Saturday, April 19, 2014

Evaluating Natural Daylight Levels

After visiting one of our new early childhood development sites, we noticed the inside of the stimulation rooms (classrooms) were a little dark. They buildings are supposed to be naturally daylit, but nobody on the design team knew anything about estimating daylight or optimizing the design. Subsequently, I’ve reviewed the design and taken light levels in the field. My initial conclusion was that the light levels in the center of the rooms at the floor level often met or exceeded recommended levels (primarily because the windows extend very low to the floor below typical vision glazing and light reaches this spot from multiple directions), but at 1 meter off the floor and in many of the corners the levels were below recommended levels. The Illuminating Engineers Society (IES) recommends 50 foot candles (500 lux) on the writing surface for schools for visual comfort and productivity. Based on the 4 sites I measured, the levels with full sun are typically:
20 - 30 FC in centre of room at 1 meter
60 - 80 FC in centre of room at floor
5 - 10 FC on bench in “front” of room

With these light levels, activities low to the ground in the center of the room (building is designed for children 0 - 6) will be well lit with daylight on sunny days and most overcast days, but children in the corners of the room will have less light than ideal. While a couple of the sites have electric lights available to help alleviate this condition (two, 13 watt CFLs without a fixture), we should advise teachers in all sites to focus art projects, reading, and other visually sensitive tasks away from the bench area.
There are a number of potential ways to improve the lighting levels if doing a re-design, as well as some options to improve levels in the already constructed buildings. We decided to go with painting the interior brick white to improve reflectance. Each site has three stimulation rooms so we painted the two longest walls white from floor to ceiling in two of the rooms at one site and re-did our testing. Light levels were almost double in the painted rooms and significantly improved light levels. All sites have since been painted (some not yet to the ceiling as in the image below). While the bench area is still darker than is ideal, the rooms are much improved as a result. At other sites that are nearing the end of construction, we're going to remove some of the brick vent holes at top of the front wall and replace with a framed, translucent plastic window, which should bring the daylight levels up to recommended levels.
The lesson learned in this exercise is that daylight modeling or at least crude daylight factor calculations are critical for buildings intended to be naturally lit.
Extra info (sent to my supervisor when trying to raise the issue):
An easy way to evaluate natural lighting is daylight factor (DF), which is the ratio of outside illuminance over inside illuminance, expressed in per cent. The higher the DF, the more natural light is available in the room.
The general rule is a room needs to achieve at least 2% DF to be considered daylit, though this is still considered gloomy and electric lighting is needed most of the day. From 2 to 5% the daylighting is better, but electric lighting is still needed up until 5% for optimal visual comfort.
Using the crudest rule of thumb method of estimating daylight factor (DF = 0.1 * Glazing Area / Floor Area), it looks like we would just be above the 2% “daylit” threshold as we get 2.7% DF (13.7 m2 glazing / 50.8 m2 floor). Unfortunately, this is overly optimistic in our case for a number of reason. Daylight factor is the sum of three components: direct lighting component (DC), externally reflected lighting component (ERC), and internally reflected lighting component (IRC) such that DF = DC + ERC + IRC. The rule of thumb metrics assume typical office building values for all variables. There are a few problems with this method as we need to account for:
  • Many of the windows and one door are shaded from most direct sunlight by roofs above
  • All of the masonry vent openings are deeper (22 cm) than they are tall (8 cm) so let in no direct sunlight most of the day
  • Most interior surfaces are dark and non-reflective and standard calculations assume partially reflective white ceilings and light colored interiors
As a result, most of our windows and openings have 0 direct lighting component because of the overhangs (good for avoiding heat gain, but also less visible light), we have very little externally reflected daylight since there are no surrounding buildings other than the others we’ve built with non-reflective exterior surfaces, and we have little internally reflected lighting as our interior materials (especially the ceiling) are darker and less reflective than a typical office. We do have the benefit of very clear glazing in the windows with higher than typical visual transmittance (VT) and of course no glazing in the ventilation openings.
Multiple field measurements on the overcast day in Site A showed a range of 1% to 2% DF in the center at 1 meter and about 2.5% to 5% DF in the center on the floor. DF calculations at other sites were not possible as it needs to be overcast and low enough direct sun levels to not overload the meter, but they confirmed the Site B assessment by showing full sun measurements in line with what was expected. 
To get up to the 5% daylight factor for the entire room (suggested target), we’d ideally incorporate a combination of increased opening size, especially up high where the contribution to daylight factor is greater, and lighter and more reflective interior surfaces. Even adding the colored paint in the current rooms has already brightened the space a lot compared to the pre-painted condition. We’ll see the impact of the white walls in Site A. 

Sunday, March 10, 2013

Earthen Ducts Question


My husband and I are designing a modest home in northern Indiana to passive house standards. We have a beautiful building site on a south-facing ridge in the middle of forty acres. Picture a house earth sheltered on three sides and open to the south to incorporate passive solar heating/cooling. We will not need a furnace or AC. What we do need is a well-designed whole house ventilation and dehumidification system. I’ve been researching these topics for over a year.

My conclusion: I want to explore the concept of using an earth tube or tubes to precondition our fresh air intake. If I can build a tube system that will achieve a 50% reduction in moisture load on incoming air in the dog days of summer, then tubes are the way to go. My math is not up to the task of evaluating the design. I’ve been trying to connect with a mechanical engineer who could give advice on the psychrometrics. If I can’t find someone to help me geek out the system variables for our site (tube diameter/length, depth, rate of air flow, pressure) then we will have to go with a purely mechanical system to be on the safe side. And then I will always wonder if we missed a grand opportunity.


I love the idea of Passivhaus, though haven’t been able to work on any projects yet.  Good luck on the project.
Calculating the benefits of earth ducts is probably above my math too (I’m not an engineer).  Not one of the easier systems to analyze – I was expecting something easier J   We recently looked at it for a project in Durban, South Africa and besides calculating the potential energy savings, issues to consider include risk of condensation, fan power, and cost of construction.
The ground temperature below a couple of meters is usually around the average annual air temperature for your location.  The ground temperature tends to lag the air temperature by a couple of months and the deeper you go the less the ground temp changes.  Using data from South Bend – Michiana Regional Airport, we get the graph below, showing your ground temperature average is about 50 degrees F, but ranges from 40 to 62 at 13 feet and from 30 to 72 at 1.6 feet (this was all in meters originally – so the units for feet aren’t regular).
 With enough surface area for incoming air to duct contact, you can eventually get the outdoor air almost to the same point (dry bulb) as the ground temperature.  This means that in September, if your duct was 13 feet below ground, you could get the air to about 60 degrees even if it is 90 F outside.  Conversely, you could get the incoming air as high as 45 degrees (13 foot duct) in February, even if it is 0 degrees outside.  Unfortunately, to change the temperature that much, you need longer duct runs.  Assuming you need about 55 cfm of ventilation (2500 sf x 0.01 cfm/sf + 4 people x 7.5 cfm/person), the graph below shows what percentage of the temperature difference (between earth temperature and outside air temperature) you could make up  with different diameter ducts and different lengths (I assumed square / rectangle ducts).  This says that 20 meters of 0.1 x 0.1 duct would make up 90% of the difference between outdoor air and ground temperature.  For example, when it was 0 degrees outside and 50 degrees in the ground in January, you could make up 45 degrees just from going through the duct.
It doesn’t look like fan power will be that big of a deal for you (if you’re providing 55 cfm), regardless of the distance.  When I looked at the project in Durban we were trying to move 8,000 cfm and the fan power penalty was significant.  It looks like you’d really only need a 1 watt of fan power (though you’ll probably have to get one bigger just because that is what is available).  1 x 365 days / year x 24 hours / day / 1000 Wh / kWh = 8.76 kWh per year (about $0.88 per year in fan energy).  I’m only looking at 55 cfm as the ventilation requirement – this air isn’t meant to heat or cool. 
Finally, the last thing you’d want to consider is condensation.  The psychrometric chart below shows one dot for every hour of a typical year (8,760 dots) in South Bend.
The same chart below shows just the dots for a typical August.  During this month, your ground temperature could be around 60 degrees (assuming 13 feet deep).  During the hour that I’ve highlighted (point 1), the air temperature is about 87 degrees, 70% Rh.  When you cool this down towards 60 degrees, you start to get condensation at about 76 degrees (point 2 – the air is now 100% Rh).  By the time you get to point 3, you will have gone from 0.019 grains of water per pound of air to 0.011, with the water that is no longer in the air condensing out in your ducts.  I honestly don’t know how big of a concern this is, but in a typical air conditioner you’d collect the condensation at a specific point and be able to deal with it.  There may be concern about bacteria if you have standing water running the length of your ducts during the summer.  You wouldn’t have to worry about this in the winter.
That’s all I have for now.  If you have specific questions about your system, I might be able to do the calculations for you.  I made a few assumptions in the calcs above (6 bends in each duct work, turbulent flow, concrete ducts for example) and adapted calculations we already had for a larger commercial system, so if we had a real system and started from scratch the numbers could be tightened up  a bit, but I think this info is close.  Thanks to Alejandra Menchaca, PhD and Director of Operations for the Boston offfice of EA Buildings, who did most of the original calculations for our project in Durban.


Wednesday, March 6, 2013

Ground Source Heat Pump Question

Wed, 6 Mar 2013

Hi Nathan,  I am writing about our house in Lexington which my son and family are living in. He has contacted a firm who is planning to install heat pumps to heat/cool 2 floors of our split 3-level house. It is estimated to cost $20,000 with an interest free loan. I just am somewhat skeptical that heat can be supplied reasonably cheaply with electricity and heat pumps in this climate. I wonder if you are able to send me to appropriate literature or home owners who are using this equipment or perhaps give me your opinion. Sam



I usually think of GSHPs in terms of their coefficient of performance (COP). A good one can get around a COP of 4, meaning for every one unit of energy (electricity) you put in you get 4 units out as heat. According to the 2009 Residential Energy Consumption Survey ( the average house in Massachusetts uses 65,200,000 Btu of energy on heating per year (652 therms about $652 cost). Since most homes use a regular boiler or furnace of around 80%, I'd estimate they only need 52,192,000 Btu of heat (the other 20% is inefficiencies from the boiler). This is the same energy as 15,296 kWh. To get this from a GSHP with a COP of 4, you'd need 3,824 kWh of electricity (about $382 cost). Conversely, if you provided the same 52 MMBtu of heat with a 95% condensing boiler, you'd use 54,939,000 Btu of natural gas (549 therms and $549 cost). GSHPs will save some energy, but the payback is usually pretty long.

As to whether or not the GSHP will work in Boston, it definitely can. We used a lot at Harvard. I know a guy in Somerville who has one for his house (case study link below). Average ground temperature in Boston is about 50. At shallow depths it ranges between about 30 to 70 (see attached from Logan). The ground temperature usually lags behind the air temperature by a month or two (the coldest months for ground temp are a couple of months behind the coldest air months). Still more efficient than trying to use an air source heat pump where the delta T is greater and less in your favor.

The GSHP will also be more efficient than a typical air conditioner, but the annual air conditioning cost is less than the heating cost. Assume you could save 25% or so on your AC bill (say going from a COP of 3 to 4).

I hope this helps.


Thursday, January 19, 2012

Paper on Energy Modeling Barriers

Holly Samuelson and team at the Harvard Graduate School of Design recently presented a paper on "Identifying Non-Technical Barriers to Energy Model Sharing and Reuse".  This is a topic dear to my heart as while I was at Harvard I chaired the Green Building Standards committee and wrote the language requiring all major renovation and new construction projects to submit as-built energy models in electronic format as part of their closeout documents.  We also recommended projects to use eQuest or Energy Plus unless they had a defendable reason why one of these programs wasn't viable.  As far as I know, this was the first such requirement and nobody really know how the industry would react.  Holly and friends took this idea and surveyed 154 energy modelers to see what they thought about sharing energy models.  A whopping 75% of the respondants indicated that they would share these files.  Those that wouldn't share gave a range of reasons including (in decreasing order of prevalence) that the models are too complicated to be understood by others, the models don't represent reality in the finished building, the model itself represents intellectual property not to be shared, and finally a couple of engineers felt that owners do not have staff qualified to receive the model (though I'm sure this is almost universally the case).  I spoke with Holly years ago when she was initially contemplating the paper and survey and she gave me a shout out in the acknowledgements for my help.  She also acknowledged fellow EA TAG member Chris Schaffner (the Green Engineer). Thanks Holly and keep up the great work.

Tuesday, January 10, 2012

USGBC Blog Post ~ Project Haiti

I'm about to get married and my fiance and I would like to share our good fortune with others. We both support the USGBC Project Haiti project to address the health and emotional needs of orphaned children and provide a pathway to adoption in that country. What's more, the project plans to do all of this with a green buildling that can be a model for sustainable development in that country. We’ve found that our friends and family are equally inspired by this worthwhile cause. That being the case, we chose to donate to Project Haiti ourselves and include it in our wedding registry as an option for others to donate on our behalf. Here's the Project Haiti website:

The USGBC recently asked me if I'd write a blog post for them about our decision. You can see the full post here:

Wednesday, January 4, 2012

Water Cooler Timers

The following is a simple Energy Conservation Measure identified for a site in Mexico:

Existing Condition: Water Coolers on 24 Hours per Day
There are at least 10 bottled water dispensers, each of which consumes 1.45 kWh per day to heat and cool the water. These units continue to draw some energy during the evenings and weekends when nobody is in the building, consuming up to 5,293 kWh annually and costing 7,430 pesos ($619 U.S.).

Recommendation: Add Timers to Water Coolers
Add a simple programmable timer to each of the water coolers. The should be set to turn on when people are scheduled to first arrive at the building and turn off when the majority of people are scheduled to leave.

It is assumed that a simple, 7 day, analog timer will be able to reduce annual energy consumption by at least 50%, saving 3,715 pesos ($310 U.S.) annually. It is assumed that these can be purchased for $15 U.S. (180 pesos) each, or $150 U.S. ($1800 pesos) total, and that no cost of labor is needed to install them.

All costs below in Pesos...

Monday, December 19, 2011

Electric Hand Dryers

Conservation Measure from Recent Utiliyt Audit - Electric Hand Dryers

$2,000 first cost, $17,200 annual savings, less than 2 month simple payback.

Existing Condition: Paper Towels in Restrooms
There are at least 9 restrooms in the building, all of which have paper towels for hand drying. Historic purchasing records show that 30 units are purchased per week at $15.42 per unit for an annual spend of $23,125.

Recommendation: Electric Hand Dryers
Remove the paper towel dispensers from restrooms and install energy efficient electric hand dryers. A good hand dryer will use 1,500 W or less and require about 12 seconds to dry the user’s hands. The Xlerator is one such option and sells for $400. It is recommended to install electric hand dryers in the two large production area restrooms. Since 90% of the building users are in the production and warehouse areas, these areas will see the greatest savings. Since the payback will be longer in the office area restrooms and the electric hand dryers can be loud, it is not recommended that hand dryers be used in these areas at this time. Most units do not require special wiring and can be installed with in-house labor. Units can be purchased online at sites such as:

It is assumed that 2 hand dryers are needed per restroom for each of the two large restrooms. The units are $400 each. It is assumed that labor and mounting costs will be an additional $100 U.S. for each unit or $500 total. The total cost will be $2,000. It is assumed all 1,900 production and warehouse workers will use the restrooms and the hand dryers 3 times per day for 250 days per year. The units will draw 1.5 kW and will run for 12 seconds per use (0.0033 hours) for a total annual electricity consumption of 7,125 kWh. At $0.12 per kWh, this is $855 per year. It is assumed that 75% of the total annual paper towel cost is attributed to the two large restrooms, which represents $18,038 annually.

This measure was identified for a factory in Tijuana. All prices were converted from Pesos.