I had a Eureka moment about 10 years ago.

Folks like us who look at buildings tend to simplify them and miss too much.  Not only are buildings 3 dimensional, the various components of the buildings also are 3 dimensional. 

When we tell a crew to ‘airseal the attic’, we are not giving them all the information they need, and we are not necessarily going to get what we want if we don’t.  The ceiling and the wall are three dimensional, when the crew goes to seal the roof to the wall if they don’t cover the whole wall plate, including the inner and outer sheathing, they will leave a potential air leak. 

I had a job once where a wet wall intersected an exterior wall.  When it got cold, the client had an ice dam right above the wet wall.  I assumed my crew did not airseal correctly and sent them back to fix it.  The ice persisted.  I figured the crew messed it up again, so I went with them and approached the problem a third time.  I could not find anything wrong, so I decided to just fill the wet wall with cellulose.  When we started blowing the cellulose in the interior second floor partition it blew back in the attic through the ventilation baffles  directly over the intersection of the wet wall and the exterior wall.  Walls are three dimensional!!!   We airsealed the top of the wet wall, but we were not contracted to remove any siding or soffit.  We didn’t know it, but the outer end of the wet wall was not airsealed behind the siding and rigid foam cladding.  Air, (and cellulose apparently) was able to move from the hollow wall into the soffit and up the intended ventilation baffles into the attic.   I do not know why the pipes never froze.  Since this job we have changed our practice, so we do not airseal top plates if we can access the wall.  Instead we just fill the wall. 

In another instance, we were asked to fix an old farmhouse.  We accessed the attic and airsealed about 12 can lights, the hatch, a partition, etc., then blew cellulose in the attic floor.   The second floor rooms all had a small sloped ceiling portion that was about 20 inches from exterior wall to collar tie, and that space was stuffed full of rock wool.  When we blew in the attic we put the cellulose up against the rock wool.  Winter came, and there was massive heat loss and associated ice damming at the second floor bathroom.  We moved the cellulose off the bathroom from the attic and found there was a furring space between the original ceiling and a new finish ceiling.  We sprayed foam over the lath to airseal the area, then put the cellulose back.    The following winter there was a melt line about 6 ft long that was very well defined right above the bathroom and ran perpendicular to the rafters.  IR said that ceiling was fully insulated, so I was stumped as to why the ice formed that way.  In the end, we realized the remodeler hadn’t removed the plaster from the original ceiling where the furring met the slope, and with our access being from the attic (the roof was  a 4/12 pitch), we could not actually get to the extreme outer edge of the furred space from the attic.  Heat was leaking through that flaw.  We ended up taking off part of the roof to access the flaw & fix it.  No Ice.

Anyway, what this has all brought to my thick head so far is each ‘side’ of the 6 sided box I call a building is actually a 6 sided box comprised of several more 6 sided boxes.  It is very easy to get lost in a box or forget exactly what side of which box is really important when you are assigned to stop air from moving from box to box.  In most cases, it is better to just fill as many of the boxes as possible so the edges don’t matter and you have effectively reduced the number of boxes you have to contend with.  Obviously, the fill material should be capable of stopping air movement. 

If you are thinking of using hybrid systems you need to apply the sealant so it covers all the holes, so a 1 inch thick 'flash' of foam inside a wall cavity may not do much because it doesn't cover the electric and plumbing penetrations.  Perhaps the reason more of these jobs do not have condensation issues is there is enough air movement in that system to effectively vent the inside of the foam.  Foam is not flat, so there will be space.  There are holes, so there will be air movement.  (10 years later I am still trying to figure out how relevant this is, but the thought is keeping me from offering flash & dash) )

But, 2 inches of foam applied external to the frame does a lot more because now all those interstitial penetrations are 'inside', and the frame is also 'inside'.    Are there any models that can depict the difference in performance this kind of thinking creates? 

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Lots of good questions and some good experiential learning.

We not only need to think in 3 dimensions, but to think like flowing heat, think like liquid water and water vapor, and think like air - each of which moves through 3 dimensions in their own peculiar way.

Exterior rigid foam is a popular method of eliminating thermal bridges, but unless it's continuous with roof or attic insulation, there can still be air channels within the envelope to the attic or soffit. It's far more challenging with retrofits, but with new design I teach that each barrier assembly (thermal, air, moisture) must be traceable with a pencil on the cross-sections, without ever lifting the pencil off the page, around all six sides of the envelope. It's the transitions which are problematic.

Another 3-D effect which is often overlooked is that, though we tend to think of heat loss/gain as moving linearly from inside to out or vice versa, heat is isotropic - it moves equally in all directions in all the 3 dimensions. So, for instance, if a roof is blown with closed-cell foam, but the 2x8 rafter bays are filled only 4" deep, half of the sides of the rafters are exposed and that surface is a major heat loss/gain area. Most of the thermal bridging will occur through the broad side surfaces of the rafters, not just the 1½" inside edge. This is also why steel studs are such horrible thermal bridges, even though the cross-sectional thickness is miniscule, because the inner and outer flanges are heat loss/gain surfaces (and the metal is highly thermally conductive).

It is also often overlooked that heat loss through corners can be almost double the heat loss through flat wall sections (and corners are often nearly solid wood). This is particularly true of slabs-on-grade, but the more articulated the geometry of a building the higher the heat loss, all else being equal - so corners and transitions of all types are the trouble spots, perhaps even moreso on roof planes.

yeah, i had to explain just yesterday to a client why framing a second wall in an attic that joined a second floor of the rest of the house, and insuating that wall would not work.  if air is moving in the existing wall, adding a second wall just hides the flaw better, it doesn't fix the problem.  he said 'nuts'.  that existing wall has shrunken uffi foam in it, so demo and removal is in my future.  fun to follow.

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