Spartan’s Introduction Guide to Building a Super-Energy Efficient Home.
Why do it?
The winter of 2013-2014 was a particularly cold one in New England. In Greenfield Massachusetts we logged over 8,200 heating degree days. Some folks tried to keep their energy bills down by lowering their thermostats. On the other hand, my wife and I enjoyed an average daytime temperature in our home of 70 degrees F. Sure, we too could have saved some by lowering our thermostat, but in a super-insulated home why bother? Our single mechanical heat source for our house is a 12,000btu air source heat pump. During this and other winter’s we spent just under $200 to heat our 1500 SF home. OK, I lied, we actually spent zero dollars. The PV array offset the entire usage, but if we didn’t have the PV array we would’ve spent $200.
Indeed, if we make our buildings super-air-tight and super-insulated they will use 75% or less energy than their counterparts. And with the necessary proper ventilation they will be healthier as well. These tremendous benefits don’t have to come at tremendous costs. As you will read below, one can save many thousands on dispensing with a central heating system. Also, there are generous rebates that can defray many thousands more of cost. So, if you find the right builder, you may very well end up with a super-efficient home that would have cost no more than it otherwise would have.
- The 20-40-60 represents R value minimums for the foundation, walls, and roof needed to achieve super-insulation. Typically foundation insulation is foam board (R5 per inch for XPS), but some builders who want to move away from foam are using Roxul (R4 per inch), which is a mineral wool based product rated for sub-grade use.
- Most every builder who is concerned with environmental impact of their buildings will use dense pack cellulose in the walls and attic/roof. It is made from recycled newspapers and is treated with borate. Borate is both an insect deterrent and fire inhibitor and is a naturally occurring mineral. Unlike fiber glass, dense pack can fill cracks and improve air-tightness as well as help mitigate moisture problems by safely absorbing some moisture just like wood.
- Dense pack cellulose has an R value of about 3.5 per inch therefore most super-insulated buildings have at least 12 inch wall cavities. Two approaches to create this thick wall are 1) double stud walls and 2) I-joists mounted on the outside of the load bearing 2×4 wall.
- There are many variants, but the most basic double stud wall is comprised of two 2×4 walls inside and out with plywood squares connecting the two. Spacing is usually 24” OC to increase the insulation content in the wall and decrease thermal bridging.
- One advantage of mounting I joists on the outside of a load bearing sheathed 2×4 wall is that the wiring and other utilities are always accessible as they are in open walls.
- All of our wonderful insulation is made useless if it is bypassed by air leaks. A meticulously detailed air barrier is imperative. By meticulous we mean if there is a nail that gets pulled out of the sheathing (which is usually used as the air barrier) one should patch it with a piece of tape. All pipe, window, door, and duct penetrations through the air barrier need to be appropriately sealed. Air barrier connections from the foundation to the wall and from the wall to the roof all need to be carefully considered. A quick helpful design tip: if one takes any section or plan drawing, one should be able to draw where the air barrier is without lifting up one’s pen. Air barrier sealing has become increasingly easy with the advent of fancy tapes coming out of Europe that can stretch and conform to complex surfaces. Check foursevenfive.com.
- One of the most common measurements for air tightness is ACH50. ACH stands for Air Changes per Hour; 50 stands for 50 pascals of pressure, which is roughly equivalent to a 20 mile per hour breeze. A building rated at 2 ACH50 will have its entire volume of air changed out twice over the course of one hour under 50 pascals of pressure. A leaky farm house might have 7-9 ACH50. My own home achieved a blower door rating of .58 ACH50—which is very tight. .58 was not difficult ot achieve and we probably could have done a lot better had I not made some mistakes with my roof design that compromised our air barrier.
- It is important to consider vapor separately from air. Indeed the barrier for one is not necessarily a barrier for the other—think Gortex which blocks air, but allows vapor to pass through. The big things with vapor are to minimize its entrance into the part of the wall that reaches dew point and to allow it to dry out (avoid vapor barriers all together). Foam board and spray foams are vapor barriers therefore, if one is using them, it is vital that there is enough thickness to ensure that the dew point is always inside the foam where the vapor can’t get to. Given our, climate this means 3” of foam. Air movement through the wall will carry many times as much vapor as compared to what would occur through diffusion. Again, dense pack cellulose to the rescue, air can hardly move through it.
- Even if one doesn’t have a super-airtight home, it is advisable to install a ventilation system (do you really want the air that you breathe to go through your walls and cracks of your home? Have you ever woken up in the morning to a bedroom that smells like a locker room?)
- Heat Recovery or Not
- Installing an HRV ventilation system that recovers heat can run $3-8000 depending on the quality of the unit. I spent ~$1500 for a Venmar EKO 1.5 which recovers 80% of the heat that would have otherwise been wasted. If the cost is too expensive one could always install a bathroom fan on a timer and have holes with dampers that allow fresh air in. Of course it will be cold near these holes, but now you have control over them and can eliminate them if the time comes. Even if the unit is too much, you may want to plan ahead for it by installing ductwork while walls are open.
- ERV’s – Energy recovery ventilators recover or exclude humidity as well recovering heat. This is not particularly necessary for our climate as our buildings don’t typically have humidity issues. On the other hand, in climates where AC is used a lot, it is very helpful to exclude humidity from the building.
- Centralized vs Decentralized
- Not all systems are centralized and have ducts. Centralized systems have a single unit, located in a mechanical space, that has ducts connected to the various rooms in the home. Typically, they exhaust from the bathrooms and kitchens, and supply to the bedrooms and living spaces.
- On the other hand units that are not ducted and get installed per room in the wall. Each unit is actually comprised of two identical components that trade the role of supplying and exhausting the air from the room. Lunos and Vents US Twinfresh are two examples of this approach.
- Ventilation can get complicated if one is venting over the stove. In leaky houses one only needed to install an exhaust hood. In a tight house one also needs to install make-up air. There are more products coming on the market that do both. For my own home, I avoided the matter by forgoing an exhaust hood.
- Wood burning appliances
- Similar to cooking, it is important to have make-up air for any wood burning appliance. The best thing to do is have a sealed duct supply air to the appliance.
- Sound transmission concerns
- If using a centralized ventilation system it would be wise to consider sound dampening in the ducts. High end models like Air Pohoda and Zehnder have sound dampening built in or as an option as well as using small diameter, non-static, flexible plastic pipe for ducts.
- Radon is a concern regardless of what kind of house one builds. Make sure that you install pipe beneath the slab and either pipe it out the roof, or make it so it is easy to retrofit.
Building Materials to Reduce
- Some building materials are particularly taxing on our planet. The following all require a lot of heat to create or have toxic chemicals as part of their manufacture:
- Forgo the basement, use 6” rebar reinforced foundation walls rather than 8” typical.
- There is one neat thing about foam. It is fairly easy to purchase reclaimed foam board from building demolitions. (See links on the right side of the home page of this blog).
- Doesn’t come up too much in construction except in aluminum clad windows and siding
Passive Solar and windows
- Passive solar heating costs no additional money to benefit from and will never break or need upkeep. It offers huge rewards when implemented correctly. Take for example my own home. On sunny winter days, regardless of the outdoor temperature, we gain enough heat from the sun to be able to shut off our heat pump entirely and enjoy 70-72 degrees in our home. On days where it isn’t sunny, heat still comes in, just not enough to allow us to shut off our heat pump.
- Orienting the building so the largest surface area of the building faces south is ideal. Also, lay out the interior so as many living spaces as possible benefit from south facing glass. In addition to solar heat gain, one will benefit from daylighting.
- Not all glass is equal. Adjusting the coatings will yield a window that can let in heat gain or exclude it. The best high heat gain windows capture 65% of the available heat.
- Mass and over-heating
- On sunny winter days, particularly if the outdoor temperature is warmer, overheating can be a concern. Appropriate passive solar design will include a properly balanced ratio between glass and mass. By mass we mean things like concrete, rock, and water. Of course the simplest of these to implement is a concrete slab floor. The mass serves as a “battery”; that can store the heat that would otherwise overheat the air.
- Over-heating is also a concern during the summer, but perhaps not in the way you think. Those high heat gain south windows actually contribute less to over-heating than east or west facing windows. During the summer the sun doesn’t have direct access to the south glass. On the other hand, for the first half of the day the east facing windows get a lot of exposure and conversely the west in the afternoon. Low heat gain windows that only let in 20-30% of the sun’s infrared radiation should be spec’ed in (this is what is ordinarily sold). Having a large shade tree directly to the east and west of the house is desirable.
- Windows in thick walls: innie, outie, or inbetweenie
- OK, so let’s say we have decided to have super-insulated walls that are 13 or more inches thick. Now we face a decision: where do the windows sit. All approaches have been taken by builders. The general consensus is that it is easiest to have “outies”. This means one gets deep window jambs on the interior. If I were to do it again, I would go with drywall on the side and top, and use a nice piece of thick hard wood or stone for the sill.
Space Heating systems
- Central oil/propane heating system
- The smallest boilers output around 25-30,000 btu’s per hour. If one superinsulates one will probably only need 15,000 btu’s per hour (as a point of reference most homes need at least 75,000)
- Since, the load for super-insulated homes is indeed so small, central heating systems are generally dispensed with. One exception is to attempt to integrate a heating element with the ventilation system. So far, I have not seen or heard of any local examples.
- Air source heat pumps
- The heat source of choice for super-insulated homes has been air source heat pumps (ASHP). This technology is energy efficient, relatively cheap, low maintenance, and quiet. A compressor sits outside the house and one or more heads are located inside either on the wall high up, on the base of the wall like a cast iron radiator, or in the ceiling like a large bathroom fan. Heads can output anywhere from 7000 – 36,000 btu’s. Though typically not ducted, there are units that can be. These units can either heat or cool and use a fan to blow the temperature into the living space. Each head has its own thermostat and can blow in a variety of directions at a variety of speeds.
- A single compressor and head will be around $4200, while a a 3-headed multi-head unit with one compressor will be around $8500. Compare to a central heating system which will probably be around $15,000.
- These are point source heating devices so open floor plans help to make them effective.
- Since they run on electricity, a lot of folks enjoy offsetting the cost of running them with a PV array.
- In Massachusetts there are generous rebates for the most efficient units that can reduce the cost by as much as 30%.
- Radiant floor heat
- We all love the luxurious feeling of a warm floor. That radiant floor feels warm because it is warmer than our skin temperature. Our skin is usually around 80 degrees F, and radiant floors are typically 90-110 F. A super-insulated home with a slab or floor at 90 degrees will quickly find that the entire house is at 90 degrees. There simply is not enough heat loss to balance the equation and make radiant work. That being said, it still could be done. Rather than heating the entire floor one could heat select locations that are frequently trafficked. Presumably one has forgone central heating which means that the radiant floor will probably be electric.
- Wood Stoves
- I too love the feeling of a wood stove cooking away. The ambiance is truly wonderful. Unfortunately, all wood stoves will over-heat a super-insulated home. That being said, one can always crack a window or two.
- Masonry wood stoves are far more efficient that even EPA certified wood stoves. By reaching much hotter temperatures more of the combustion gasses are burned and there is significantly less particulates and ash. The downside of the masonry wood stove is that it too can cost as much as $15,000.
- Solar hot water space heating
- Could be done, but, like central heating, is not worth the cost.
- Basements are only useful if they are dry. In areas with high water tables and when one can’t drain the footing to daylight, I would strongly advise against a basement. If you find yourself missing out on a basement for this reason you can take solace in using less concrete.
Domestic Water Heating
- Assuming one has forgone fossil fuels, there are really only two choices: Air source hot water heat pumps and solar hot water.
- Since we live in Massachusetts and benefit from generous rebates for solar hot water, the choice is easy: solar hot water. After rebates and incentives (good until Dec 31 2016) a solar hot water system will probably only cost around $3000. An electric element will serve as back up.
- Air source hot water heat pumps are not nearly as efficient as solar hot water, but are certainly far better than straight up electric resistance water heating. They work by using the heat pump to draw heat out of the basement air. In a normal house the basement will warmed by the ground, which is always at least 50 degrees below the frost line, or from waste heat from the boiler. In a super-insulated basement with no boiler… you see the problem. They also make some noise and need at least 100 SF of floor space to draw warm air from, so mechanical rooms on the first floor are usually not a good location.
- Plan for it with conduit and location, but spend your money on insulation.
- Local contractors: Northeast Solar, Hatfield and PV Squared, Greenfield
- A Tier 3 Energy Star certified house will earn you $7000, contact CET during the design phase for details.
- Solar hot water: up to $4500
- Air source heat pumps: up to $4250