Numbers

I recently gave a presentation to the Super Insulation class at Yestermorrow Design/Build School about building and living in our house. This presented a convenient opportunity for me to take stock of the building process, consider our experience living in the house so far, and to put together some performance numbers. We've been in the house just under two years and have established a track record for firewood consumption and are starting to get some reliable numbers on our propane and electric costs.

When our home was completed Efficiency Vermont (the state energy conservation organization) tested and evaluated our house for energy performance and determined a HERS number, which was presented as a part of their evaluation. HERS stands for Home Energy Rating System and is the standard methodology for comparing performance across buildings and regions. Many factors go into the rating system, including air-tightness, window performance, efficiency of the appliances, lighting and heating systems, as well as design specs like insulation R-Values and so on. The rating is adjusted for climatic conditions in a given location.

This is a generic chart showing the HERS rating scale. The "This Home 65" has nothing to do with our home.

In the above chart, 100 equals the energy use of a new home built to conventional standards in 2006. This is used as a base line by which to evaluate the spectrum of performance across buildings.  A rating of zero means the house requires no energy at all, while a rating over 100 means the house is sub-par in relation to current norms.

Our home earned a HERS rating of 38, which is a "5 star +" in the star rating shown below.  The scale somewhat curiously goes from 1-5 stars and then has a + category.  (Maybe they needed to add a new category as more efficient homes are being built -- I don't know.)

The HERS rating laid out estimated energy use predictions for our home as follows (calculated in 2011):  

             26.6 MMBtu heat  $990

             10.1 MMBtu hot water  $375

             17.9 MMBtu lights & appliances  $704

             HERS estimated annual energy cost:   $2,188

Based on our actual usage, these are our costs:

           $100-$150/yr firewood

           $750 propane hot water/cooking

           $768 electric

           Our actual annual energy cost:   $1,668

In the nearly two years we've been living in the house, we have only heated by wood. There is the radiant system in place should we ever choose to use it, but for now we like heating by wood; it's easy and feels good.

Although we know from experience that our heating expenses are tiny, it is useful to see them laid out together with the other home energy requirements. We plumbed the house in preparation for installing a solar hot water system, and seeing these numbers is a good motivator for costing out the completion of that project. It would feel good to bring the hot water energy numbers down. 

Dedicated Air

Air to supply the woodstove comes in through this duct hood...

...then passes under the first floor and up to the woodstove...

...then into a little add-on air-supply box under the woostove where the stove draws it's air. Once combusted, the fumes travel up the stove pipe...

...into the insulated pipe in the second floor living space and up through the roof...

...and back out to the great outdoors, having never mixed with the interior air

A key element required of a super-tight super-insulated house is insuring a dedicated air supply to all combustion. Likewise all combustion needs to have a dedicated vent route, also isolated from the interior air.

Two examples of this principal are the propane boiler in the basement which has a short in-and-out combustion air supply and the woodstove on the first floor. The woodstove, unlike the boiler, is located in the middle of the house so there is a good measure of ducting running from the outside supply hood to the woodstove and then exhausting from the wood stove up the stovepipe to the insulated stovepipe and through the roof. I've been quite careful to make sure the whole run is tightly sealed so that the woodstove is only pulling air from outside and only venting to the outside. I don't want any leaks.

The not-really-resolved aspect of this equation is the propane-fueled gas stove-top range. The air supply is the ambient household air, and the venting, such as it is, will be a hood mounted over the stove top. This will all work okay, but it isn't ideal. If we run the hood, we'll need to open a window or door to allow air in so as not to create a negative air pressure in the house which could then result in a back draft in the woodstove. We have carbon monoxide detectors on both floors even though they are not required by code since there will be a small amount emitted by the stove.

One of our submarines...

This propane tank will soon be buried somewhere in the vicinity of the house.