Inspired by so many discussions here on natural building and breathable walls (and so we are all on the same page, by "breathable" I mean vapour-permeable), I designed a breathable wall system for our new house that is somewhat unconventional around these here parts. Since we were going with conventional stick-frame construction and had a skookum crew of trustworthy and talented builders, I wanted it to be simple enough that time would not be wasted training anybody on novel techniques.
The end result is a stick-frame wall with Roxul mineral wool insulation on either side and NO vapour barrier! We had to get an engineer to sign off on it, since here on the rainy West Coast of Canada, the industry is currently involved in an intense love affair with vapour barriers and trying to keep moisture out of walls (while living in what amounts to a cold sauna, ha!). And when the framing crew arrived, I received much praise on my design. They said they'd seen nothing but trouble with people relying on vapour barriers, and predicted that the hangover would soon prompt a change towards more breathable homes (and not, instead, on costly mechanical dehumidification systems).
The exterior insulation will be attached in two layers. The first involves vertical 2x2 furring strips with 1.5" of Roxul comfortboard in between. Then a second layer of furring strips will be attached horizontally, with another layer of 1.5" Roxul comfortboard placed between those. The screws for the furring strips will be placed such that no one screw penetrates through both layers, so the only thermal breaks will be a 4 inch deep 2x2 bit of wood. On top of that will be a layer of rainslicker rain screen material, then our vertical board-and-batten siding (which will be secured to the horizontal furring strips).
As a former medical research scientist and current stay-home mum, I have zero experience in building, so I am particularly proud of this wall! I also want to give a shout-out to Jay C. White Cloud, who patiently answered my endless questions while I was coming up with the design.
Here is a link to my blog article about our walls: https://freelearners.wordpress.com/2016/03/19/breathable-walls-using-conventional-construction-techniques/
"breathable walls (and so we are all on the same page, by "breathable" I mean vapour-permeable)"
Hi Mariah, no I and other pros are not on "the same page" that is only part of the definition of "breathable" walls and why we have so many issues and a big lack of understanding in mainstream construction. There are two other important vapor transport parameters that define a three axis system, an air change through the wall and radon mitgation system, and enthalpy, you can read about by searching my "breathable Walls" thread where there is lots of proven data and interpretations, and a review of a awesome book by Guru George Swanson's "Breathable Walls". If one does not understand this term in it's entirety it can lead to design issues over time especially in wet climates.
It sounds like you are getting your rain screen from a product "rainslicker"...the best are created by the right size air gaps behind the siding on vertical furring strips that are continuous through the roof vents. The best way to break thermal bridges is with IS rigid boards between the studs and furring strips as outsulation, no vapor barriers or retarders needed. You can find proven data on it on my "Air Sealing Buildings" threads by a university professor and PHD named John Straube and Engineer named Joe Lstiburki both from Building Science Corp. It shows just about all house wraps or smart vapor retarders are not as effective or provide enough air gap/flow for vapor drying and water resistance despite the mfg claims and limited testing. What is really boils down to is, air flow in CFM to allow drying your product may not provide or should be tested at installation with a manometer. In your case, you'll still want to make sure your interior relative humidity does not exceed 35-40% so drying can occur in that direction if nothing else, if it does exceed those values naturally you know your walls are failing by being saturated past their max water/vapor holding capacities, and your cooling loads will show it. This has little to do with microbails growth, that is another subject based on matted material reactions, in this case the best design has no other mating materials but mineral wool and pure wood. It may be a good to have a back up plan to have provisions for an active whole house dehumidifier if it is not too late. Finding you need one after construction will not be easy. Also, if your walls do fail go after the engineer that probably(since you did not mention it) put his license on the line without the proper data like a CFD model using WUFI, etc...but rather guessed which is practically impossible, backed by errors and omissions insurance you can file claims against. Mineral wool does posses a formaldehyde that is not compatible with some materials and acids. There are cases of black mold. Jay is not the best source of info here.
It sounds like you have created a horizontal and vertical lattice grid of furring strips infilled with IS boards attached by fasteners to stick framing. The only issue I see in that I think you mean above in thermal bridging not thermal breaks? Your engineer should have been able to calculate a u-value. 3" of IS Board is R-12, I hope you have another R-23 batt to add to that in CAN.
But yes, for the most part way better design than most of mainstream or DIYers including earth constructions seen here on permies.
I really like what pro SIRE (Rammed Earth) has done in wet-cold CAN with hygrothermal mass, I am looking closer at that now along with magnesium cements. I use IS boards all over my portland cement foundations and walls now which is way too expensive you can see here(http://www.permies.com/t/52818/natural-building/Craftsman-Home-Plan-Review) , I hoping to get rid off soon, although this MGO cement needs very little due to low u-values depending on design. I'm working with a chemist and university to develop that, get rid of imported mineral wool and lower cost.
Kudos on looking into moisture movement in wall construction and dew point location in the wall.
A question and food for thought
What are you planning for interior wall finish? The reason I ask is if you are planning on painting the walls with readily available paint, my research showed that after maybe 3 coats of paint, the wall is close to vapor impermeable. So, if there is 1 coat of primer, and 2 finish coats, which is normal, changing the color with 2 coats of paint runs the risk of seriously lowering the permiability of the wall. Almost like glueing a vapor barrier on the surface of the wall.
I have spent a good deal of time talking to the big paint manufacturers and found normal paints and primers have a perm rating of around 4, more natural, old world looking paints can go to 12. Drywall can be in the 40 to 50 range, mineral wool in the 30's. So paint deserves a good look in order to not seal the wall from the inside. I used a mineral primer called Roma, which has perm rating close to drywall, and clay plaster as a finish. You can have the Roma primer tinted and used as finish coat (flat sheen) on exterior walls, and multiple coats do not appreciably reduce the perm rating of the wall. Natural clay is also highly permeable, and moisture regulating (I used american clay).
I have knowledge and training in the passive house technology, and have been researching, and modeling wall constructions, and have built a passive house and am working on designs for more.
It looks like you are planning for your wall to dry to the outside, which is ok, provided that all the applied materials allow this. It's good that you are using a rain screen. My question is this: is the rain screen material vapor open? Will it allow moisture to move out of the wall into the rain screen?, or seal the roxul? I built a rain screen cavity of 3/4" min, vented top and bottom to allow good air flow to dry cladding, prevent capillary action, and keep moisture away from exterior wall surface.
My wall is designed to dry to the inside, air sealed from the outside. My climate is different, but perm ratings of materials and finishes are still critical. It is also critical to keep as much moisture out of the wall as possible. We can design a wall to dry either to the inside or outside, given environmental conditions, moisture may move in unplanned directions, it's good to keep that in mind.
You mentioned condensation on windows, that speaks to dew point on interior side of window, same as why one would put insulation outside of exterior sheathing like you are planning. The problem is simply cold surface meeting warm interior moist air and resulting condensation. Dehumidification will reduce moisture in the air, but won't fix the underlying cause. Better windows, triple pane, with better air seal and insulation value will have an amazing effect. In my current project, 5 degrees outside, 70 inside, 40 % humidity, triple pane passive house cert windows, with an ERV - absolutely no condensation! I see condensation in homes in my area with leaky, poorly insulated windows.
Are you taping or sealing plywood seams to prevent air currents and subsequent moisture in the insulation cavity?
Did your engineer model the wall design?
I think what you are attempting is great, just want to bring up some points that I have seen overlooked. I don't like vapor barriers either, and wouldn't want you to end up with a vapor sandwich after going thru your design process.
I was aware that latex paint is a vapour retarder, but my understanding was that it was a pretty poor one. However, I did not consider the effect of multiple coats of paint. I will definitely look carefully into our choice of primer and paint to ensure I don't end up overdoing it on the inside walls, thank you for that tip.
I understand that under certain conditions vapour may move from outside to inside and I'd like the walls to be breathable in both directions, although our summers are pretty dry and high humidity occurs almost exclusively in the cool season.
The rain screen is Slicker (material looks a bit like a plastic dish scrubbie) and yes, it is vapour permeable. And it will be open at top and bottom as is required by our code (nearby Vancouver, my home town, was home to the "leaky condo crisis" of the 1980's, which I think birthed the rainscreen industry!).
As for those nasty windows, I was referring to our current housing situation which is a 30 year old mobile home that is deteriorating pretty quickly (when we bought this property it was supposed to be "temporary housing" but it took us longer than anticipated to start building the new house). The windows are the old style double panes where each pane opens separately and there is just a big air space between them. I'm quite sure the seals have long gone bad, which probably explains the condensation inside. Our new house has fantastic windows.
As for the plywood seams, no we are not taping them. I confess the topic of sealing a house airtight is where my opinion departs from the current green trends, and I've had many a lengthy discussion on green building forums about this, lol. We are using house wrap as an air barrier, but we are not going to any extent beyond code in that regard. I'm pretty stubborn about not wanting an airtight house, but I can afford to hold that position because of our mild climate and short heating season!
You asked about whether the engineer modelled the wall system. I'm not sure I understand the question. He provided detailed drawings for the building inspector so that it would get approved. But I specified the structure and our builder specified the attachments (how it would all be put together). Roxul, the manufacturer of our mineral wool insulation, has already approved the use of their product in this fashion and confirmed that when done so properly, no vapour barrier is needed. So really the engineer's job was just to confirm for our building inspector that it would work the way we said it would.
I too do not subscribe to drying in one direction since the hope and theory is that vapor will move to a lower humidity interior. I also do not subscribe to moving a dew point to foam that is not inert, I have yet to see the dew point people claim measured and an actual location. An HRV/ERV does not stop microbial growth or suck it out of wall cavities or foam wraps, nor other toxic solid particles floating around or attached to materials. Another industry misconstruction. IS board is far superior to foam, not just perm rating, it also has 70xs better compression for under slabs, and much higher deflection, but as I said not cheap and as available yet. Once rigid foams cracks that becomes a real issue water and vapor trap, rot and fungi.
WUFI or CFD(Computation Fluid Dynamics) computer modeling is good but not completely accurate, some predicts mold that never exist, it tries to predict based on moisture content where mold risk are, however, it does not understand all the ways that microbials grow. WUFI Plus is a good product with the right person running it like an engineer with a fluid dynamics background. At the end of the day, a trained experienced eye will be needed to interpret data, WUFI will tell you that. In most cases, most codes, the inspector can opt out of a vapor barrier or retarder requirement with proven data, or, code allows a third party inspection.
Breathing in all directions should equalize the vapor pressures so that drying can occur in any direction, that will depend on anode-cathode. I did not see the blog in this case but, I think they plywood bond glue or bond line will be an issue for microbial growth since even a urea formaldehyde is not inert. I definitely would not air seal the seams with another toxic glue line in this or any case so that the glue lines can dry out to the rain screen, that is IF moisture can get past the edge seals/glues. That will depend on how well they vapor diffuse in the transverse direction along the grain, not the factory tested direction through the glue. Another misconception is glue or bond lines never thermally or structurally fatigue over their live cycle. I can't wait for someone to check that .6 passive house cert in 25 years to find it is now 5 ACH and all the glue lines have to be replaced.
The 1x 3/4 gaps means little unless CFM is verified. This gap does not prevent capillary action, flashing or a capillary break does that, if wicking of liquid water does occur up or down a wall or roof material, or condensation occurs, the CFM through the gap will phase change from a liquid to gas and dry IF it is functioning properly.
A credit card thick American Clay is not going to do alot and it is expensive. You can make their product with 30% kaolin clay, 60% magnesium bentonite clay, and 10% sand. 2" is an average, 4" better, just depends on the holding capacity or MC(Moisture Content found in some data sheets). You can get just about as much hygrothermal mass with a thin hot mud coat of gympsum, or better lime, or a lime clay mix, best a magnesium plaster. Add kaoline clay to get the sheen you want more gets more sheen and workability. AC product I had issues with dusting that tells me they are using a high sand or kaoline clay content or filler to cut cost. Kaoline is not permeable and has no moisture holding capacity. Unlike lime that gets hard as rock clay will break down over time and is not as abrasive unless it is modified. You can also paint their product with a mineral silicate paint despite their claims that you have to buy more product to change color. That has little to do with breathable whole walls and is more of a surface vapor storage systems that phase changes.
Kevin, where are you getting the tripple panes? So far I have not found enough of a u-value difference between them and double for the money. Some want $1000 or so more for a 20% increase in u-value I don't see doing alot or worth it. I think as more window companies start offering them anywhere in the US and not just cold climates competition will drive cost down to reasonable. In the mean time put the money into the walls.
Roxul has not had a presence in the US long and is not that familiar with our building practices. Brick and mass dominates Europe where they have thier roots, US insulation and lots of unnecessary layers that don't mate well. USA its all about perm rating, dew point, and r-value very limited tribal knowledge. US really struggles with air flow in stud cavities and WUFI has simplified that flow on my Air Flow thread I discuss that. I pretty much ignore their details and use my own models and knowledge. My stick design is 100% breathable using just mineral batt, IS board, and wood, no glues or vapor barrier, retarders, house wraps, plastics, foam, anywhere, less is more approach. In humid climates I add a natural plaster to the interior. I'd use ForeverBoard MGO Fire Rate board but the cost is too high, no plaster, just a ROMA or Hiem paint. The salt content and compatibility gets tricky here I'm working on with a chemist and a MGO shotcrete is being tested next week. Hiem or ROMA no longer warrants their paints on mag boards due to the junk from China. I have some Magnesium Oxide coming to develop the same effect cheaper but not liking the shipping either, it will be a magnesium chloride or phosphate plaster. Next a cement for my foundation with much higher insulation and strength not needing the IS boards. MGO cost about 3x more than lime and portland cement but you need 3X less to get the same or better properties. Nevada is the only place it is mined, little manufacturing, I think maybe one other location out north east, were as we have alot of lime mining and manufacturing. The CO2 omissions are less too. Rome had alot of MGO building's and plasters that stood for centuries. It has an affinity for cellulose aggregates to bring up it's insulation not needing costly IS boards. It also dries in 24 hours not slowing construction, needs no wet curing, great as a binder for wood chips, straw, hemp.
Terry, you are correct as usual, but sometimes your tone is a bit abrasive for those who don't know that you do have the best of intentions.
A breathable wall assembly transports moisture in both directions in order to accommodate drying in all seasons. Drying is important because no builder, myself included, is perfect and we get a little water in our walls sometimes. However, as Terry points out, we want to limit vapor permeability and have a nice repository for temporary excess humidity inside the house. This is so after you shower, cook, etc. and RH (relative humidity) increases, you need a place for that excess moisture to be held up and reintroduced into the home if RH decreases in a timely fashion or transport the excess moisture through the assembly. If this does not happen and RH stays high within the home, then mold will grow on the walls surface.
The other point Terry makes is that enthalpy(stored heat energy) moves within the moisture and too high or low of a vapor transport rate will negatively affect the energy efficiency of your home. I'm going to leave that there since it gets extremely complicated and I don't fully understand all the complex interactions that can occur within the wall assembly of an actual home being lived in.
My first choice for interior wall finish is a 3 coat lime plaster. The first 2 coats are the repository and the finish coat is burnished or soap scummed in order to reduce pore size and reduce imperfections in the matrix of the CaCO3 crystals which reduces wet diffusion through the matrix. Most people just can't afford to pay all the labor for this ideal wall, so they end up with a compromise. I feel that lime, clay and gypsum can be mixed with sand, pumice or crushed carbonate rocks or shells to create a plaster that has the mix of qualities that best suits your particular biome, which can be applied over common 1/2" drywall. As Terry pointed out, gypsum breaks down in the presence of moisture; not the first time but over a long period, the crystal matrix of CaSO4 breaks down and wet diffusion is no longer possible. I have found that the paper on drywall actually resists long term moisture better than the gypsum core.
The real dilemma here is to reduce the size of our homes so we can afford to build it right, with the best, most natural materials so we have healthy, energy efficient homes that reflect our relationship with the land that we live on/from.
Terry, I have read many articles by John Lstiburek. In fact, it was his articles that inspired my slab design: we put 3" of Type II EPS rigid insulation under the slab and then another 2" of XPS around the slab perimeter. You can read about that on my blog here. https://freelearners.wordpress.com/2016/04/10/insulated-concrete-slab-foundation/
I am not surprised that I'm using the word "breathable" in a way that is not used by the industry. Perhaps I should just stick with "vapour permeable" though the former has a nicer feel to it. Some of what you talked about went above my head, but I can address a few points nevertheless.
Summers in our local microclimate are very dry, much more so than when I lived just across the Salish Sea in Vancouver. So I anticipate that my walls will have a long time to dry off in-between winters which I hope will prevent any potential issues with mold though I don't expect any. Our dry summers is reason why I don't anticipate many situations where outside humidity is higher than that indoors, but it was nevertheless my intention to have walls that can breathe in both directions. I will have to look into the paint situation but it is doubtful I will go with lime plasters or anything else like that - very labour intensive, small pool of skilled professionals here that can do it, and my budget is tapped out. Plus I have my pretty paint colours picked out already.
Interior humidity is really only a problem here in winter when the doors and windows are closed up and that lasts from late October to early April. In fact, for the last 3 weeks now I have had the windows open and the front door open during the day in our little mobile home. Even before that we were in a situation where the heat was needed in the morning and evening but off during the day. Our new home will have a woodstove and I anticipate that when it is fired up we will often end up opening doors or windows by midday. This short heating system and no need for mechanical cooling in the summer means I can get away with a more lax attitude towards sealing my home up airtight or needing an HRV system (or as I call them: iron lungs), because for most of the year the windows can be open.
With that said, of course the new house will have exhaust fans in the bathrooms and kitchen. We were also forced by new code regs to install an HRV system with ducts leading to all bedrooms and the main living areas - was not my desire to do so because of aforementioned short "closed up" season but it had to be done. So we will have the ability to dehumidify if necessary, though I'm betting we won't need it. In fact, I'm anticipating that we will barely need to run the HRV system at all, which is why I was ticked off that I had to shell out $5000 bucks for one.
And yes you were correct about me meaning to say "thermal bridges" and not "thermal breaks" in regard to fastening the exterior insulation system to the outside of the sheathing, my mistake. And I don't know that the engineer did not use any computer modelling, I didn't ask. And yes, while the outside insulation has an R-value of 12, there is also insulation on the inside of the sheathing, between the studs - 5.5" of Roxul batt insulation R = 22. Our building code requires wall insulation of 20, while Buildinggreen.com recommends 30. We have total of 32 so I'm pleased with that.
I'm a huge fan of Roxul which is why we used it everywhere except under the slab due to the limited underslab testing, as Terry described. My builders love the stuff and say it is fast growing in popularity here.
Let me know if I've missed anything. Some of what was said I did not understand.
Be careful what your read and is recommended by some of these building scientist, I been banned by some, and on my threads you can see why I challenge them and the products they sponsor. I mentioned several far better materials they don't push on this thread alone that are low cost, very low in some locations, like earth, lime, mag, etc. I did not mean to discourage IS board under slab, as I said hands down it is far superior to foam products that have more failed testing, again on what I referenced. If you suspended the slab ok mechanically, that leaves chemically mated to OPC concrete, some soils, plastics, etc, that usually does not go well. That is why I am looking at magnesium cements w/bassalt or FG rebar, that have an affinity to what they mate with, cellulose, not resist or poorly react.
Don't feel bad, with the proper education and tools it is hard to understand and errors are made. All and all you did good. Just realize throwing some mineral wool insulation in wall cavities does not make the envelope breathable, inert and chemically stable. It's not that simple, there is a myth out there. Nor is natural building simple.
Thanks for tip on KEIM, I will look into it. I spent a lot of time talking to Croft at American Clay about the problems people were having with the reformulation of drywall paper. Seems that there was a good deal of delamination issues. That is what got me looking into ROMA. I agree that removing the paper from the wall is probably best, I haven't seen PCM drywall or Mag boards I my area, I will have to ask if I can get them.
I like lime and clay finishes, not only for the moisture holding properties, and permiability, but the esthetics as well.
Are you running WUFI Plus? I am looking into going to the training this summer.
I used Prosoco products to air seal plywood seams, Joint and Seam Sealer for gaps, Fast Flash for window bucks, and Air Dam for window sealing. They are advertised as vapor permeable, and were easy to work with - A blower door test in 10 yrs will be very interesting! I have also used The Zip system with their tape, was faster than wet flashing, but again long term adhesion is questionable. Zip panels tended to cup a little at the edges, not sure if that is normal.
I see lots of rainscreen products, tiny airspaces that get sandwiched between sheathing and cladding that have great claims about air transport. I have dismissed them because I have problems believing that they won't get compressed during installation and provide even less airflow, as well as it being hard to believe that they can provide sufficient air flow under perfect conditions. I may be totally out to lunch! My initial reaction was some marketing guy decided to repurpose a material for another industry to take advantage of the new hype on an old technology. Sort of like the polybutlyene fiasco. I sort of like being able to just create the rain screen cavity detail without another expensive product.
thanks for the clay recipe, I am definitely going to play with that! And I am happy to hear you can use mineral silicate paint to change the color of clay walls, I knew they claimed you had to just replaster with another clay color.
I got the triple pane windows for my last project from Zola windows in steamboat springs. They are imported from Europe, built in Poland I think. When we were pricing the windows, we found they were comparable to higher end Marvin windows the architect was familiar with. There was another project using the same windows, so we coordinated schedules and split the shipping cost, which significantly reduced the price. The project I am working on now has Kolbe windows specified (US Made, www.kolbe-kolbe.com), haven't seen them yet. The entry doors I used were built locally. The door builder "schiavonewoodworking.com" builds doors based on german design for passive house. He also has been working on passive house comparable tilt and turn windows. They again, are based on a german design with imported hardware. I have seen the prototypes and think they are as good if not better than the European imports. Great to be able to have these windows and doors built locally. Hammer and Hand in Seattle is also building passive house doors. I attached a pic of one of the locally built doors I used, with triple pane lite and profile pic showing one of the 2 seals, they are a tad over 2.5" thick, multipoint locking hardware. I watched him build similar doors before we chose him, there was a lot more that goes into them than I thought. The triple pane lite came from a glass supplier, not as good as what is in the tilt and turn windows.
Dan Rockhill: https://en.wikipedia.org/wiki/Dan_Rockhill
Dan's Builds: http://www.studio804.com/
Be careful with PCM and Mag drywall there is A LOT to know there.
I like lime and clay finishes, not only for the moisture holding properties, and permeability, but the aesthetics as well.
It does look nice and functions well, finding the trades not easy in most locations. I attached some American Clay rendering's I did in my model of a Multi_Gen that is gaining popularity just to get a look see I am thinking about replacing with Rammed Earth if I can still make the plan square feet work. I have not had the time to turn those images into realistic high def yet. One builder went from selling 300 in 2014 to 1100 in 2015 Multi-Gens in Tucson, do the math on that profit. I'd put them in all bathrooms min, Mariah would be wise to especaiily the exterior walls where drying inward may be the only choice. Or, get with Bill he knows what to do here. I can't find that clean of pottery clay needed for such thin coats on drywall less than what AC can sell to me for yet. Drywall I found here to be a cheaper lath but the paper is junk! ROMA Biogrip does nothing to change that. The gypsum grades some use has too many fillers degrading adhesion to the paper, more binder is needed surface ROMA primer does not provide since it cannot get to the bond-line and modify it. That my friend would take heat and pressure but, if there is not pure high quality natural gypsum binder it still won't work.
Next Gen Growth: http://sandiegouniontribune.ca.newsmemory.com/publink.php?shareid=19e182ea6&pSetup=sandiegouniontribune#.Vx1H8Lt2Ik0.gmail
I agree these rain screen products don't have enough CFM gap, proves my point how some of these building products these Building Scientist like Joe push without knowing for a fact.
Are you running WUFI Plus? I am looking into going to the training this summer.
Not yet and not sure I will buy into it or wait for a plug into my CAD software, or DOE/BEOPT is decent. I'm currently doing HVAC design work for a corporate giant on the east cost that is paying well to fund my building R&D. I run FLOMASTER and SOLIDFLO CFD. I need an update from ASHRAE 160 before I decide, and I need some MAG properties from the chemist to make it worth my while since we can define our own material properties. As I said, it's just software that needs a qualified person running it ask WUFI, that won't happen by training at PHIUS, trust me!
I'm looking at WUFI detail products and others to manufacturing my own natural products, since I have a 30 year background in it, perhaps we will see.
Thanks for the window info, I figured you had to be getting them from Europe, and door. We need to talk on the phone soon, I'll PM when I got some time.
It's like anything else that is complex you just can expect to dive right in and swim right off the batt as much as the sales hype says you can.