Category: Insulation

Air Infiltration - Blower Door Test

5/31/2014

A blower door is basically an industrial fan mounted in a frame that fits into an exterior doorway. The fan pulls air out of the house which lowers the pressure inside. The higher outside air pressure causes air to flow into the house through all unsealed cracks and openings.

The blower door unit has pressure gauges to calibrate the pressure difference being used for the test and a manometer that measures the total amount of air flowing in through cracks and openings.

Air infiltration is measured in cubic feet per minute (CFM) and is then calculated as a number of Air Changes per Hour (ACH)

Georgia currently utilizes the 2009 IECC (International Energey Conservation Code) which mandates that all new homes must have an energy audit and must achieve an air infiltration rate of 7 ACH or less. As I wrote in a previous post (Air Infiltration - How Tight is Good Enough), this is a pretty poor standard and we are aiming to do much better. The 2012 IECC standard sets a much stricter standard of 3 ACH or less.

Normally, a blower door test is done after the house is entirely complete. We decided that we would do two tests: one with just the flash insulation and a final test when the house is complete. This allowed us to fill some leaks and make a few improvements before the drywall goes up. It also allows us to measure the effects of drywall and finishing on the overall performance.

Keith Jensen from Home Energy Shield did the test.

The total volume of the house (counting the basement) is about 45,000 cubic feet. Code allows 7 ACH which would be roughly 315,000 cubic feet per hour or 5247 CFM.

Our test came in at 2294 CFM, which is approximately 3.06 ACH.

We are very happy with this result. Tight construction, quality windows and flash-batt insulation have resulted in a package that just about meets the future IECC requirements and we don't even have the drywall installed yet.

Is it Cost Effective??
Based on the computer models, moving from ACH=7 to ACH=2 will save about 20M BTU of heating per year, which will save about $350 per year in electricity (neoTerra is all electric). This would be a savings of roughly $7000.00 over a 20 year period.

On the other hand, spray foam insulation is expensive. You have all the cost and labor of installing batts and even more expense and labor for the spray foam. You essentially insulate the house twice. The spray foam is labor intensive since all of the windows and floors have to be covered in plastic and taped to protect against overspray.

For neoTerra, the flash-batt system cost more than twice as much as just installing fiberglass batts. The fiberglass came to roughly $2.00 per square foot while the spray foam was more than $3.50 per square foot. At $7400.00 the cost of the spray-foam is sort of on the edge of being cost-effective if we only look at the savings on electricity.

As an additional consideration, however, reducing the heating load by 20M BTU/yera allowed us to save about $2000 on HVAC equipment.

My conclusion is that flash-batt insulation is cost-effective and will improve the comfort of the house.

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Flash Batt Insulation

5/29/2014

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Insulation is a big factor in any energy efficient design. As I wrote in a much earlier post, air infiltration would account for about 40% of our heat loss if we used conventional insulation so we looked at ways to build a tight envelope.

In the early design stage we looked at building with Structural Insulated Panels (SIPs). The SIP panels are typically made as a sandwhich that has OSB sheathing bonded to an insulating foam core. With SIPs you can construct very tight, energy efficient shells but the cost was just prohibitive.

We also investigated using spray foam to fill the wall and ceiling cavities but the foam is very expensive.

We decided to go with a "Flash-Batt" system. This technique uses a layer of spray foam against the outside walls to get an airtight seal and then conventional fiberglass batts, which are much less expensive, are used to fill the rest of the wall (or ceiling) cavity to get the desired R-value.

The spray-foam creates a vapor barrier against the oustide wall. The system has to be designed for the specific climate to avoid having moisture condense inside the walls, which would create mold or rot problems.

We used 1" of closed-cell foam in the walls (about R-7) along with R-19 Batts for a theoretical total of R-28. However, the 5 1/2" batts are being compressed into a 4 1/2" space so the actual R value is probably closer to R-24.

The ceilings have 2" of foam and R-25 batts for a total of R-39.

The basement walls are precast concrete panels from Superior Walls and have 2" of Dow blue board cast into the walls. Along with R-19 batts this provides a total R value of R-31.

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Air Infiltration - How Tight is Good Enough?

3/9/2013

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Air Infiltration Standards
There are several energy standards to choose from. Each one has a different intent. A standard that works for Germany or Minnesota might not be the most appropriate for Miami or the Bahamas.

Consequently, the standards for air infiltration vary dramatically and don't provide good guidance on their own. The three main standards are IECC, Energy Star and Passive House. Passive House is the most strict while the IECC is absurdly lax.   Here are the maximum ACH50 rates for the various standards.

    - IECC (Georgia code)                          7
    - Energy Star for climate zones 3,4       5
    - Energy Star for climate zones 5,6,7    4
    - Passive House                                  0.6

So, Passive House is more than ten times better than current code and about 8 times better than Energy Star requirements.

But how much is good enough? We can do some calculations based on the previous Baseline design to estimate the energy savings. As a spoiler alert, fans of Passive House might want to skip this section.

The table on the right shows the annual BTUs and cost of air infiltration for different levels or air tightness. This is assuming an Air Heat Pump with a cost of $1.76 per therm.

There is, of course, a law of diminishing returns. Cutting from 7 -> 2 (roughly a factor of 3) saves about $350 per year. Further improvement from 2 -> 0.6 (roughly a factor of 3) only saves an additional $98/year.

It would seem that meeting the Passive House air infiltration standard is not a cost effective goal. We can probably find less expensive ways to save another $98 / year on energy.

Going forward, I'll assume a goal of hitting an air tightness of ACH50 = 2. Recalculating the heat load and cost shows that this reduces the annual heating load by about 20M BTU and that air infiltration now accounts for only 12.82% of the heating load.

By far, the windows are the greatest heat load (almost 45%). In future posts I'll look at the solar gain and show that the south facing windows can collect more heat than they lose.

Primary Sources of Air Infiltration

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Baseline Design Part III - Estimating Energy Costs (without solar)

3/3/2013

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In this post I'll calculate the approximate cost for heating the Baseline design. At this stage, I'm still not accounting for any solar gain.... this is the worst case analysis if the sun never shines or if we choose windows with poor SHGC characteristics.

The table below, lists the energy content and approximate cost for different solutions. Natural gas isn't available where we're building, but I included it in the table for comparison. The efficiencies listed for Air and Geothermal heat pumps are just approximations.

In a moderate climate, the Air Heat Pump is a cost effective alternative to burning natural gas or propane.

The next table shows the approximate BTU heat load for various aspects of the structure and the cost of providing the heat using different solutions. As mentioned in previous posts, the biggest opportunity for savings is by making the structure more airtight and improving the air infiltration.

Clearly, LP and resistance electric heating are poor choices so I'll eliminate them from further consideration. LP, in particular, has been through volatile price swings over the past several years so we've decided that neoTerra will be all electric with a fireplace or wood stove for backup heat.

To summarize, heating with an Air Heat Pump will cost approximately $1300 / year, based on the estimated baseline heating load and current cost of electricity. We will keep improving the design from this point.

In addition, going all electric will avoid the costs of installing an underground propane tank and plumbing the house for LP.

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Baseline Design

2/24/2013

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There are many, many tradeoffs to consider when you're designing an unconventional house. To help us evaluate different strategies for building a sustainable home we decided to start with a baseline model that would meet minimum IECC and Georgia requirements. This allows us to make changes to the model and see the effect. My primary focus has been on heating and passive solar design.

Aside from the size and orientation of the house itself, the major factors that go into the baseline model are Climate, Insulation, Air Infiltration and Windows.

Climate

Climate, of course, plays a major factor in the design of an energy efficient home. An approach that's appropriate for a cold climate may not work for a hot climate.

North Georgia is in the southern edge of IECC 2009 Climate Zone 4.

We are lucky to have a small weather station on a mountain just a few miles from our site and at the same elevation (about 2800 feet). http://www.canoodlenest.com/weather/

The station has established an average of 4147 Heating Degree Days (HDD) and an average of 857 Cooling Degree Days (CDD) over the past ten years.

For a passive solar design it's also important to have an estimate of the solar energy available at your site. The National Renewable Energy Lab (NREL) publishes detailed solar data for all 50 states here http://rredc.nrel.gov/solar/pubs/redbook/ and has a dynamic map visualization site here http://www.nrel.gov/gis/

Insulation and Air Infiltration

The DOE recommended insulation levels for Zone 4 are:
    - Attic or Ceiling                     R38
    - Walls                                  R21
    - Floor over unheated space    R25
    - Slab                                    R5

Georgia code requires a maximum ACH50 score of 7 air changes per hour (not very good).

Windows

There is some confusion on the allowable characteristics for windows. The prescriptive window requirements in the 2009 IECC specify U of 0.35 for Climate Zone 4 and place no restriction on SHGC for Climate Zone 4.

See the Efficient Windows Collaborative link here http://www.efficientwindows.org/code_overview.cfm

Energy Star, however, has somewhat different zones and sets different standards than the IECC. The Energy Star North-Central Zone roughly corresponds to IECC Climate Zone 4.

The table lists the Energy Star maximum U-Factor and maximum SHGC allowed for each region.

In future posts I'lll compare the overall heating loads for a base design with no solar consideration and then compare the effects of high SHGC windows versus the low SHGC windows madated by Energy Star.

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Air Infiltration - Blower Door Test

Flash Batt Insulation

Air Infiltration - How Tight is Good Enough?

Baseline Design Part III - Estimating Energy Costs (without solar)

Baseline Design

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