Energy model

The road to a design workshop for our sustainable rehab took us to an energy model for the house (see also 09/09/2009 post). Our goal is to have a super insulated and airtight building envelop. But how much energy, CO2 emission and dollars will this save us and how much heating will we need?

I provided Corbett Lundsford from the Green Team Group with building floor plans and some building parameters that he input into the REM/Rate™ Home Energy Model:

Building shell:

  • Insulation of building shell: R-21 in 2×4 stud wall, 16 inches on center
  • Insulation of basement slab: R-5
  • Roof insulation: R-52
  • Windows: Double glazed, low E wood frame windows,  U-value of 0.25 and Solar Heat Gain Coefficient (SHGC) of 0.5
  • Exterior doors: Front door with U-value of 0.27 and all other exterior doors with U-value of 0.19
  • Infiltration: 0.2 natural Air Changes per Hour (natural ACH)

Mechanical equipment:

  • Heating: radiant heat (hot water radiators) with natural gas fired 90 AFUE boiler at 60 kBtu/h capacity
  • Domestic hot water: natural gas fired water heater at 0.63 EF with 30 gallon storage tank
  • Cooling: We decided against any air conditioning system, but would like to use ceiling fans and have a small window unit as a backup if needed. Over the past few years, we had a window unit in our un-insulated and drafty Elmhurst apartment. We may have used it three or five days in a year, usually when we had visitors. Knowing that we will now switch a very well insulated and airtight envelop, we don’t see the point investing in an air-conditioning system we won’t use.
  • Ventilation: Energy Recovery Ventilator at 300 cubic feet per minute (cfm) capacity and 250 watts

Renewable Energy:

  • Solar hot water: 128 square feet of evacuated tubes with 150 gal storage tank
  • Sun room for passive solar: Our enclosed back porch faces due south and offers the best and only opportunity for passive solar gain in the cooler seasons. We have the idea to create a ‘sun room’ with glazing on most of the south elevation, and awnings for shade in the summer months.  The infiltrating sun in the cooler months would heat up the building’s rear brick wall. We hope to use this thermal mass and energy to precondition air with which we would ventilate the rest of each floor.


Corbett run three model scenarios for us, each with a slight modification to the above mentioned sun room:

Unconditioned sun room, full glazing

  • Design Load (kBtu/h): 53.2
  • Annual Load (MMBtu/h): 63.1
  • Annual consumption (MMBtu/h): 70.7

Conditioned sun room, full glazing

  • Design Load (kBtu/h): 52.3
  • Annual Load (MMBtu/h): 61.8
  • Annual consumption (MMBtu/h): 69.2

Conditioned sun room with ½ window sizes

  • Design Load (kBtu/h): 48.6
  • Annual Load (MMBtu/h): 57.3
  • Annual consumption (MMBtu/h): 64.2

I have to admit that I was surprise. I did not expect that bringing the sun room into the conditioned envelope would reduce our energy needs. In other words, heating the sun room will help us to save energy. Sounds somewhat counter intuitive, doesn’t it? That the reduction in window size would result in better efficiency was more intuitive, as windows are the weakest link in the envelope.

Here is another interesting result that adds to the feel good factor: Corbett compared our performance targets (outlined above) to those of conventional building performance levels. Lo and behold, we could save as much as $1553.00 a year on energy cost (and that is with current energy prices…).


This energy model is by no means the final word, but provides us with a baseline that we can use for the workshop, design and decision making processes. I was sieving through the report details looking for opportunities and found them in the component load summary. This part of the report identifies how much energy (in Btu) is needed by or lost to various building components.

The single biggest energy loss is attributed to infiltration, in other words, having warm air leaking out of the building and cold air leaking in. This loss occurs at a rate from 28.8 to 30.5 MMBtu/yr., which is anywhere from 45 to 53% of the annual heat load! I like to think that this has opportunity written all over it if we take objective of a super insulated and air tight building envelope seriously!

What is:

ACH (Air Changes per Hour)

AFUE (Annual Fuel Utilization Efficiency)

Btu (British Thermal Unit)

EF (Energy Factor)

Low E (Low Emissivity)


SHGC (Solar Heat Gain Coefficient)


About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

4 thoughts on “Energy model

  1. Marcus,

    Since you have most (if not all) of the floor joists exposed, have you considered underfloor radiant hydronic heat? I wonder what the model would predict for that vs. radiators? Underfloor heat offers the advantage of focused comfort -> if our feet are warm on a 80 degree F floor, then we are much more comfortable in a 65 degree room than we would be in a 70 degree room without a heated floor. Usually, the overall heat input is lower with a heated floor because of this comfort effect.

    Our house in Maryland is a 55 year old house with plenty of glass. We live primarily on the lower floor and I can access all of the floor joists from below. Radiant hydronic heat is definitely on my wish list for energy improvements!

  2. This is the post I’ve been waiting for: It is very interesting and helpful to know of your methodology in addition to seeing the work and output.

    I have heard of ideas for air conditioning powered exclusively by a dedicated solar circuit. Apparently, a chunk of the cost of solar panels is the electrical element which prevents solar AC power from backfeeding up the line to the utility’s electrical grid. A dedicated circuit removes the need for this element. You can only run your air conditioning on sunny days, but it’s assumed this is when you’d mostly need it anyway.

    Like John, I have been looking at radiant floor heat solutions, since I’m in a space with easily accessible floor joists underneath. I found a company called Radiantec which looks promising ( One of their more interesting ideas is using a super-efficient water heater for both radiant floor heat and domestic hot water. Anyone work with them? Anyone know of other quality vendors?

    I look forward to your plans for sealing the building envelope: We are also in an older, Chicago brick building.

  3. Justin, what you describe there is called “direct PV applications”. Basically having a PV array dedicated to power a specific utility. I have started to look into direct PV applications for de-humidification or air conditioning. There are units manufactured to RV’s or yachts. I am unclear to date if they are sized large enough for a 1500sf floor… More research ahead!

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