Window comfort and condensation

 

Graham Wright

Graham Wright

At the 10th Annual North American Passive House conference in Chicago, Steven Bluestone and PHIUS’ Lisa White made a presentation about the design of the Beach Green North (BGN) project in New York City, a seven-story multifamily residential building of some 94,000 square feet and 100+ dwelling units. The design featured double-pane windows with R-5 glass and R-4 frames.

A number of people were surprised that R-7 triple pane windows were not in the design. Low performance windows can indeed lead to comfort and condensation problems, and we did look into that. Here, PHIUS Senior Scientist Graham Wright posts about window comfort and condensation, both for the BGN project itself and how that may inform the PHIUS certification protocol going forward.

Background

Performance of building components such as windows has not been in the category of “hard-and-fast” requirements for certification. The hard requirements have been on overall building performance – the “three pillars” of space conditioning loads, primary energy, and air-tightness.

Though we have been adding more requirements over time, window performance has remained in the category of recommendation, not requirement. Since 2013 we have been recommending window performance by climate according to this table. You can see that for zone 4A, we are indeed recommending an R-7 window.

To be a bit more granular about it, New York (LaGuardia climate) has 12-h mean minimum temperature of 6.44 F, which would require a window U-value < 0.16 Btu/h.ft2.F or R-6.3, to maintain interior window surface temperature of 60.8 F (within 4 C of a 68 F air temperature, assuming an interior surface film resistance of 0.74 h.ft2.F/Btu.)

In terms of comfort, the first line of defense is the limit on peak heating load. Window U-value has a strong effect on peak heating load and, as explained in the report on the standard-setting process for PHIUS+ 2015 , we now require that projects meet limits on both annual heating demand and peak heating load (same for cooling). Moreover, the model building used in the standard-setting studies had its window U-values constrained so as to maintain 60 F interior surface temperature under winter design conditions (12-hour mean minimum outside temperature).

Thus the peak heat load criterion for the building overall is predicated upon windows that good, and it serves as an indirect curb on bad windows, while allowing the designer some flexibility to meet the overall peak load in different ways.

That is working well for buildings that are not tremendously larger than the study building. But BGN is fifty times larger, in terms of floor area. In the standard-setting report, we anticipated that there would be consequences of applying the same energy-per-floor-area criteria to all sizes of buildings; that is, larger buildings with lower surface-to-volume ratio (or surface-area-to-floor-area ratio) would more easily meet the criteria. We opted for giving this allowance to larger residential buildings, because such forms of housing are more efficient in terms of their materials usage or embodied energy.

Because of that large-building break, the BGN design could meet the peak heat load criterion with windows as low as R-2.5. The best double-pane windows available are in the R-4 to 5 range and the team asked if those would be acceptable. So we did some additional calculations on comfort and condensation.

Comfort analysis

The comfort analysis was done largely using the ASHRAE Comfort Tool, a standalone software that allows you to put an occupant in a room, then compute the radiant and operative temperatures and the human comfort metrics Predicted Mean Vote and Predicted Percent Dissatisfied (PMV and PPD).

The comfort standards ISO 7730 and ASHRAE 55 are in agreement on fundamentals. Their development was coordinated and they share the same models for the PMV and PPD metrics for overall bodily comfort, as well as “local” discomfort on different parts of the body. ISO 7730 is focused on analysis and assessment methods. ASHRAE 55 draws pass/fail lines whereas ISO 7730 just makes recommendations; it sets up categories A, B, C of design criteria for consideration.  The ASHRAE 55 pass/fail lines correspond to ISO 7730 category B. In terms of overall body comfort, the criterion is less than 10% dissatisfied. (Even at a predicted mean vote of zero, that is, on average feeling neither warm nor cool, there are still 5% of people dissatisfied.)

Worth noting is that our protocol of modeling buildings with a heating setpoint of 68 F already pushes the winter comfort to the cool end of the acceptable range of Category B or ASHRAE 55, even if there are no windows in the room at all. The clothing and activity level of the occupants are factors in the PMV, but even with Clo=1 (long sleeves and sweater) and a little activity of Met=1.1 (seated, typing), in order to get a PMV near zero you need an operative temperature of around 22 C (71-72 F). The Building America default heating setpoint happens to be 71 F.

At 45% relative humidity (RH) and an operative temperature of 68 F, the PMV is minus 0.6 and the PPD is 12%. Because the operative temperature is approximately the average of the air temperature and the mean radiant temperature, it will be lower than the air temperature in the winter when the window surfaces are colder than the walls. Thus, under the scenario shown in the first column of Table 1, the lower setpoint for air temperature has used up the design margin for comfort, leaving none for windows. If the activity level is a little higher but the RH is a little lower, as in the third column, then there is some comfort latitude remaining to accommodate windows.

Table 1. Winter comfort at two heating setpoints.

Air temp F

68

71

68

71

Radiant temp F

68

71

68

71

RH %

45

45

30

30

Air vel ft/min

20

20

20

20

Met

1.1

1.1

1.2

1.2

Clo

1.01

1.01

1.01

1.01

PMV

-0.6

-0.2

-0.43

-0.08

PPD %

12

6

9

5

There are two ways to look at this

One is that, under our current recommended way of modeling things, the improved comfort that high performance windows would bring is taken back with the lower heating set point, traded to save energy.

Another way to look at it is that as a general point, really good windows really are required for comfort at a 68 F heating setpoint.

However, to use this type of analysis to derive an absolute minimum window surface temperature criterion from the comfort standards, it would have to be carefully contrived to allow any windows at all. One would have to be specific about the activity and clothing and activity level of the occupants at least in an average sense, and small differences could make the difference between say, an 8 F margin between the window surface and the air, to no margin at all.

For the BGN window comfort analysis we instead performed a relative comparison, first constructing a baseline scenario wherein the occupant comfort was close to neutral with no windows, and then adding windows and checking the difference in comfort.

Typical main-room dimensions for the Beach Green North project are 10 x 22 feet. There are corner rooms with one window on each of the two exterior walls. Windows are 3’ 3” wide and 5’ 2” tall. Geometrically, an occupant in the corner with the windows also near the corner would feel the greatest comfort impact.

Results:

Baseline scenario (See Figure 1):

Occupant is seated, quiet (1.0 met), typical winter clothing ensemble (0.9 clo). Air temperature and radiant temperature both 74 F, humidity ratio 0.010.

Comfort is almost exactly neutral: PMV -0.10, PPD 5%

Figure 1. Baseline comfort scenario

Figure1

 

Windows scenario (See Figure 2 and 3):

Room 22 x 10 x 8 feet. Occupant seated in corner 3.3 feet from each wall, facing the short wall. Window jambs are 2 feet from the corner on each side. Window inside surface temperature is 50 F, corresponding to window U=0.4 at 6.44 F outside.

Mean radiant temperature drops to 71.2 F. PMV drops to -0.30 and PPD increases to 7%. This is still within the ASHRAE 55 or Category B criteria range of PMV +/- 0.5 and PPD < 10%.

Figure 2. Mean Radiant temperature with windows in the corner at 50 F.

Figure2

Figure 3. Overall comfort in window scenario, shifts PMV 0.2 cooler.Figure3

Conclusions:

Going from no windows to U=0.4 windows caused the PMV to shift cooler by 0.2, and the PPD to increase from 5% to 7%.

There is also local discomfort to consider. Even if the whole wall was at 50 F, this would still be just within the acceptable range for cool-wall radiant asymmetry.

We communicated to the BGN team that windows up to U=0.29 would be acceptable, splitting the difference between U=0.18 and U=0.4 (figuring that the shift in PMV would be even less going from U=0.14 to U=0.29 than it would be going from no windows to U=0.4.)

One of the other benefits of keeping the window surface temperature up within 3 or 4C of the air temperature is that there is less pooling of cold air under the window, and no need for heat under the window to prevent discomfort due to head-to-foot temperature difference. Some loss of amenity is occurring in this respect, but we did not attempt to quantify it.

One interesting point is that head-to-foot is indeed one of the local discomfort criteria in both ISO 7730 and ASHRAE 55 (per clause 5.3.4.4.) but, in ASHRAE 55, none of the local discomfort criteria apply unless the occupants are at Clo<0.7 AND Met<1.3. With that low of a clothing level, they would not be overall comfortable at 20 C (68 F) air temperature anyway, so likely they are bundled up to 1.0 Clo and the local discomfort isn’t as important.

Anecdotal feedback has been mixed. Our Canadian friends tell us “no one is complaining about comfort here with R-6 windows” even where we would recommend R-8. Our Lithuanian colleagues say “double-panes are uncomfortable; no one uses double-panes in Lithuania.” Lithuania is 10 F colder than NYC though, with a 12-hr mean minimum temperature of 3.8 F below zero versus LaGuardia at +6.4 F.

As to the certification program going forward, the matter was brought before the full PHIUS Technical Committee at our October meeting,

For the time being, the Committee decided to refrain from imposing a “hard requirement” on inside surface temperature for winter comfort, or directly on window U-value, and to continue in the category of recommendation.

We have already collected the data to set recommended winter-comfort-based U-value maximums for all the climate locations on our criteria map, and could make those show up.

A possible approach for certification could be to require that those recommendations are followed but give an option to do a more detailed comfort assessment like the one shown above.  This very kind of material-cost versus analysis-cost tradeoff is done elsewhere in the program, for example with thermal bridges. One can do a conservative design following simple rules , or make an edgier design and do more engineering work to verify whether it meets criteria.   Such an approach would require the development of some additional calculation protocol.

Condensation analysis: Background

One of the “hard requirements” PHIUS has added pertains to avoiding mold growth on interior surfaces caused by thermal bridges. Even if a thermal bridge is tolerable in terms its impact on the space conditioning loads and demands, it is not tolerable if it can lead to mold growth on the inside. Our protocol follows ISO 13788, and one of our calculator tools follows its methods. Just as in calculating the energy impact of a thermal bridge, we make a THERM model of the detail. But instead of calculating the extra energy loss, the critical result is the point of lowest temperature on the inside surface, and the criterion is that at that point, the interior air, when chilled down to that temperature, should be at less than 80% relative humidity.

ISO 13788 addresses how to determine the appropriate boundary conditions – the outside temperature and the indoor relative humidity. This is based on consideration of the monthly average outside temperature and humidity for the climate. The outdoor humidity is added to an indoor source that depends on one of five building humidity classes from low to high.

For each month, a psychometric calculation is then done to determine a minimum inside surface temperature needed to keep the RH at the surface below 80%.

The critical month is the one in which that minimum surface temperature is farthest from the outside temperature and closest to the inside temperature, because that requires the detail to be the most “insulating.” This “surface temperature factor” (fRsi) of the building element is defined mathematically as

fRsi = (inside surface temp – outside temp)/(inside temp – outside temp),

with a surface resistance at the inside surface of Rsi.

(Usually the critical month is also the coldest month but not always – depending on the climate it might be in October, for example)

ISO 13788 also addresses assessment of condensation on “low thermal inertia” elements such as windows and doors, using a similar procedure, but with some differences: instead of keeping the RH below 80%, the goal is to avoid outright condensation (RH=100%), because windows and doors have impermeable surfaces that aren’t as subject to mold, but vulnerable to rot and corrosion if outright wet. But the outside design temperature is more severe – instead of a monthly average, it calls for the lowest daily mean temperature of the whole year.

For our BGN analysis, we:

1. Used the 13788 procedure for “low thermal inertia” elements to determine the required minimum surface temperature and fRsi to avoid condensation.

With an interior RH of 48% in the coldest month, the dew point of the interior air was 47.7 F, so the inside surface must be warmer than that.

2. We then did a one-dimensional calculation with the frame U-value at 0.28 to determine if that was the case. Instead of the lowest daily mean temperature, we used (for convenience) the ASHRAE 99.6% design temperature, as AAMA does for their Condensation Resistance Factor. This was 13.8 F.

With an interior temperature of 68 F, and an inside film resistance of 0.74 h.ft2.F/Btu, the inside surface temperature then is 68-(0.29*0.74)*(68-13.8) = 56.7 F, that is, 9 degrees above the dew point.

Of course, this does ignore the fact that the surface temperature could be lower right in the corner where the frame meets the glass, because of the conductivity of the spacer, but 9 F provides a comfortable margin. ISO 13788 does caution that one-dimensional calculations aren’t generally good enough, but it is a place to start. We will refine the “low thermal inertia” version of our 13788 calculator and publish that soon.

We’ve also been asked whether we can specify an NFRC Condensation Resistance rating (CR). The AAMA recently published a good summary paper [AAMA CRS-15] that explains the differences between NFRC’s Condensation Resistance (CR), AAMA’s Condensation Resistance Factor (CRF), and the Canadian temperature Index or I-value, per CSA A440.2.  All of these are 0-100% higher-is-better ratings, but they are not directly comparable to each other.

From that paper it is clear that the CRF and the I-value are the same kind of thing as what ISO 13788 calls fRsi – ratios that indicate how far some critical inside surface temperature is towards the inside air temperature. Therefore, if that data is available for a window of interest, those ratings could be compared directly to the required fRsi from a 13788 calculation for “low thermal inertia elements” for an indication as to whether a window is good enough in the climate location of interest.

The AAMA white paper indicates that the I-value is generally more conservative/stringent than the CRF due to differences in the temperature sensor placements. Both of these are physical tests.

AAMA provides an online calculator that takes a given outdoor temperature, indoor temperature, and relative humidity, and computes the dew point and the required CRF, so it is making the same kind of calculation as called for in ISO 13788. (The disclaimer for it makes many good points.)

http://www.aamanet.org/crfcalculator/2/334/crf-tool

The NFRC Condensation Resistance rating is more complicated and harder to interpret, except as a relative ranking. It is basically the percentage of the window frame, glass, or edge-of-glass area (whichever is worst) that is below dew point under the standard test condition temperatures, averaged over interior RH levels of 30, 50, and 70%. It is based on modeling rather than a physical test.

At the October Technical Committee meeting there was consensus on the general matter of setting a definite requirement to avoid condensation on windows.

The next issues then are: under what circumstances a window condensation check should be required in project certification, and what the passing criterion should be.

PHIUS’ Certification staff are working out those details and plan to phase in the requirement.

In the meantime, a determination about “when to check” should key on risk factors such as:

  • Window U-value significantly above the comfort requirement.
  • Frame U-value significantly above the glass U-value.
  • Presence of aluminum spacers.
  • Lo-e coating on the inside surface of the glass.

Our recommended passing criterion is that 1-D calculations on the surface temperatures or fRsi of the frame and the glass, or an AAMA CRF rating, should meet the ISO 13788 minimums at the ASHRAE 99.6 design temperature for the climate, with some safety margin, or that a CSA I-value meets it without a safety margin.

We will also consider adding to our window rating data the fRsi calculated at the worst-case location, the inside corner where the glass meets the frame.

 

California here we come!

I’m just back from San Francisco where I spent five days to meet with stakeholders and contributors to discuss plans for the conference coming up on September 10-14, 2014.

The talks were amazing and super encouraging! PHIUS+ projects in the Bay Area are exceptional – they all are also zero energy or positive energy buildings highlighting that passive building is the ideal starting point for going zero or positive. This growing trend – passive to positive energy — will be one of the major themes for conference sessions!

Combining passive design in buildings plus renewables is one of the strategies identified by carbon reduction groups to help mitigate and adapt to climate change. The latter because passive buildings are especially resilient in weather extremes and power outages.

San Francisco has long been aiming at carbon zero goals by 2020, looking to identify a clear set of tools on how to practically and cost effectively implement them. That’s why we chose San Francisco for this 9th Annual North American Passive House Conference. We think it can be the catalyst for a tipping point, a special moment in time when the concept is catapulted forward thanks to favorable factors in the Bay Area. With plenty of high quality high-performance projects designed and built by the pioneers in the passive community, we have an excellent opportunity to make the case to make to the city and its residents that passive design is the best path to their goals.

On my trip I have spoken with various stakeholders and thought leaders and have seen nothing but honest excitement about the possibilities of the conferencce. And better yet, if San Francisco get’s it, you know that the rest of the state and then the country will eventually follow, hence it is critical to make this a big success that radiates beyond the borders of California setting a definite sign: we are in the transition toward a new energy economy and buildings, passive and renewables will play an important role in it.

We’re excited that William Rose, a building science pioneer, will deliver our keynote and that Achilles Karagiozis, Director of Building Science for Owens Corning and WUFI developer will speak at the closing plenary. Also: Joe Lstiburek will present a daylong workshop on building science fundamentals during pre-conference sessions.

Of course, success of the conference – as always –will depend on the dedicated members of our community. We’ve collected dozens and dozens of terrific presentation proposals (and we’re a week or two behind in our review, please accept our apologies; we’ll be in touch soon), and the content of our breakouts will be terrific, as always. We also have a great range of pre-conference sessions (which als earn CPHC CEUs), including a daylong session with five CPHCs who are leading the way in multifamily builldings. Visit the conference website often for updates.

And if you’re available to volunteer to help, email conference@passivehouse.us with your availability and any special expertise. We could use help with everything from registration desk staffing to videography.

Full schedule — and more announcements on some great presenters — are on the way, stay tuned!

Katrin

 

 

PHIUS Certified Data for Windows program online

Graham Wright, who heads up the PHIUS Certified Data for Windows program, joins us today to provide clarifications on some key features of the program. And to clear up some misinformation.

I get and see online a lot of questions about the PHIUS Certified Data for Windows program, and how it differs from PHI’s Euro-centric program. Apologies—it’s clear that we haven’t communicated the program as well as we should have—but we are catching up to ourselves.

I’m happy to report that we’ve got data and climate recommendations for a nice range of windows online here.

The table lists products, climate zone recommendations, full data certificates and supporting THERM files.

There are a lot more coming—and we’ll be converting this static table to an online database soon.

For manufacturers and suppliers, we invite you to download a detailed description of the certification process (with an application form), and the document is also available at the program overview page.

In the meantime, I hope to clear up misconceptions and concerns about the program:

I heard that the PHIUS Certified Data for Windows program doesn’t account for whole-window R-value. Is that correct?

In fact, the program does provide recommendations based on whole-window R-value, and all have the force of criteria as far as manufacturers are concerned. Also the program provides recommendations / criteria on solar heat gain coefficient.  (They vary by climate from about R-5 to R-9.)

Why doesn’t the program address surface temperature factor (“fRsi”)?

Eventually, it probably will – but on the list of important future program improvements for North America, it’s pretty far down the list, after data publication, NFRC harmonization, air-tightness-durability, and Canadian Energy Rating. That’s partly because fRsi isn’t pertinent to hot climates (and there’s a lot of hot climate zone in North America), and we decided to pay more attention to solar heat gain coefficient.

We cover the condensation / fRsi issue by providing the THERM files, which allow consultants to calculate it if they wish, and more precisely, with respect to the expected interior humidity conditions for their particular project and climate. (I made an ASHRAE 160 + ISO 13788 calculator for computing fRsi requirements climate- and project- specifically. We make it available during CPHC training, and if you email me at graham@passivehouse.us, I’ll be happy to send it to you.)

Moreover, both the fRsi criterion and the single-height bar on U-value (at about R-7) look to be legacies of the single-zone origin of PHI’s window program.

Setting that high bar at R-7 has certainly spurred innovation. But our multi-zone system does the same thing – people want to “level up” from zone 3 to 4 or from 4 to 5.  A single standard, when it comes to windows, fails to inform a shopper whether a window is appropriate for a passive house project because it’s overkill in some places and under kill in others.

It also hurts manufacturers—many of them mainstream producers of very affordable windows—who, right now, offer windows that will work well in mild climates. Designers, builders and clients should have those options.

Moreover, we find much to admire in the NFRC system and would like to get the best of both worlds. Funding for such harmonization work is being applied for.

Do we really need the PHIUS window program?

Absolutely.

PHIUS’ window program is moving toward a critical goal: producing data in the format that passive house consultants need, and that enables direct comparison to windows rated by other EN-based outfits like PHI or say, IFT Rosenheim. Both window industry representatives and passive designers have told us this is critical if we want to energize the market. To be sure, the programs’ fundamentals are aligned, but the presentation and recommendation level is different in a number of ways. For example, PHIUS’ program is more fussy about solar heat gain and zone granularity. PHI’s is more fussy about horizontal / vertical.

At PHIUS we believe that passive house principles apply universally, but a single criterion does not. From their Greenbuild presentation, it was clear to me that PHI recognizes the need for climate-specific recommendations for components, including windows.  But as I understand it their window certification is still pass/fail at one level. PHIUS has moved more quickly on this front. Going forward, if you hear “this is a passive house window” people should know to ask:  for what climate?  Be wary of claims about passive house windows that don’t show any numbers or label or certificate, it might just be loose talk.

Why such a long name? Why not just PHIUS Certified Window, for example

We settled on the name with care. The AAMA or NFRC would say, you don’t have a “window certification program” unless you address air / water / structural issues. So PHIUS is not certifying windows (and neither does PHI by such lights).

We’re certifying certain data about windows, namely thermal performance, modeled. Hence, “PHIUS Certified Data for Window Performance Program.”

We did not include the term “passive house” because while designed with passive house in mind, the data—even for windows that don’t receive recommendations for passive application—will be extremely useful for designers.