Wednesday, April 30, 2014

Cavity insulation and finish work

To complete the insulation, we first sprayed a layer of closed cell foam on the basement ceiling. Closed cell is quite a bit more expensive than open cell but the basement is outside the thermal envelope of the house and the closed cell foam will provide the air barrier.  We hired a company called Green Cocoon to do the work.

The closed cell foam has a slight greenish tint to it.  I only had a single layer (about 2") applied to save on cost.

After installing the foam, we added batts of Roxul mineral wool for a big boost to the R-value at a relatively low cost.  The Roxul also provides fire resistance, which otherwise would have had to be provided by spraying a fire resistant coating onto the closed cell foam.  The coating actually costs way more than the Roxul but provides no additional insulation.

Upstairs we sprayed in open-cell foam since we weren't depending on the foam as an air barrier.

In the picture below you can see that they do every other cavity, to make it easier to trim the overflow.

Prestained knotty pine arrives from Duragroove.  Two palettes remain in the picture below but there was a third one when we started.  The boards are all 16 feet long and since I couldn't find my extended fork lift we had to unload it all by hand.

And here's what it looks like installed.

In the picture below you can see the indoor unit for the heating/AC system.  That one unit is supposed to be able to heat the entire house.  Above it you can see the vent from which fresh air will come into the house. Since that air will be colder (in the winter) than the air in the house, putting the vent above the heating unit will hopefully temper any cool breeze from the fresh air vent.

Sunday, January 26, 2014

Midwinter Update

Here's a current picture of the house:

There's a front porch now, the roof has been shingled, the windows and eaves have been trimmed out and most of the siding has been installed.

The siding is cedar with a factory applied coat of Defy Extreme Stain in light walnut. It'll need a second coat in the spring and hopefully only every 5 years or so after that.

The trim is a composite called Boral TruExterior Trim.  It's a polymer composite with fly-ash filler that won't rot, takes paint well and is very dimensionally stable. The crew said that it worked like wood and was much easier to deal with than PVC trim.  All the trim will get painted a medium brown in the spring.

The roof shingles are GAF Camelot II shingles in Barkwood.  This was an upgrade over the standard 30-year architectural shingles.  They're heavier, have a longer warranty and, of course, cost more.  What I didn't know when I opted for them, was that they are significantly more complicated to install. They're too heavy to weave together in the valleys so painted metal flashing is required instead. But what really threw the crew for a loop was that rather than every shingle being identical there are about 10 different shingles that need to be applied in a particular pattern.

Here are a few pictures of the house from the lake:

Friday, November 15, 2013

Heat loss calculation

I'll be using a ductless mini-split heat pump to heat the house.  A heat pump is a device that moves heat from a colder area to a warmer area, just the opposite of the way heat flows naturally.  An air conditioner is a heat pump as it takes heat from a cooler area (inside your house) and moves it to a warmer area (the outdoors). If you've ever stood outside next to an air conditioner, you've probably felt the hot air exhaust.  That's the heat it pulled out of the indoor space.

Now imagine you took one of those air conditioners that sits in a window and instead of taking it out in the winter and storing it in the basement, you just turn it around in the window so that the hot exhaust air blows into your house. Well, now you'd be using your air conditioner heat pump to heat your house. Please don't actually try this as window air conditioners are not designed to work in that situation but with a little tweaking manufacturers have, in fact, made heat pumps that work quite well for heating.

Modern heat pumps are more like central air conditioning systems where the compressor and the blower are split such that the compressor sits outside the house and the blower is inside, with tubing running between them. A very nice bonus is that these units are designed so that with just the flip of a switch they can run backwards and cool the house in the summer.

To decide what size heat pump I'll need, I first need to calculate the rate at which heat leaves the house. I'm not going to go into all the details of how that's done but here's a link to the spreadsheet that I built to do the calculation. It's worth a look if you're at all interested in the details.

The calculation showed that it will require about 12,000 BTUs of heat per hour to keep the house at 72°F when it's 0°F outside. To put that in perspective, that about the amount of heat that two 1500 watt space heaters generate (12,000 BTUs per hour = 3512 watts). So, in theory, a couple of portable space heaters will be able to heat the whole house on a really frigid day.

So why not just buy a couple of cheap space heaters to heat the house? Well, space heaters work by directly converting electrical energy into heat energy. So a 1500 watt space heater requires 1500 watts of electricity to generate 1500 watts of heat. A heat pump, on the other hand, uses electricity to move heat from outside to inside and it turns out that a modern heat pump only requires about 500 watts of electricity to move 1500 watts of heat into your house. So using a heat pump I'll be able to heat the house for a third of what it would cost to heat it with space heaters (aka resistance heaters).

One other thing I saw on the the spreadsheet was how much heat will be lost through each area and I was surprised at how much heat will be lost through the floor into the basement. By increasing the insulation in the floor I should be able reduce the overall heating load to below 11,000 BTUs/hr.

The upshot of all this is that I'm going to try to get by with a single small heat pump, specifically the Fujitsu High SEER 9RLS2 which is rated at 12,000 BTU/hr when heating. For those rare nights when it gets really cold I may need to augment with a space heater but hopefully those nights will be few and far between.

UPDATE 11/24/2013

I've updated the spreadsheet to reflect additional insulation in the floor which reduces the total heat loss, and to somewhat take into exfiltration, which increases the total heat loss. Exfiltration is simply interior air that leaks directly out of the house through the myriad of tiny cracks and holes that exist in any house.  Of course, any interior air that leaks out is replaced by cold exterior air that leaks into (infiltrates) the house. The problem with calculating heat loss due to exfiltration is that there's really no way to do it.  At best there are rules of thumb which were mostly developed in the days when houses were an order of magnitude leakier than this house will be.  At some point we will do a blower door test which will tell us how leaky the house is when it's depressurized by a powerful fan but there's no rigorous way to extrapolate from that number to the actual leakage the house will see.

So I made a more or less wild guess that the actual exfiltration would be about one tenth of the value we get from the blower door test, which itself is just an estimate at this point.

An empirical observation though:  I was in the house today when the wind was really howling (white caps on the lake) and the outdoor temperature was in the 20's.  Inside the house, it was at least 20 degrees warmer and there wasn't a trace of a draft.  And the house isn't even completely sealed yet.  There are still open holes where the electrical wiring goes through the walls.  And the interior insulation has yet to be installed.

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.