Winter condensation on windows in a house that is very well insulated.

This is my first post and I hope I have the formate correct, I live and build in central Pennsylvania. I built a house in 2004 using what I thought were the best practices at the time to insure it was tight and very well insulated. Took great pains to properly vent all bathrooms, dryer and range. I used Tyvek house wrap and a 6 mill vapor barrier on walls and ceiling. Used a pre-fab basement wall system that creates a very dry basement. Customer works for a propane gas co. and had a high efficiency gas hot air system installed.  They are having a very hard time with condensation on their windows in the winter. They are very unhappy with me because they feel the house is to tight and I should remove the plastic vapor barrier.  The only thing I changed was using Crestline windows instead of Anderson.                              As a side note I built 6 new homes using all of the same building techniques in my area and had none of these problems. We have discussed the use of exhaust fans allot and they assure me they use them. It is just Mom, Dad and a teenage girl so not an excessive amount water usage.     What should the RH be in the winter and should I just buy them a de-humidifier.  Oh, Yea they do have a heat recovery system they use when the temp reaches 50 degrees. Any help would be welcome.

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Nice JOB!!!  

What was the cfm50 number? 

not exactly sure but they said 1.5 is ok for the energy test they do for a local home building company to get energy star rated. I was showing .05. I will talk to them and get exact numbers 


.05 ACH is great, but ACH is like telling someone how much they weigh per inch of height.  Leaves a lot of opportunity for error or gaming or simply confusion.  

CFM50 is the raw number.  The total weight.  Very transparent measure.  Pretty hard to hide anything in that.  

Going forward you may always want to get CFM50 leakage as well as whatever other number they typically share.  

Actually, as raw data, CFM50 by itself is pretty meaningless, as a very small but leaky house could have a lower number than a very large and tight house.

What matters is ACH50 (CFM50 compared to whole house air volume), and CFM per square foot of envelope, which indicates the actually leakiness of the building system.

ACH50 can be somewhat misleading as a comparison tool because it is biased toward larger volume homes, while CFM/SF envelope is a more accurate comparison of the actual air-tightness of the structural elements.

Normalized ACH is the best measure for determining the actual air exchange in local climatic conditions, given the height and exposure of the house.

Actually volume is NOT the measure, it's surface area.  Leakage occurs at surfaces, not at volume.  But the blower door as a tool doesn't have the level of accuracy that would be necessary to split those nits.  That's taking obsessive to the 5th power.  I don't want to queer people with that level of detail, at least not up front. I don't think Eric gives a rats behind about measuring at that level.  Most want a yardstick, not micrometer.  

Square footage and CFM50 are PLENTY to get you to the neighborhood for understanding relative value.  Is it a highly accurate model, no.  But we are hitting a big target on the side of a barn rather than having no idea where the barn even is.  Not hitting the third nail on the fifteenth board from the left.  

We need the average Joe to start understanding leakage, and if you make it absurdly and unnecessarily complicated for them to get their heads around, they'll tune out.  After looking at 3-400 energy bills, square footage and energy consumption have a pretty good correlation to cfm50.  

Not enough to build a model on, but enough to understand the size of the opportunity.  Taking that conversation to the homeowner helps determine whether further efforts are even warranted.  

I don't know what you could possibly mean by "volume is NOT the measure" (of what?). ACH50, which is a volume air exchange metric, is the only one referenced in building codes and efficiency programs:

2009 IRC/IECC                                      ≤ 7 ACH50

Old Energy Star                                     ≤ 5 ACH50

Energy Star V 3.0 (2011)                       

Climate Zone 1,2                                    ≤ 6 ACH50

Climate Zone 3,4                                    ≤ 5 ACH50

Climate Zone 5,6,7                                 ≤ 4 ACH50

Climate Zone 8                                      ≤ 3 ACH50

USDOE Build America Program              2-3 ACH50

Canadian Standard                                 1.5 ACH50

And there is a hot debate in the building energy-efficiency community about whether ACH50 (which penalizes small homes) is the best metric versus CFM50/SF envelope, or above-grade envelope as in the Minneapolis Leakage Ratio (MLR).

One can compare CFM50 before and after retrofit in a single building to measure the value of the improvements, but it says nothing in terms of comparison to other buildings or to a universal standard of tightness.

This has nothing to do with levels of accuracy (or "nits"), but only with the appropriateness of various metrics.

The older Energy Star (pre-2011) required no more than 5ACH50. 1.5ACH50 is extremely tight in today's new home market. The world's strictest standard - Passive House - requires only 0.6ACH50. No house can achieve 0.05ACH50.

You might want to double check those numbers.

My in-laws have a similar problem in their home in Des Moines, Iowa. The temperature differential between the window and the indoor air is too great causing ventilation. The only way to battle is to put your HRV on auto-pilot. You should install a humidistat coupled with a timer that lets the HRV run for prolonged periods of time. At least 1hr increments.;

Are the flue and condensate drainage systems for the heating system working correctly? Propane gas produces a lot of moisture. The vapor barrier should definitely not be removed as there is always a risk of interstitial condensation in highly insulated buildings. The cost of running a whole house domestic HR ventilation system should be minimal and should be more than offset by the heat saved.

The vapor barrier should not be removed, but not for the reason you suggest. 90% or more of interstitial moisture is due to air movement, not vapor diffusion. So what is needed is an air barrier. The current International Building Code (IRC) requires only a warm-side vapor retarder of 1 perm (latex VR primer like Benjamin Moore SuperSpec meets this standard).

A vapor barrier is just as likely to prevent drying out of interstitial moisture as it is to prevent it from getting into the envelope, particularly in a mixed heating/cooling climate.

The cost or running a HRV system depends heavily on how it's designed and the quality of installation. Using a HEPA filter can add 25% to operating costs and integrating with an air handler with inefficient high-wattage fan can add 50% to total HVAC operating cost. ECM motors are much more efficient than conventional. There is a wide spectrum of system types and associated costs. Annual HEPA filter replacement can be $100.

In one Chicago study by the Consortium for Advanced Residential Building (CARB) in 2004 of three very tight new houses (300-350 CRM50) with different ventilation systems (an ERV integrated with the air-handler, an AirCycler™ supply ventilation integrated with air-handler, and an exhaust-only bath fans on timer control), both integrated systems were about double in total operating costs over the exhaust-only system (including electrical consumption and additional heating).

This was corroborated by an LBNL study the following year:

"The results show that a single port exhaust system is the least expensive to purchase and install. This is supported by Wray, et al (2002) who also found a mechanical exhaust system the least expensive to operate." - Review of Residential Ventilation Technologies, Lawrence Berkeley National Lab (2005)

Thanks Bill, The furnace is PVC vented and supplied. I agree hopefully they will to. Thanks for your input. Eric


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