Thursday, January 11, 2007

Hooray! Our other house is done and on the market. That means I now have loads of time to work on the farm house. Well… a few hours per week.

When we bought the farm house, I anticipated that it would not be very energy efficient. Sometimes, there are disadvantages to being right. I also hoped that we could document the before/after energy use. So far, the “before” looks pretty bleak. We burned an entire tank of propane (approx replacement cost $700) by the 1st of December.

This, at a time when we are having one of the most mild winters in decades. On top of that, we keep the thermostat set at 60F when we are in the house doing stuff, and 55F when we are sleeping or absent. It could easily have been double the propane cost if winter had been colder than average, and we set the thermostat at normal comfortable levels.

Oh well, this will make our amazing tremendously efficient outcome all the more impressive and convincing.

Our main topic for this installment is weak spots in conventional house construction and how to avoid this energy inefficient trend. (Thank you David H. for the timely reminder. Accountability rocks!)

There is a right way, and a wrong way to add insulation to your house. As mentioned in a previous entry, the conventional approach is to use 2x6” studs (rather than 2x4” studs) and call it a day. This would moderately increase insulation and reduce energy use, but ignore the big picture.

If you’re going to build a house from scratch, you need to consider some other issues:

1. Orientation
2. House shape and geometry
3. Airtightness/penetrations
4. Heat loss below ground
5. Heat loss through the band joist
6. Properly insulating the attic, even over the exterior walls
7. Movable insulation for windows

1. Orientation matters. In a conventional house, the insulation is so poor, you need a big capacity furnace to heat it, and the sun coming in through the south windows and the heat given off by the people, cats, appliances, lights, etc, cannot really make a significant contribution to heat. But once you attain superinsulation levels, all these sources really do matter. They all work automatically to help heat your house, except for the solar heat contribution from your windows.

In order for the windows to contribute the maximum solar heat gain possible, they have to face south more or less. Most houses are not perfectly square. If it’s a rectangle, you really want to put the long dimension East/West. That puts the longest wall facing south, and will allow for the best use of the normal windows as solar collectors. It doesn’t have to be exactly south, but should be within +- 20 degrees or so. You can certainly have too much glass. This will cause overheating during the day, and lots of heat loss at night.

In fact, for many climates, even a south window may be a net energy loser. It leaks more heat out overnight than you can collect from the sun during the day. Better windows lose less heat. Movable window insulation turns them from a liability to a big asset. There are various ways to accomplish that. Even a “good” window will only have r-5 or 6 at most. Compared to the wall at r-40, this is a gaping hole in the insulation. Old fashioned single pane windows and crappy double pane windows provide r-1 to r-2 insulating “power”.

Insulated window panels can easily bring that up by an additional r-7 to r-21. This can have a disproportionately large effect on perceived comfort. The actual energy savings may be moderate, but the room is suddenly far more comfortable and magically lacks drafts. The improved comfort and lack of drafts may allow you to turn the thermostat down and still feel comfortable.

A google search for movable window insulation will be profitable for everyone who dislikes cold feet and large heat bills. This can be as simple as a slab of 2” blue Styrofoam cut for a snug fit in the window frame. Glue a pretty fabric on both sides, or a reproduction of a nice Monet watercolor. Nobody says they have to be ugly. Handyfolk can build shutters based on this idea. We will expand on this at a later date.

As far as total window area, there is a rule of thumb. Shoot for around 10-15% of floor area as window space. Put as much of that as you can on the south wall. Windows with north exposure are a bottomless pit of energy loss forever and are to be avoided or downsized at every opportunity.

2. Size really does matter, and so does shape. Remember how you told your math teacher in high school that you would never use this stuff in real life? You were wrong.

All other things being equal, a big house takes less energy to heat PER CUBIC FOOT, than a small house. Further, a squarish/blockish house shape takes less energy to heat than a long skinny house. Both of these facts derive from the relationship of surface area to volume. Let’s consider some examples:

Compare two houses of exactly the same shape, but one being twice the size and several times the volume of the other:

House 1 is 20’ x 40’ x 8’. Let’s call it the cabin. Simple geometry tells us that:

Surface area is: 2,560 sq. ft. That counts all six surfaces, since they all lose heat.

Volume is: 6,400 cubic ft.

Volume to area ratio is: 6,400/2,560 = 2.5 cubic feet per square foot surface area

House 2 is twice as big in every dimension. Let’s call it the castle.

40’ x 80’ x 16’

Surface area is: 10, 240 sq. ft.

Volume is: 51,200 cubic ft.

Volume to area ratio is: 5 cubic feet per square foot surface area

The take home message is that we increased the volume by a factor of 8, but only increased the area by a factor of 4. The castle will require half the energy to heat, PER CUBIC FOOT, compared to the cabin. Despite this little geometric oddity, we still want a house that is no bigger than needed. In absolute terms, the bigger house always takes more energy to heat.

This is one of the reasons malls are so ubiquitous. They are inherently easier to heat and cool than all of those stores built as stand alone buildings. This is also why really big people are often hot and really small people are often cold. The small person has more surface area PER POUND. This is also why big cyclists have a slight advantage on the long flat legs of the Tour de France, while the little wiry riders tend to have a slight advantage in the mountains. The big guy has less surface area (per pound of course) to create wind resistance.

One way to take advantage of this effect without building a house the size of a mall is to build a duplex. You get the inherent thermal efficiency gain of a big house, without having to pay for the entire “mansion”. Effectively, your neighbor (or mother-in-law) helps reduce your heat bill since you have one exterior wall you don’t have to heat. This physics stuff about heat loss pops up in all sorts of places. In big apartment buildings, you generally don’t have to pay the heat bill. That’s because they are relatively efficient due to size.

Now, about shape…

A long skinny house has more surface area per enclosed volume than a blocky cubic two story house. Of course, the best possible shape in the world at reducing surface area per enclosed volume is the sphere. I was very close to building a dome house, which can reduce energy use by 1/3 simply due to reduced surface area. But we will press on and ignore the dome and look at three houses of identical volume, but very different shape:

House 1 is long and skinny, like a mobile home.

16 x 70 x 8’

Square feet: 1,120
Surface area: 3,616 ft2
Volume: 8,960 ft3
Vol/area: 8960/3616 = 2.48 ft3/ft2 surf. area

House 2 is a more “normal” one story house of exactly the same square footage (1,120) and volume. But the surface area has changed.

32 x 35 x 8’

Square Feet: 1,120
Surf. Area: 3,312 ft2
Vol 8,960 ft3
V/A 2.71 ft3/ft2 surf area

Thus changing from a long skinny house to a squarish one story reduces the surface area about ten percent. This would reduce your heating/cooling costs ten percent or so, and it would do so forever.

House 3 is a cube shaped two story with the same square footage and volume, but with further reductions in surface area.

23.66 x 23.66 x 16’

Square feet: 1,120
Surf. Area: 2634.516
Vol: 8960
V/A 3.40

This more “cubic” house has 1000 ft2 less surface area to enclose the same square footage and volume as the “skinny” house. This represents a 28% reduction in surface area to enclose the same volume. This is a free reduction in heating and cooling costs for the life of the building. Coincidently, this suggests that the classic long, skinny, one story ranch floor plan has average to crummy geometry as far as energy use is concerned. Long skinny mobile homes are even worse.

3. Airtightness

All the insulation in the world won’t fix a house that leaks like a sieve. Today, we have lots of technology to improve airtightness in construction. In some early superinsulated houses, airtightness went too far and a few people even got sick because of air quality problems. Air quality problems are easy to avoid though, as we shall see.

You should aim to attain a perfect polyethylene vapor barrier on the inside. This requires a few simple techniques. Seams can be overlapped by one stud bay. Seams should also be sealed with acoustic sealant, a non-hardening gooey nasty stuff dispensed from a caulk gun.

Of course, right after you do this perfect job, you intentionally cut a bunch of holes in it at every outlet. There are electrical boxes that are designed for energy efficiency. Some have flanges or lips to allow for sealing goop on the vapor barrier. Some have rubber gaskets that seal around the wire as it is pushed through the hole. The next windy day, go stick your hand up in front of an outlet on an outside wall, or a lit candle, to check for drafts.

Air pollution can be much worse in your house than it is in your back yard. The traditional fix was automatic, aka, a leaky house. Older homes can leak air at a fantastic rate. Clever people have devised various ways to measure this. A drafty non-insulated farm house might have 10 air changes per hour (ACPH) if it’s windy. That means, ten times per hour, all of the air inside the house is exchanged for outside (cold) air. You can recognize this house easily. The curtains move (inside…) when it’s windy (outside). They are also fairly to exquisitely uncomfortable in the winter. They are virtually the definition of a drafty house.

A more “modern” insulated house with average construction will cut that number to 2 or 3 air changes per hour. This is a dramatic improvement of course, but not where we would like to be. Current thinking/research suggests that you need approximately 0.25 ACPH (Air Changes Per Hour) to keep the air in your house fresh and healthy, without wasting too much energy. It takes exceptional care to build a house tighter than 0.25 ACPH.

Once you get your new house sealed tighter than a drum, then you fix the indoor air pollution problem. The first step is to not bring/make pollution inside your house. Don’t smoke! Don’t fry stuff so that it makes the oil smoke. Don’t store pesticides, herbicides, gasoline cans, and so on inside the house. Then, buy an air to air heat exchanger. This runs 24/7 and does just what it says.

It acts as an exhaust fan for the whole house. It removes stale air, and brings in fresh air to replace it. It does this at the perfect rate, so that you don’t have too much fresh air (aka energy waste) nor too little air (stale air, or worse yet, sick building syndrome.) Further, it recovers about 2/3 of the heat in the outgoing stale air and uses it to heat the incoming cold fresh air. It sometimes goes by the name HRV, for Heat Recovery Ventilation.

These are not shockingly expensive, and with installation, ductwork, etc, run less than a thousand dollars. Here’s the first one I found on Google, and it is pretty typical. They are not too difficult to install if you are a little bit handy.

These are only worthwhile if your house is really really airtight or you are sensitive to even small amounts of indoor air pollution. They can be added later as well.

The construction techniques that give us airtightness are threefold.

First, we need a vapor barrier, specifically 6 mil polyethylene sheeting. After the framing is up, and the house is roofed, sheathed, insulated, wired and plumbed, we are ready for the vapor barrier. The plastic is installed on the warm/moist side of the wall, just under the drywall. It prevents warm humid air from escaping into/through the walls/ceilings.

This saves energy of course, but also prevents that moist air from causing moisture problems inside the walls. This can cause mold issues, rotting, funny smells and gross increases in heat loss. One of the (many) disadvantages of fiberglass insulation is that it loses 70-100% of its insulating power if it get moist/wet.

Every seam of the plastic sheeting is overlapped onto the next sheet and sealed with acoustic sealant. Every penetration by wiring, junction box or pipe is sealed with caulk or expanding foam. Then the drywall crew does their magic. It is good to inspect the vapor barrier prior to drywalling, as drywall hides a multitude of sins.

Second, we need an air barrier. This is generally known as housewrap, and the most common brand is Tyvek. It is resistant to air exchange and liquid water intrusion to prevent “outside” moisture from damaging the wall, but allows water vapor to pass out of the house. It is installed on the outside, just under the brick or siding. It does not have to be totally sealed like vapor barrier, just overlapped.

Third, we need good workmanship. Both the Tyvek and the poly vapor barrier need to be installed neatly, thoughtfully, and sealed/stapled/caulked where appropriate. A sloppy job here will negate half or more of the value of these products.

I am out of time at the moment and will finish up the list next time.

Finest regards,


1 comment:

Rachel said...

actually David said...

Troy - now that credible winter weather has arrived your efficiency improvements will be all the more apparent... next year. For now just keep your rabbit close by.

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