x8 diagonal solid wood (hemlock) sheathing, 2-3” of external tongue and groove wood pulp insulation (Timber HP’s timberboard—listed at 40 perms per inch), air gap/drainage plane/rain screen with furring strips, followed by 1” solid wood siding (likely hemlock). There will be 2’ eaves on the load bearing walls and 18” on the gable ends. No house wrap.

There will be a wood stove or ideally a masonry stove (Walker tiny cook stove).

Most likely the cabin won’t see more than 30-40 days of occupancy each year but I would like to be prepared for potential full time living if plans were to change.

Given all the natural materials, would you agree that it’s safe to omit the vapor barrier?

Thank you for any thoughts!

I don't have the knowledge to advise, but I look forward to seeing what other people say. And welcome to Permies!

Welcome to Permies!

It is always good to see another fellow New Yorker. Hello from Washington County.

Hi Gregg,
I will start by saying that I am not a building envelope expert, nor even a professional builder. I do a lot of home projects and I read a lot.

It appears that TimberHP's products are vapor open. And the gaps in between the solid wood sheathing and siding would also allow "breath-ability". I would personally not even add the air gap. Those seem to be added to make sure moisture doesn't sit next to a vapor barrier. If you are going with natural products to allow the vapor to dry to either side without a vapor barrier (a superior way to do walls, I think)...  I would put up the studs -> sheathing -> insulation -> siding, and not bother with the other layers. The airgap and rain screen are needed when building walls with vapor barriers and small to no eaves. It's like mixing two different wall systems, one natural and the other modern.

If those were the materials I had to work with, I would keep it simple.  

Hi Gregg,

I'll also be curious to hear what others have to say.  Here are my thoughts, but I’d check with local builders, describe your wall assembly to them, and ask what they think.  It may be that others have attempted something similar.

The wall assembly as you describe it harkens back to pre-modern era construction methods before plywood sheathing replaced diagonal wood sheathing in N. America around the time of WWII, and somewhat later, when house wrap became a thing in the late 1980s and 1990.  

The big difference is your use of wood fiber board insulation on the exterior, but as it's highly vapor permeable, I think the resemblance holds. The reason many older buildings are still standing is because they were quite leaky--air moved in and out of all those gaps and cracks in the diagonal sheathing and siding and whatever loose insulation filled the wall cavities contained by interior lathe and plaster or wood paneling. All of these leaks allowed water vapor to enter the wall on air currents, and it also allowed it to escape the wall assembly before it could do any damage.  

But it also meant that the structures were energy in-efficient; during cold weather they needed a near-constant supply of heat from wood burning in a fireplace or stove (and later a coal or oil burning furnace) to keep them warm.

Once any wet lumber dries out, the primary source of interior moisture is human activity.  We breathe, sweat, cook, and bath. All of those activities raise interior humidity.  Water vapor moves across a gradient from higher pressure to lower pressure, so if the water vapor pressure outside is the same as inside, water vapor tends to stay put. Think summertime warm, humid conditions inside and out (assuming no air conditioning). If, however, the vapor pressure is lower outside than inside, as during the winter months when it’s warm and humid inside when you’re at the cabin, but cold and dry outside, that moisture will try to escape the building envelope through openings, cracks, gaps, and vapor permeable materials.  
Most water vapor traveling through a wall is moving on air currents passing through gaps or cracks in siding or plaster on the exterior, or interior drywall or wood paneling, and the tiny holes in electrical outlet boxes, and openings around plumbing perforations and vents. Very little moisture actually migrates through a vapor permeable material like drywall or plaster.  

However water gets into the wall, so long as it travels completely through the wall assembly and doesn’t get trapped either by a vapor impermeable barrier or a low enough temperature (the dew point) that causes it to accumulate and condense to form liquid water, no problems.  
If I understand your wall assembly, you have 2x4 studs and diagonal 1x sheathing exposed on the interior, i.e. no cavity insulation or finish material like drywall, lath and plaster, or wood paneling.  

The 2” or 3” of wood fiber board insulation will give you at most R-6 to R-11insulation, but I’ll wager that the building code for your area calls for at least R-21 insulation, and more if you want an energy efficient structure.  

That means the interior side of the diagonal 1x sheathing is likely going to be quite cold in winter—probably cold enough that while you are actively living in the structure the water vapor you generate will condense on it just as it will condense on cold windows.  The many exposed gaps between the 1 x 8 boards will also readily pass moist air to the interior side surface of the cold wood fiber board where it could also condense.  In other words, if it’s cold enough, the dew point may be at or around the interior surface of the sheathing, and water vapor could condense there.  

So long as it stays cold not much will happen—the microbes present on most cellulose based materials are dormant. But if the water is still there when it warms and the microbes wake up, they’ll start eating.  

I understand that you’ll be heating with wood if you’re there in the winter months, and that surely affects on how much water vapor the interior air can hold, and how warm the interior surfaces of the diagonal siding and wood fiber board will be, but I’m not sure how to weigh that variable. It may depend on how cold it is outside, and how warm you keep the interior while you’re there.

If you do a good job of roofing, adding gutters, and installing the siding, and keep the walls elevated above grade so you don’t have snow up against the siding or rain splashing back, you probably won’t have moisture problems with this wall assembly caused by outside water intrusion.  

The only time I think you might create a moisture problem is when you spend time in the building during the cold winter months. Instead of leaving the studs and diagonal sheathing exposed, you might consider adding cavity insulation like rockwool, fiberglass, or cellulose, then sheathing it with drywall. If you mud and tape the edges and seams you’ll have an air barrier, and a few coats of primer and latex paint will also give you a water vapor retarder, drastically reducing the amount of water vapor carried into or onto the exterior sheathing and surface of the exterior insulation panel during winter months. Instead of sheetrock you can also apply wood lath and clay, gypsum, or lime plaster for a similar performing interior finish.

Jim
Many Hands Builders

I really like what Jim said about the dew point. I had forgotten that piece of the puzzle. And the rest of it is really good too.

There is only one point I might not suggest for you. I would use the lime plaster rather than sheetrock. The sheetrock, mud, and paint would create the air barrier as he says, but that is a modern building technique combined with your otherwise natural wall. A modern wall technique works. I don't personally care for all the materials used, but it works. A natural wall structure that can "breath" also works. Whenever I have seen people mix the two, there are problems. Each method makes certain assumptions. If those assumptions are not correct, you can end up with a really big problem.

A lime plaster (which is vapor permeable) on the inside, with some sort of cavity insulation that is vapor permeable, should be able to increase the insulation enough to avoid the dew point, while still allowing the water vapor to dry to whichever side it needs to.

I agree Matt, it can be tricky mixing modern building materials and methods with more natural materials.  I revised my post to make it more clear that a plaster could be applied to the interior instead of sheetrock. The plaster itself won't increase the wall's insulation value, but a 1" thick plaster of any kind offers roughly twice as much distributed thermal mass as 1/2" sheetrock, which would help keep interior temperatures stable. After reading Matt's post I realized I forgot that you could also use cotton or wool batt cavity insulation. If you use commercially available products both will have been treated with a flame retardant and insect repelling material like borate, but are easy to work with.

Again, I think you'd need to insulate only if you're planning to use the cabin during the winter, or the "heating season," however long that lasts in your area. There are probably millions of rural vacation cottages and hunting cabins in North America that fit this description--they can last a very long time with no moisture damage to hasten their decline. These structures often see very little winter-time use, so when it's freezing outside, it's probably freezing inside as well, and the vapor pressure (humidity) is similar inside and out, so there's no vapor drive.  It's only when an interior moisture source (human occupation) and heat enters the picture that moisture mechanics begins to work against a poorly insulated, poorly air sealed building by driving water vapor into the walls where it can become trapped and condense against a vapor barrier or the dew point.  If you're there for a winter visit and you see frost on the interior surface of the diagonal wood sheathing and water dripping down the interior of the windows pooling on the window stool...that's the moisture you're generating trying to get out and running into either a vapor barrier (the window) or the dew point (the very cold surface of the inside of the wood sheathing, and probably also the wood fiber board).  Will a weekend-long winter-time visit cause irreparable damage?  Probably not. Water eventually finds a way out of vapor open wall assemblies. But a month-long winter-time visit may drive so much moisture into the walls that it overwhelms the dispersal capacity.

Jim
Many Hands Builders

You definitely want the rain screen, just make sure the gap is screened for bugs and mice at top and bottom. It creates a calm air space so wind driven rain stops and gravity takes over. The gap is good for a minor r value boost as well. All wood siding should have a rain screen.

If you want a little less permeable, old fashioned tar paper as a backer to the rain screen will still breathe between the seams but stop the wind pressure. Where I am at I would need that, but a cabin tucked in the woods probably doesn’t see that much wind actually hit  the cabin.

Hitting dew point in the wall is bad, that is why they thought plastic vapor barrier was a good idea. New technology has smart vapor control layers that change their permeability based on conditions and are one way—they let vapor out but not in. Really cool stuff but most assume a powered HVAC system to deal with the majority of interior moisture.

Look up vapor diffusion ports. It’s a southern thing but the ideas can translate to the north. Basically all interior vapor in a low permeable house eventually rises to the ridge, so you build most of the house zero perm and then spend a the money for the fancy smart barrier only at the ridge.

Thank you all for the extensive feedback. I am thinking I could increase the amount of external insulation without great cost. They make the insulation boards as thick as 9”, so I could potentially go to something like 5” of insulation, which would get me up to about R18. The only issue is I’m trying not to increase my footprint past 144 sq ft since below that I don’t need a building permit. 5” would get me right to the edge of that limit, I think.

Hi Gregg,
For what its worth, most places measure the square footage by the inside footprint. So adding more exterior insulation would not affect the square footage in all the places I have ever lived.


**Edit - For future readers, it seems my experience is the minority, and that most places actually measure the exterior dimensions, NOT the interior.

I am a builder by trade and my first thought is why omit house wrap?  It is cheap and fast to put on, an easy way to add one more layer of protection to your investment.  I know it gets a little extreme (I am an extreme person myself and can appreciate the outliers) on here with wanting to never ever under any circumstances use any synthetic material ever for any reason, BUT there are good reasons that building technology has developed the way it has over the last century and it would be foolish to not take advantage of some of it some of the time.  Use the 80/20 rule and try to use the synthetics where people won't come into contact with them (house wrap being one of those applications).

I agree with others here that cavity insulation would be a good idea with whatever interior cladding method you decide to use.

Then again, if you are just looking for something only slightly more habitable than a shed for occupancy less than 10% of the time, and not in colder months, then build it cheap and fast and don't worry about it.

Have you considered going underground?  I just read the Mike Ohler book "$50 and up Underground House," and from that book it appears that semi-underground cabin construction would be cheap, and fast and you don't have pesky things like house wrap to deal with (just pond liner which is non negotiable in that situation).

Hi Gregg,

If you go that route, you'll still have water vapor moving into the wall assembly during the heating season since the interior surface (diagonal plank sheathing) has lots of gaps. Most of it may get no further than the interior surface of the fiber board where it will hang out until it disperses back towards the interior or works its way through the fiber board as conditions allow.  

Also, my understanding is that permeance ratings are usually given for a given thickness of the material, often the standard application thickness.  For example, 1" of clay plaster has a perm rating of between 15 and 20 US Perms.  2" of clay plaster has a lower perm rating, and 3" even lower.  If the 40 perms for the wood fiber board applies to the 2" or 3" thickness that you mentioned, a 5" thickness will likely be lower.

Finally, 1" wood siding is pretty heavy, and it will be cantilevered from the wood studs.  Wood fiber board doesn't have nearly the holding strength as solid wood, so I'd think of the furring strips as spacers only (to provide the rainscreen air gap) and not rely on them to affix the siding to. That means to attach wood siding over any thickness of wood fiber board you'll need to use screws long enough to capture the siding (1"), furring strips (1/2"), the fiber board thickness (2" - 5"), the diagonal plank sheathing (1"), and embed deeply into the wood studs.  A 5" thick insulation panel might require screws around 10" long.

A work around is to oversize the structure's foundation so it supports the furring strips resting on a sill plate that is external to the sill plate you'll see from the interior.  If the furring strips are beefy enough, say 1 1/2", you could secure the siding to these.  There should be a top plate for these furring strips to anchor to as well, connected to the underside of the rafter or truss tails.  

Whichever method you choose, as mentioned above, make sure air flows through the cavity, and that it's screened to keep insects and rodents out.

If building a wall around a wall or driving 10" screws through four layers of material sounds a little complicated, you might revisit the option of insulating the stud wall cavities and installing some kind of interior vapor retardant finish (plaster, drywall, wood paneling). The technique of wrapping a building with external insulation was initially meant to reduce thermal bridging at each stud while providing a better air barrier, not as a building's primary insulation layer.

RE Matt's post about square footage.  Perhaps it has changed in very recent years, but in my experience most building departments measure square footage by exterior footprint. I hope that changes because it penalizes those who opt for thick wall construction methods, which if done well, are usually more energy efficient.

Jim
Many Hands Builders

RE Matt's post about square footage.  Perhaps it has changed in very recent years, but in my experience most building departments measure square footage by exterior footprint. I hope that changes because it penalizes those who opt for thick wall construction methods, which if done well, are usually more energy efficient.  

Interesting. I am coming from the side of dealing with small towns and whether you need a permit for a building or not. And many of them say if its less than 100sqft, you don't need a permit. And the ones I spoke to were talking about usable space, because a 10x10 with 2 stories was not allowed. I just assume it is the interior space when I look at real estate, but I don't really know. I think it would be important for Gregg to confirm whether it is the exterior or interior square footage.

Matt,

I too worked mostly in a rural area with small towns, but my colleagues throughout the U.S. West report square footage for permit requirements measured as exterior footprint.  

A few relatively forward-thinking building code jurisdictions in California (e.g. Sonoma County) will permit a straw bale building with 18" or 24" think walls as though it had 6" thick walls--same as most conventional residential construction.  The rationale is that builders who construct super-energy efficient homes that store carbon using the lowest embodied energy material on the planet shouldn't be penalized.  They're not actually using all that wall thickness for living space, and they're doing the rest of us a favor. I have heard of builders and homeowners elsewhere getting an exception like this for thick-walled construction (interior square footage plus 6") but it's far from universal.

RE exceptions for structural permit requirements. I believe that Jackson County in S. Oregon is typical for the rural U.S.  There is no requirement for a building permit for structures under 200 sq. ft.  Most county planners and building code officials don't care if you plop a 200 sq. ft. or smaller shed on your multi-acre property to store tools, animal feed, firewood, etc. However, should those buildings be used for living space with electricity and plumbing, in Jackson County they require a structural permit and both electrical and plumbing permits.  I understand that some rural counties in California have a 400 square foot agricultural building exception.  

I'm guessing that more urban areas are much stricter about this. Lots are smaller and any size structure could have impacts on neighbors or block egress for fire fighters, etc.

Jim
Many Hands Builders

Yes, very good points about supporting the siding…I looked up the town’s building permit application just to double check and it says that buildings (no reference to accessory or otherwise) “with a gross floor area of 144 sq ft or less” don’t need a permit.

I tried looking up a definition of gross floor area in the NY state building code, but couldn’t find anything. I’m guessing it’s probably inclusive of the exterior walls. I left a message for the tax assessor to see if they have the answer. The code enforcement guy didn’t get back to me.

As far as using the house wrap, I’m on the fence. I feel like buildings lasted a very long time without it for so long…doesn’t that mean it isn’t necessary? Especially with vapor permeable materials, large eaves, and a rain screen? I’ll keep thinking about it.

And I would add just a couple more finishing thoughts:

The land where we are building is 3.5 hrs away and I have a 5 and 7 year old—so the time I get to take to go work on the place has been sporadic. I get maybe two full days of work done every 6 weeks. I was able to finagle a week away this spring and have one more week this summer. So I’m hoping to just get the walls up and sheathed and a roof on by the end of the year (have the two load bearing walls up already). Then next year will be windows, doors, insulation and siding. So all that to say that I’ll probably do the external insulation first (most likely not more than 2-3” since I’m already at 128 sq ft without it and seems likely that the exterior wall measurements will be included in the gross floor area) and see how it goes and then I can add cavity insulation/vapor barrier after the fact if our use or discomfort justifies it.

I appreciate the detailed comments from everyone, hope the discussion has been helpful.

Oh, and yes! I had gotten excited about underground construction and have also read the book you referenced…our soil is super rocky, high clay content and there is a lot of moisture…basically, I’m intrigued but the learning curve seemed too steep for a first project…might work as a second small cabin later on down the road.

Good that you checked around Gregg,  It may be different everywhere, but I expect most jurisdictions issue permits based on exterior dimensions.

I have never used house wrap, but then I didn't do much conventional construction--almost all of my work was on straw bale or straw-clay buildings, or structures insulated with blown-in cellulose and sheathed in plywood.  All of these were plastered on the interior and exterior with lime and/or clay plasters.

I understand there's a vapor membrane used in extreme cold climates on super energy efficient structures.  Jacob Deva-Racusin at New FrameWorks in Vermont was telling me about it--can't recall who makes it....Mento?

The only "paper" I have used is the 2-ply paper (top layer is asphalt paper, bottom layer is a dimpled or wrinkled vapor permeable synthetic membrane of some sort) commonly used for lime or cement plasters. The paper is only 39" wide so it's lapped in 3' horizontal strips over sheathing--not really an air barrier. But that, combined with a well detailed exterior stucco and carefully applied interior lime or clay plaster did a pretty good job, along with air sealing electrical and plumbing perforations, gaps and cracks around windows and doors.  

The handful of my projects that were blower door tested had pretty good results for not trying that hard--between 1 and 2 ACH (air changes per hour).  For readers not familiar with this metric, it means that the air inside the home that we spend money heating or cooling doesn't escape the home very easily through leaks in the building envelope, only to be replaced by outside air that we need to heat or cool.  The Passive House standard is .6 ACH.

For reference, the average Oregon home built in the 1990s and early 2000s tests at 7 ACH, which is pretty leaky.  I'm not saying that all buildings made with natural materials are inherently airtight.  I have also been in many straw bale and straw-clay homes that had blower-door tests much the same or worse than 7 ACH.  They often still felt comfortable because the distributed thermal mass of interior plasters moderates temperature fluctuations, but they weren't nearly as energy efficient as they might have been had more attention been paid to air sealing.

Many in the natural building world would prefer a leaky building to installing some kind of mechanical ventilation system.  It is more natural that way, but also wastes a lot of energy. I liked to make buildings as tight as possible, and prepare for if not install a mechanical system like an HRV (heat recovery ventilator) or ERV (energy recovery ventilator).  That way, people who initially didn't want a mechanical system could more easily add it later, or they could just crack a few windows if they felt the need for fresh air.  At least they have some control.  Leaky buildings always just leak.

Jim
Many Hands Builders

To my pleasant surprise, I just got a very quick (and favorable) emailed response from the NY State code department people, in case there are other New Yorkers reading this. The language could be easier to understand, but I’m pretty sure that the area “within the inside perimeter of the exterior walls” must not include the thickness of the exterior wall itself.

Here’s the response as it was written to me:
Thank you for contacting New York State’s Department of State Division of Building Standards and Codes (DBSC).

Your question (paraphrased):
How is gross floor area defined?

Our response:
As per the 2020 Building Code of NYS, Gross floor Area is defined as:
[BE] FLOOR AREA, GROSS. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, ramps, closets, the thickness of interior walls, columns, or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.

Thank you,

Keith G. Burger,II  Assoc. AIA
NYS Code Enforcement Official, Division of Building Standards and Codes

Although I suppose if I’m trying to think like a lawyer,
this definition is only relevant to those who are in a situation where “gross floor area” and not some other term, like square footage, is at issue.

I know I'm a little late to the party, but I have nothing to do for a few minutes and I am a licensed residential builder in two states as well as a building code guru for a major city in Arizona.
So as far as building codes and what is required to have a permit, I can say this.
Most U.S. municipalities that have an adopted building code have adopted the International Codes (one for Residential and another for Commercial buildings). The base code exempts buildings from permit based on "Floor area", which is defined in the code the same way NYS has done it.
Huge caveat: Every municipality (could be the state, a county, or even a city or town) also amends the base code and changes sections to suit their own purposes, their environmental conditions (political and seasonal), and their history in the art of building. ALWAYS check with the local jurisdiction you are building in to see if they have an adopted code and what their amendments are. A federal court ruling a few years back said that any municipality that adopts a building code by way of ordinance MUST publish their amendments free to the permies. They cannot charge you money to get their amemndments. You do not have to pay to know the law. However, the court did not say they had to publish the base code, because that is copyrighted material owned by the International Code Council.
These amendments are usually available on the internet.
Next caveat: There is a little section in the base code that basically says even if it is exempt from permit, it isn't exempt from code requirements or any other law of the jurisdiction. (Section R105.2 for those of you who happen to have an IRC code book handy).
So, even if the structure is exempt from permit, you still have to build it to code requirements, at least technically speaking. Is anyone going to serve you a violation notice because you didn't add that vapor barrier? Probably not. It depends on whether the local enforcement is the Barney Fife type.
Yet another caveat: The permit exemption in the base code is for "one-story detached accessory structures". The structure is supposed to be an accessory to a primary use structure. Again, check the local ordinances and building code amendments, but if there isn't already a primary structure, that little shed isn't an accessory to anything and isn't covered by the exemption. This is typically not a building code issue. It's usually more of a zoning/land use issue.

Now for something actually helpful......
There is a group of master craftsmen out of Latvia (northern Europe on the Baltic sea) who build using very traditional, old-world methods.
I love these guys. Here is a video of the owner of the company, building his own house. You will see how he did the walls, roof, foundation, everything.

https://youtu.be/RV7pmE4MC-I?si=mWc8xZLi50ZmmTXY

There is also another fellow on YouTube who does some great small buildings using traditional methods. His foundation work is pretty basic, but incredibly strong.
https://www.youtube.com/@MrChickadee

Keep in mind, R-values are designed to make fibrous insulation look good.  And to do that they make a bunch of assumptions that often are not true.  Two of those assumptions are that there is no air movement through the insulation and that the insulation is dry.  Both of those assumptions require it to be part of an entire wall system that stops air and moisture movement, which is what the vapor barrier is for.

If you don't have a vapor barrier air will be able to move from inside to outside and in some situations that air may also carry moisture.  Without a vapor barrier your actual R- value may drop substantially.  I don't know how much air can get through your fiberboard, it would probably work better than fiberglass in a situation where you don't have a vapor barrier.

I have worked on lots of old leaky houses.  Insulating them without also sealing them up usually doesn't improve their heating efficiency much.  Sealing them up without insulating them often is better.  And sealing and insulating them is the best.

Often old buildings can not be easily retrofitted to have a plastic vapor barrier installed, so it is often done with calk and a thick paint to act as a retarder instead of a barrier.  In areas where the wall covering and ceiling needs to be removed for other reasons a vapor barrier is usually put up before new sheetrock gets installed.



Now, to show what a hypocrite I am, I am in the process of building house with no vapor barrier on the main floor.  With that said, I am going into it knowing it may be a problem, and if it does end up being to much of a problem I have a plan to retrofit it and put a vapor barrier and insulation in from the inside in the future.  I am putting vapor barrier on the roof and loft walls.

Hi J,
That is one of the most succinct explanations of vapor barrier I have come across. Thanks.

I will point out though, that vapor barriers can also cause problems if there is an issues with the wall system. At some point, something will fail, we all know that. I want the wall system that is the most resilient and can bounce back easier when something fails. I feel like not having the vapor barrier is more resilient, while maybe not being as insulative.

Vapor barriers.......we could spend the next several years discussing the pros and cons.
For years the building code was hyper-focused on making new construction as tight and impervious to air movement through the building envelope as possible. They did it in incremental steps, adding new requirements every code cycle (3 years). Nobody ever thought there was such a thing as "too-tight construction". The train of thought was always, we also require minimums for natural light and ventilation, so people wil naturally open some windows and refresh the house air.
Well guess what?
It turns out that most people don't do that, like ever. Suddenly, "stagnant air syndrome" became the hot topic. People were actually getting sick from stagnant air inside their homes.
This is where the funny part comes in. They curently require a specific level of air-tightness and make people do this blower-door test (someone above mentioned it) and the house cannot leak past a certian level. BUT, they also require mechanical ventilation to produce a specific amount of air changes during a given day.

So, OK. I get it. We build houses to be super tight and not leak air. Then we cut holes in the building envelope and install electric fans to bring outside air in and push inside air out automatically.
The ironic part is this is in the "Energy Conservation Code".......uh huh. Right.

I appreciate what you're saying Joshua, it's frustrating.

People were getting sick from a number of things. Some buildings were tight and the finish materials were off-gassing into the interior air space.  Other buildings weren't tight and water vapor carried on the leaking air condensed inside the walls and mold grew. I have been in poorly air-sealed buildings on sunny, windy days and noticed dust clouds dancing in the interior air. It was bits of fiberglass dust sifting through the T&G wood ceilings, which had been installed without any kind of air barrier between the wood ceilings and the insulation.  Imagine breathing that air for a generation or two!

The mechanical ventilation systems recommended for airtight buildings are called Energy Recovery Ventilators (ERVs) or Heat Recovery Ventilators (HRVs) and are designed to exchange the heat from the outgoing air so you're not bringing in cold or hot outside air which you then need heat or cool. Energy Recovery Ventilators also exchange humidity.  Extreme cold climates generally use HRVs, but a quick internet search offers several sources that offer guidelines for which one is best for any given location.  A relatively inexpensive and easy-to-install (in new construction) HRV unit from Panasonic works well for small homes.  It cost around $600 in 2022.  Placed in the central area of a home--say a kitchen-living area with bedrooms on either side--they do a pretty good job.  

I'm set up to install an HRV here, but for now my wife and I are running a portable HEPA air filter whenever we close all the windows. Our straw bale house has R-30+ walls and R-50+ ceiling insulation.  I haven't had a blower door test done, but since it was the first straw bale house I built I wasn't paying as much attention to air-sealing as I did during my building career. I'll guess we'd blow at around 3 or 4 ACH.  Could be better, could be worse.  On hot days when we closed the windows the space stayed cool but the air seemed stale; the HEPA air filter changed that.  It does nothing to address moisture (neither does an HRV). To help expel moisture we try to remember to run the kitchen range vent when we cook, and also the bathroom fan when we shower.  

But as you said, people forget to open and close windows, or turn exhaust fans on. Still, I'm not too worried about moisture inside our home. We have 1 1/2" thick interior clay plaster that functions as a hygric buffer--absorbing and releasing interior moisture. I also office in a straw bale woodworking shop where a hygrometer hangs on the wall.  In fifteen years the needle hasn't moved much.

Jim
Many Hands Builders

I don't want anyone to take me wrong. I am not dissing the building codes. I have made a pretty good career out of building by them and for the better part of the last 17 years, enforcing them. As a building code official in the 5th largest city in the USA, it's been an interesting ride. When you have participated in as many code adoption hearings, code ammendment hearings, and building code classes as i have, you can get a little jaded. Sometimes it's like watching the dog actually catch the car......

That being said, ERVs and HRVs are great ideas, but they aren't what the code requires. You could literally install two ducts at either end of the house. One has in internal fan set on a timer to blow outside air in and a damper that opens when the fan pushes air. The other duct is just a plain duct with an output damper. Once the input fan pressurizes the house the output damper opens and closes when the input fan shuts off. Voila, mechanical ventilation.When this requirement came out in 2012, I saw dozens of plans by production builders that simply had a pggyback unit on the HVAC that did just that.

It's good to know that some builders out there are doing it with energy conservation in mind.