Heat Bridge or 3-Dimensional Pressure Boundary?
I stumbled on this interesting image a while back.
I noticed that the caption refers to “heat bridges from structure and roof fasteners”….
The fastener locations are obvious….
But the linear pattern does not look like a structural pattern(series of wood or steel rafters or joists).
It looks more like a pattern of rigid insulation and the voids between the insulation.
I think the pattern is more likely related to 3-dimensional Air flow as mentioned in this Insight
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/files/bsi-036_complex_3d_airflow_networks.pdf
I think what is missing in the photo is an air barrier on the warm side of the insulation.
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Replies
John,
I think there is insulation missing as well, roof or attic insualtion should be continuous along with an air barrier as you say. Thermal bridging should be avoided on all building planes.
JB,
You could be right.
It looks a lot like Dr. Joe's barn.
Pictures like these make me think that in cold climates, the best place for the pressure boundary is at the warm (in winter) side of the envelope.
Lucas,
yep... I noticed that you have your pressure boundary in the right spot
Lucas(the heretic),
have you read this Insight-56 ?
http://www.buildingscience.com/documents/insights/bsi-056-leiningen-versus-the-ants-redux/files/BSI-056_Leiningen_vs_the_Ants_Redux_rev2.pdf
I wonder if the Ant-Farm moisture was really the result of rain getting past dumb flashing or interior moisture finding it's way thru a 3-D network ?
Since you have me thinking about 3D airflow networks today John...
In Hot/humid mixed climates with air conditioned houses...
Maybe the pressure boundary should be on the "warm in cooling season" side of the envelope...
At least if a 3D network exists on the cold side of the pressure boundary, the airflow should be relatively dry...
JB,
That's a good question.
In bsi-036 Dr. Joe says (talking about his barn roof):
In bsi-056 Dr. Joe says (talking about his poly air-barrier):
So I suppose it is possible that in some parts of the envelope convection loops could have, in fact, been "coupled" to the interior space - leading to more "catastrophic" effects...
Seems like "the perfect wall" can be a "turd" too sometimes...
Here's a Riversong Snipit
can't remember where I snipped it from... perhaps it was Martin's 1 vs 2 Air Barrier blog
Good 'ol Riversong...
You got a much more detailed conversation going when he was involved. I realize how difficult he was, but I learned a lot from the discussions that resulted.
Lucas and John,
Great thread.  let's unravel it a bit  further:
The points that you bring up are where the discussion on airsealing usually stops . This is evidenced by the all of the cold climate walls  where the pressure boundary is the exterior layer of sheeting (be it taped, liquid applied WRB...). When the pressure boundary is the exterior sheeting in a cold climate cavity wall, then there is often no further consideration for the unchecked vapor drive into the wall assembly.Â
Martins "two barrier or one" blog was the US being introduced to a cold climate cavity based system, where it is understood that the interior vapor drive needs to be stopped on the warm side of the cavity by membrane or sheet goods with near zero infiltration rates.Â
(INSERT your favorite air infiltration argument here: 0.6 ACH 50 Â is excessive, or 3 ACH 50 is not enough... HERE.)Â
The cold side of the envelope then just needs to block air movement to keep each cavity isolated and 3D thermal looping in check. The cold side becomes more durable when the pressure boundary is on the warm side. Add an increasing permeance to the cold side and then the cavities can regulate their humidity to the exterior conditions. Case in point: Thorsten Clupp's Artic wall.
But what about when there is an air and vapor control layer on the inside but the exterior sheeting is not very permeable.? Isn't that where it gets tricky? The problem, gets compounded In my view when the interior control layer is created by poor choices: drywall and paint. These are things that are going to be modified by the homeowner over the years. Every time a picture hanger is added, another penetration is added and the infiltration rate into the cavity in increased. But now because attention was given to air and vapor infiltration at the drywall, by adding Polly or low perm paints, the ability to dry to the interiors is reduced and another "wet sandwich" is created.
I like the builders like Doug McEvers and Thorston Clupp who are consistently taking on the issue and creating an interior low perm  pressure boundary in an assembly that maintains an airtight, high perm exterior layer. They take the issue seriously and do not add a layer of OSB or plywood sheeting and thus make the assembly that uses an interior pressure boundary perform and retain the ability to "self regulate" it's moisture content.
Albert, Speaking of Thorsten Chlupp
The slides for 3 of his presentations are at this page
http://www.passivehouse.us/phc2011/main-schedule/
I noticed one of the projects using SIGA
Albert,
I agree that permeability matters...
I might call what Robert Riversong describes in JB's snipit as "bending like a willow".
Whereas I might describe "the perfect wall" as "strong like an oak".
Proverbs have it as being wiser (better resiliency) to "bend like a willow".
Even with a well defined and properly located pressure boundary I think the "eggs are all in one basket" when the applied materials are all low perm.
From bsi-056:
Albert,
I think you make a good point about cold climate air barriers...
If we assume that it is best to locate the pressure boundary at the "warm in winter" side of a cold climate envelope then maybe it is also best to protect the integrity of that pressure boundary - since it is somewhere near where the occupants can get at it.
I am a big fan of the service core for this reason...
Despite some drawbacks.
I think one reason Robert Riversong likes ADA is that it is "repairable".
True, but what homeowner will actualy follow through on air barrier repairs?
Poly can't be repaired at all...
Is the service core really such a bad thing?
JB,
For your climate zone, what would you consider to be a reasonable wall assembly R value?
I wish I could have attended "Beyond Pasivhaus - Sustainable shelters in an unsustainable world".
I think Herr Chlupp sees the "writing on the wall".
John,Â
Yes I've sent Thorsten up a prodigious amount of Siga tapes and membranes and was in the audience during his PHIUS presentation. We had a fun discussion over a couple of Jagemiesters about his movement from PERSIST to his developments of dense packed cellulose assemblies with a high diffusion gradient to the exterior. I'm really happy that people like him are out there taking these issues seriously. His willingness and ability to break out of the norms are a call to all of us.
Lucas,
Personally, building a cold climate assembly with an increasing diffusion gradient to the exterior is my "perfect wall". Sadly it is really tough here in North America. We don't have much of a range of high perm materials for the exterior sheeting. A BIG problem.
Guys like Doug McEvers "got it" a long time ago and have found what they feel is reasonable fibre board that they say (and I believe them) have the high permeability but are dense enough to not bow out when dense packed.
I'm not there yet. I'm going to the lengths of importing the high perm fibre board from central Europe for the exterior sheeting. Once you know it exists, you can't (or is it just that I can't),turn away. It's as hard as regular MDF, but is rated at 18 perms. It is what Herr Clupp would use ( in his own words) but getting it up to him is not economically possible at this point.
I've got a few projects that this is going on and I'm really hopeful that the builders like it. My first container lands late January and we've still got some rain screen details to master.
I have great misgivings about creating all of this embodied energy by importing WOOD from Europe, but I have an even greater fear that we (the US) will skip looking at cellulose based assemblies that can perform well - and solve the "super insulation question" by chemistry: meaning foam production.
It seems to me that it's headed that way. Â To really do super insulated cold climate walls that will last in the 100's of years, the only option that appears to me to work is an assembly that drys to the exterior. That means a permeability of higher than 10 and we really don't have a wood panel that is dense enough to retain its shape under pressure and has that level of permeability. Without the right exterior sheeting available, we will give up head towards foam.
Even with emerging low GWP blowing agents, foam is pretty dirty when you consider it from cradle to cradle. The sad thing about foam to me is that exterior foam assemblies negate the need to learn most aspects of building science. With exterior foam, it's just a question of adding enough. Once most of the insulation is in the foam layers, then the  Pressure boundary is at the inside of the foam and the wood can remain warm and wet. It's certainly unattractive to me. As Herr Clupp puts it: it's like wearing a plastic bag on your body vs the permeability of a good Gortex jacket that can work to manage the interior humidity levels.
Wood panels that are both high perm and dense on the other hand, are to me, an area that we absolutely need to explore. Even if the early start is just with stubborn little guys like me. If we can communicate the potential benefits to our wood producing regions, then they can create new products to produce instead of the production going to foam producers or importers.
This is just the early stage in understanding these materials and how they fit into assemblies over here. The more recent versions of these products have the density reduced which then increases the R values to near 3 per inch. Now the high perm exterior sheeting is also breaking the exterior thermal bridge much like an inch of foam.
This stuff is far to beneficial to ignore and not suffer a little risk in exploration.
Albert,
You wrote, "We don't have much of a range of high perm materials for the exterior sheathing."
What about boards? We've been using them for three hundred years in North America. I sheathed my house with diagonal 1x6 rough-cut boards, locally purchased from a sawmill a few miles away. None of these fibers crossed the Atlantic Ocean.
Lucas,
 I'm glad this probably an obscure thread. I can go at length without fear of boring too many people.Â
I disagree about ADA being a good solution because it is repairable. (edited from original posting. I don't really know first hand what RR thinks about ADA.)
The point of it being repairable is a bit foolish. If your building big thick cavity walls in cold climates, a service cavity is certainly the way to go. Put the pressure barrier way from the finish surface so that it doesn't need to get repaired.
I look at this as increasing understanding and capability. It makes sense that as the needs of Airsealing come into play, they occur on existing layers. Drywall is a handy place to start. It's right there and accessible. In a thin wall, if it leaks then the damage potential is not too large because the exterior sheething will still be kept warm from various sources of interior heat. So... A failure in the pressure barrier is not catastrophic. Once you thicken that wall up to the thickness that the sheeting will live cold, then a failure in the pressure barrier can bring in enough moisture for a serious problem. As we know that those thicknesses and conditions can vary widely based on location and climate, it no longer is rational to use a pressure barrier that is both accessible and commonly punctured  by homeowners.Â
Martin has produced good Canadian studies that show ADA homes retaining the infiltration rates over something like 15 to 18 years, but on reflection, wall penetration that allow moisture to pass into the cavity don't necessarily mean that the whole envelope rate will dramatically change.Â
The "repairable ADA scenario" requires an understanding by a homeowner that something needs to be repaired. A very unlikely occurrence. Whereas putting the issue out of reach is certainly more reliable.Â
I of course have sympathy for the cost of adding a service cavity. These things have to move by progression and it's tough to add cost. Consumers are driven by finishes and not by the quality and durability of the pressure barrier. However, let's not skate on the issue with wishful thinking.
Martin,
Duh! You've said it enough times I don't know why I've not added it as a good candidate for the application. I recall you talking about using ship-lap. Correct me if I'm wrong: 3/4" flat sawn 6 to 8" wide boards with a 3/8" rabbit on opposite faces.
While the boards don't have the permeability, the rabbit joints are closed enough to reduce wind washing but open enough to let air + vapor move as needed to stay in equilibrium with the Great Outdoors. Add a high quality house wrap that is really truly taped (including staple heads with stuff that will stay in place and the wind washing is now blocked. The gaps let the wall dry while the membrane blocks the air and bulk water. What an excellent system.
Do I have the scenario right? Did you use a rabbit or was it just gapped?
The wall detail for our 1958 rambler in NW Minnesota was plaster, 2x4 studs, fiberglass batts, 1x8 shiplap sheathing, 15# felt and redwood 3/4 x 10 bevel siding. I made the comment several years ago that when I opened a wall to add an addition, the inside of the wall looked like new. The common nails holding the sheathing in place were still shiny, the plaster walls are acting as a good warm side air barrier. This wall dries to the cold side, the problem is the overall R-value is 10 or less and is not near what is needed in a 9,000 hdd climate with heating oil at $3.50.
When I see a new house being built in a cold climate with a single wall, using a low perm, low R exterior sheathing I cringe. This is an opportunity lost, the thermal bridge is great and this wall is expected to dry against the flow of heat. Why not enhance the same type of wall assembly that has proven to be durable for the last 50 years? Add more insulation, minimize the thermal bridging and you have a modern wall system that is sure to perform.
Great discussion guys. I really like how using a service cavity makes it so easy to create an air barrier that is easy to build and requires no maintenance. It also allows rewiring and re-plumbing without disturbing the pressure boundary. The costs however, are not insignificant. Apart from the obvious costs of the materials and labor involved, we must not forget that we are giving up significant interior floor space. This unusable space must still be conditioned the same as the rest of the interior. For example, on a 30 X 40 building with a 3.5" service cavity, we have to give up about 40 sq ft of floor area. I still like RR's desire to use the least amount of materials...
Albert,
The sheathing on my house is rough-cut 1x6, square-edged, right of the mill. Never been planed. No shiplap.
On outbuildings (barns and woodsheds), I have often covered this type of sheathing with asphalt felt, followed by vertical shiplapped boards for siding. Around here, shiplapped boards are all planed on one side.
Lucas,
You asked about a reasonable R-value? for my climate (North Texas) 3A --Warm/Humid
................
to use a Marc Rosenbaum (and Spinal Tap) concept.... "Dial"
I think the wall assembly "dial" should go from R-20 to R-70
the ceiling dial from R-50 to R-100
I think it is time to start thinking beyond the limits of a 2x6 wall
That's why I like variations of the double-wall (vapor open)
including Larsen-like variations
Once the concept of a double wall is considered.....
then the incremental cost of adding more r-value between the 2 walls is not-so-unreasonable
https://www.youtube.com/watch?v=XuzpsO4ErOQ&feature=related
JB,
The "dial" concept is one I've been thinking about lately too...
I've been thinking of it as "scaleablity".
I've been trying lately to think outside my climate zone...
Maybe I'll change my mind at some point but for now I think mixed climates are the hardest to design for...
Albert,
I understand where you're coming from.
Ever scince I learned of "Euro" wood panel products I've always wondered "why not here"?
I've said it before, I think waste wood will probably serve us better as insulating panels than as another form of combustible fuel (either hog or pellet).
There's not enough of a market... foam reins supreme in the mainstream.
I'm glad there are guys like you out there trying to carve out some market-share.
In my case I've decided to omit an exterior sheathing layer altogether.
Garth,
I agree that it makes sense to use the least amount of material possible.
I think the issue of the extra cost of a service cavity is a little over-blown.
The loss of interior space is a fact...
But what wall system doesn't involve a compromise of some type?
I think another pay-off for going with a sevice core is that the air barrier can be constructed of "ugly" yet far more durable panel-type materials (ie: exterior grade t&g plywood).
Fascinating conversation. Forgive me for what maybe a dumb question, but where would the pressure boundry be in a SIP wall construction? Would a SIP system with service cavity, not unlike a timber frame be a good system?
Aaron,
The quick answer is the interior OSB. Its by far the most important layer in my opion. A great, well cross checked discussion is here: https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/how-protect-structural-insulated-panels-decay
Garth & Lucas,
Try splitting a 2x6 to create the service cavity. 2 5/8" is just enough space and 2x6 for us are the best buy. Less interior space is lost to the cavity.
Also... It takes some willingness for adventure, but think of framing the service cavity with split 2x6's horizontally rather than vertically. It's faster to wire and it's really only to hang the drywall. Once your just adding furring strips, there is no reason that they need to be vertical. They are only a nailing plane for the drywall.
Albert,
I have noticed that the Europeans will often run the furring horizontal
I think it helps reduce thermal bridging and convection in the cavity.
Albert,
I will be installing 2x3s horizontally for my service core...
Faster wiring (not boring holes) was what really appealed to me.
One thing I haven't decided on yet is screws or nails?
I'm leaning towards screws.
John,
I had considered the thermal bridging but not the convection. Nice point!Â
And btw... Thanks for starting the thread. It's a good one. It's not too often that one is able to get into this level of "alternative detail".
To me, an  air tight envelope is the minimum. Pick your number but I've learned this year that as the wall gets thicker, the infiltration rate needs to get smaller. That rule becomes pointless if the pressure barrier is on the wrong side of the wall.Â
I'm certainly going with: that it's the interior side in cold climate. The exterior side in warm/humid climates (but I dont really know because I have zero warm climate understanding. I never even lived in one...). As Lucas was musing: mixed climates can be the trickiest. I've heard from some Passive House builders and designers in the San Francisco bay area that they are considering placing the pressure barrier & vapor retarder layer in the center of the wall. That could make for an interesting WUFI project.
Maybe the best way to stop convection in the service cavity would be to throw in some cheap batt insulation before drywall...boosts the total R value a little as well...
I think it is useful to at least attempt to visualize where the pressure boundary(ies) actually is/are
I think pressure boundary is a better "concept" than "air control layer" or "air barrier"
There are several Insights/Articles at the BSC website that consider Pressure and 3-D networks
When designiing a Weather Barrier it is useful to think like a raindrop.
When designing a pressure boundary it is useful to think like a gazillion very,very small ants.
Garth, I have seen photos of Mineral wool in the service cavities
Lucas, we seem to think along similar lines. I like it.
I'd really like to use screws in that application. I've even done a few test with a pocket hole on the top side of the 2x3. If its unfamiliar, look up a Kreg Jig. The thing I like about it is that it can be a pretty quick hang for screws if you are planning on pre-drilling. By using a pocket hole, the screw goes in at a 15 degree angle inclined from level. Or, at 105 degrees from the stud face instead of 90. It's kind of like pre-loading the fastener since its "oddly face screwed" in shear.
I hate fighting bad screws through long depths of framing lumber that's not pre drilled.
Of course nails are so much kinder to the guy that comes behind for a later remodel...
I like adding insulation into the service cavity. It's an available cavity that is ready to fill. The only real issue is having the right thickness available. To take the idea a little farther, for the Passive House designer, if they don't insulate the cavity, it counts as treated floor area in the PHPP. Not that it's all about Passive House, it's just illuminating that it counts either as insulated assembly, or as heated floor area. It's not something that can be set aside and ignored. It has a value in either retention or consumption of the heating/cooling load.
Comprehensive answers are what help make the forum worthwhile. Thanks for giving in to your impulses.
JB,
I agree.
I know BSC lists "pressure boundary" in their glossary.
I googled "pressure boundary definition" just for fun.
Seems that the most hits are related to nuclear engineering...
Here's a snip from an engineering forum that I thought was interesting:
I guess this fellow has never heard of ADA...
Albert,
Thanks for the tip.
Sounds like "pre-sagging" the screws when fastening through super-thick foam...
This may be true for some type of reactor vessel...
I think for enclosures that seperate the "outdoors" from "indoors", it might be more accurate to say something like:
...the boundary between relatively static indoor pressures and dynamic outdoor pressures...
Not sure why this thread moved back to the top...
I did not edit the original post....
someone else editied under my name at 15:06 today
John,
Like you, I was surprised by the edit. I assumed it was you.
Big Building Science Brother?
Posturing while Identity Thieving?
Erie...
Green..... monsters?
Getting back to Pressure Boundaries
has anyone looked at Insight 004?
http://www.buildingscience.com/documents/insights/bsi-004-drainage-holes-and-moderation/files/bsi-004_drainage_holes.pdf
and considered "pressure moderation"?
I think the magic of "pressure moderation" is especially useful at the roof cladding (and roof cladding sheathing)
I think it is one of the reasons that Vented Attics Perform so well.
I think Compact Vaulted Roofs with Vented "Over-Roofs" should have a similar advantage.
also "vented" siding
JB,
Insight 004. Pretty elegant stuff. I wasn't aware of the physics behind the pressurized chamber and baffle. It's a pretty interesting read and worth the time. I find it ironic that the high end PH Windows that we deal with are designed with the same baffled pressure chamber shown, but I've never understood the concept to this depth until reading this. Even after a trip to the factory in Germany. One does have to apprciate GBA...
It sure seems to me that we take an "Irish boxers stance" (apply brute force) when a "kung fu" (re-direct the energy) approach might be more successful when it comes to h2o in it's various forms and light frame residential assemblies.
Not that my "fight card" is that impressive...
So... Now that you've brought it up, any new details come to mind???
Albert, I don't have a particular detail in mind.
I thought it was an interesting Insight and on topic with Pressure Boundaries
John,
Regarding "vented" siding for houses with a well defined interior side pressure boundary...
Does a wall cavity filled with air permeable insulation provide "pressure moderation" without "vented" siding?
Consider the wall cavity as the "cup" and the interior pressure boundary as the bottom of the "cup"...
Is a taped plastic WRB installed on the exterior of the wall a good enough pressure boundary to prevent wind pressure from being taken at the bottom of the "cup"?
"Vented" siding will obviously drain better than unvented...
But I agree with RR that in many climates "vented" siding may be an unnecessary expense that complicates construction and potentially increases risk if not detailed correctly...
Lucas, good questions
usual disclaimer...
I am only ATTEMPTING to visualize this stuff..and thinking outloud
The wall you describe"well defined interior side pressure boundary...
sounds like the wall detail for your personal home.
http://ourhouseuponmoosehill.blogspot.com/p/details.html
I think I understand your point ...
The back side of the siding in a wall like yours may be wind pressurized even without an intentional gap (chamber)
WHERE is the wind pressure "taken" (resisted) in the Lucas-Wall?
Is wind pressure resisted at the Siding, the WRB or at the Interior Plywood?
My guess is.... All of the above
Does Pressure Moderation actually improve cladding performance?..
I am not sure... I suspect that it helps
One of Joe's footnotes in the article suggests that Pressure Moderation of the cladding is a not-so-important motive for "gapping"/draining the siding.
John,
As usual, I'm also just "thinking out loud"...
I think you got the gist of what I was thinking...
I was imagining that wind pressure would be taken by the back side of what is the "interior side pressure boundary" - "the cup"
And the back side of the exterior cladding - "the baffle"
However, for this to be the case, the exterior cladding needs to be "leaky" enough for wind pressure to have an effect "in the cup"...
If exterior cladding is too tightly sealed then wind pressure would be taken by the exterior of "the baffle"...
Exterior cladding in this case becomes another pressure boundary...
So I wonder how good a pressure boundary a taped plastic WRB is?
Most people would not rely on this as an "air barrier" - pressure boundary...
But is it leaky enough for "pressure moderation" to occur?
Also thinking about 3D airflow networks...
If "pressure moderation" is in play in a wall with a good interior pressure boundary and a not-so-good exterior pressure bounday...
3D airflow networks would be of the type driven by convection...
And not from "windwashing"...
Maybe...
By "windwashing" I mean...
Air changes driven by dynamic exterior air pressure.
Lucas, what about the top of your wall? where the siding/baffle stops?
What stops wind and or "the outside" from entering the 3d-network?
Are you "connecting" the WRB to the plywood?
The 1x2 pulls the WRB taught across the gap between the two rows of studs.
It is a not-so-good exterior pressure boundary...
Thanks, Lucas
I overlooked it (the WRB connection)before... I see it now on your exploded view