thermal bridge

EHH week 15: continuous exterior insulation

The big problem with insulating stud walls is…the studs.  Wood only insulates to R-1 per inch of thickness, while most insulation used in wood stud walls is more like R-3.5 per inch.  So every 24 inches we have a stud that lets out 3.5 times as much heat as the insulation next to it.  These passages for heat to escape are called thermal bridges.  They reduce a 2x6 at 24” stud wall with R-21 insulation from a potential R-23 wall (including all the material layers) to an effective R-20.5 wall, an 11% reduction.  If the studs are spaced closer at 16” it is even worse, an effective R-19.4 wall, a 16% reduction.


In order to reduce thermal bridges in the walls of the Eastside Harvest House, we decided to wrap them in rigid foam insulation.  At 2 inches thick, the foam adds another R-9 to the wall, for a potential R-32.  Significantly, the wrapped wall is an effective R-30, only a 7% reduction and one and a half times as good as the effective R-20.5 of the unwrapped wall.


Rigid foam is plastic resin foamed with a gas that can contribute to global warming. The worst offender in this regard is XPS (extruded polystyrene, often colored pink or blue or green) which is foamed with a greenhouse gas over 2000 times worse than CO2, the major climate change gas.  We used EPS (expanded polystyrene, usually white) instead which is foamed with pentane, only 3 times worse than CO2.

EHH week 25: exterior siding

While most of the exterior siding is corrugated steel, there are some areas of fiber cement boards and accents of laminated wood panels as well.  The fiber cement is installed as usual with hidden nails, but for the laminated wood we chose exposed stainless steel screws.  I really like the way the different materials complement each other.


In order to assure that any rain that gets behind these sidings can freely flow down and away, we install them with a rain screen shim.  Coravent makes a great product for this purpose, a black plastic hollow strip that is fairly thin, impervious to water, and keeps insects out of the hidden space.


Just as we want to avoid having rain collect behind the siding, we want a clear air space between the steel decks and the siding, too.  Because the siding has 2 inches of foam insulation behind it, we installed steel standoffs to securely hold the deck ledger out away from the siding.


The exterior skin of this house is state of the art.  Its outer layer is durable materials that need no maintenance, except for infrequent painting of the fiber cement siding.  All the siding has a rain screen space behind it to allow it to dry easily.  A vapor permeable wrap keeps the rain out but allows the wall to dry if needed.  The rigid foam adds R-value and eliminates thermal bridging at the framing.  And the plywood sheathing with taped seams is a robust air barrier.