Monday, October 28, 2013

Overhangs and wall insulation

In standard construction, the overhangs/eaves are created by simply building the roof planes to extend beyond the tops of the walls. This makes the carpentry simple but but makes it difficult to air seal and all but impossible to insulate outside the sheathing.  Building the shell of the house without overhangs solves both of those problems.  The corners where the roof planes meet the walls can easily be taped and the whole house can be wrapped in rigid insulation.

But the aesthetics of a traditional style house requires overhangs. As mentioned in the previous post, the top layer of plywood is installed in such a way as to extend beyond the roof plane.  The plywood then needs to be supported as shown in the picture below.  Looks simple but the carpentry has to be done carefully in order for these faux overhangs to align properly with the roof planes.

In the picture below the overhangs on the left side are complete. On the right side, you can see one of the "ladders" waiting to be installed.

As you can see in the pictures above, the windows were also being installed at the same time the overhangs were being built.  With the windows in, the crew can now complete the application of the polyiso (the rigid foam insulation) to the walls.  For the walls, I've opted to use 4" of polyiso, rather then the 6" on the roof, providing an insulation value of R-24 to the walls.  On the interior the stud cavities of the walls will be filled with dense-pack cellulose which will yield about another R-13.  With the sheathing, interior finish and the foil-facing on the insulation, the total insulation value of walls will be about R-40 when complete.

The polyiso is held tight to the house by screwing strapping through to the studs inside, similar to the way the plywood on the roof is screwed all the way through to the rafters.  The strapping also provides a base for attaching the siding.


Finally a word about how foil helps to reduce heat transfer. The three mechanisms of heat transfer are conduction, convection and thermal radiation. Insulation's primary job is to reduce heat transfer via conduction through the walls, the roof and the floor.  The air barrier that prevents the conditioned air inside a house from escaping is the primary defense against heat loss through convection.  And a shiny surface, like foil, reduces heat transfer via thermal radiation.  Thermal radiation is why you feel hot when standing near a fire, and why you can feel cold in an uninsulated house even when the heat is cranked up. In the latter case, you are radiating to the cold walls.

All surfaces radiate heat to a degree and all surfaces absorb radiant heat to a degree.  Foil surfaces radiate and absorb less than other surfaces and that's why they reduce heat transfer. A key word here is surface. If the foil is sandwiched between two other materials it can't radiate/absorb. So in the roof assembly the foil facing on the polyiso doesn't help at all.  But on the walls where the strapping will hold the siding away from the polyiso the foil facing will help a bit.  I'm counting it as about R-2 but that's really a guess.  If anyone knows better, please leave a comment and let me know!

Friday, October 25, 2013

Roof details

The roofing assembly we've chosen is somewhat complicated.  Working from the bottom up, the first layer is the sheathing - in our case the red Zip board.  Properly taped, the Zip system should provide an air barrier that is the key to minimizing air leakage into and out of the house.  In the case of the roof, air leakage tends to be out of the house as the air pressure high in a house is greater than it is lower in the house due to the stack effect.

The roof with just the sheathing

Later in the roof assembly screws will be driven through the Zip board and into the rafters below. Unfortunately this results in penetrations through the air barrier. To prevent this from compromising the air barrier we applied a layer of Ice and Water Shield - a membrane designed to seal around and nails and screws driven though it.

Polyisocyanurate rigid panel
Photo courtesy of Fine Homebuilding

Above that goes 6" of rigid polyisocyanurate (polyiso) insulation which in our case means 3 layers of 2" thick boards. The insulation value of polyiso is generally quoted as R-6 per inch, so 6 inches will contribute R-36 to the roof.  If you want a sense of how complicated building science can get check out this paper that examines how the R-value of polyiso can vary over time and temperature.

One thing I would have done differently had I thought about it ahead of time, is to have spec'ed fiberglass-faced polyiso ather than foil-faced. The foil is a vapor barrier and will keep the roof assembly from drying to the exterior. Also the foil will hinder cell phone reception inside the house.

On top of the ISO goes another layer of sheathing, in this case just regular plywood. This will be the sheathing to which the shingles will be nailed. The plywood is attached with long structural screws that go all the way through the sheathing, the insulation and the original Zip board and into the rafters. You can see the heads of the screws in the picture below.  You can also see that there are quite a lot of them.  These are, of course, the aforementioned screws that required the Ice and Water membrane on the Zip board.

Now I'm lucky if I can hit a stud in a wall behind a half inch of drywall so how, you might wonder, do the carpenters manage to hit the rafters through two layers of sheathing and 6 inches of insulation? Well, it's a bit of an art, from what I can tell, and they do sometimes miss and have to adjust.

And therein lies the story of one of those moments of panic that, I'm guessing, happens in every custom building project. The plywood went up on a Friday. The next day it was raining and when I made my daily check on the house, the roof was leaking in multiple places! It appeared the Ice and Water membrane wasn't even sealing around the screws well enough to keep the rain out, let alone provide an air seal.  David assured me this wasn't normal and guessed that when some of the "screw misses" had been backed out, some of the holes that resulted had not been properly sealed.  On Monday, the crew went over the roof, found multiple such holes and sealed them.  Luckily it rained again that night, so we were able to determine that the leaks had indeed been fixed.

Here's a detail at an inside corner where you can see the roof assembly on edge.  Just below the plywood you can see the edges of the three layers of polyiso.  Just below the polyiso you can see the black Ice and Water Shield folded down over a course of vertical polyiso installed at the top of the walls.  That blob of spray foam up in the corner is there to plug some gaps where the polyiso comes together in the valley on the roof .  The plywood extends beyond the polyiso to form the roof overhangs.

The leaks were gone but I still had a a concern about the screws that hold the plywood down.  Being situated on the east end of the lake the wind hits the house pretty hard.  During a big storm it's easy to imagine rain getting forced up underneath the shingles.  The screws are drilled tight to the plywood, but if water were to get under the shingles, I worry that water could wick down along the screws and get into the insulation. I'm probably being paranoid but I went ahead and had another layer of Ice and Water membrane applied to the plywood.

Wednesday, October 9, 2013

Windows and doors arrive!

The windows and doors arrived today.  Overall they look great.  Yaro sent two people to uncrate and help unload them, and then to install one window for instructional purposes.  Dave and crew took advantage of the extra hands to get the big window into the house and installed in the rough opening.

For the technical details refer back to the post Windows and Doors

September 10: A truck takes a container
with my windows to a port somewhere in Europe.
Photo courtesy of Yaro Window and Doors.

October 8, the container arrives in my driveway
The windows are screwed directly to the bracing.
It took about 30 minutes just to extricate the windows
before we could start unloading.

The big window in the house and waiting to be
installed.  Those black round things are the suction
grips that make it a lot easier to manage these
quite heavy windows.

Leveling and shimming the first window.

The trim is actually dark but is covered with a
white protective tape.

U-factor 0.13 = R-value of 7.7
COG stands for center of glass so the glass itself has
an R-value of 11.3.  The overall R-value of the window
is reduced because the R-value of the wood frame is
probably closer to 4 or 5.

The clips that hold the window to the framing.

Close-up of a clip.