High-Performance Windows for Avoiding Condensation and Mold
I understand the two different points of view about whether high efficiency Passivehouse quality windows are worth it in cold climates, compared to investing the difference in solar, etc. I am not so concerned about energy efficiency, but about durability. My #1, #2 and #3 goals in building this house is to avoid mold since I get massively sick very quickly in moldy buildings.
I took Passivehouse training and learned about fRSI values, and how “not good enough windows” cause mold and condensation, and that is my primary concern. But many of you builders and architects continue to believe triple glazes European windows aren’t “worth it.” So I’m trying to get some real world experience and try to sort out how big of a risk this is. I’m sure you don’t think it’s “not worth it” to have a non-moldy house.
I will have a very tight house in CZ 6B. But we are almost entirely a heating climate, we will have a whole house balanced ventilation system, the climate is super dry, and most of the time we are only 2 people in a large house. Passivehouse recommends an R8 window and I am thinking only R6 or R7.
There are tons of moldy houses in my town but all the ones I’ve been in were built in the 70’s through 90’s when we were building particularly badly. And even then I’ve never encountered rotted walls or moldy surfaces. The mold I’ve found is always related to basements, crawl spaces and HVAC being done wrong.
Then again these are not tight homes. So in the real world, how important is it really in a tight home to have super insulated window (esp frames) to avoid mold and condensation?
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Condensation occurs when air comes into contact with a surface whose temperature is below the dew point of that air. There are two ways to keep that from happening: lower the dew point of the air (which means making it less humid) or raising the temperature of surfaces.
In the winter, more ventilation means a less humid house, the outside air has a lower dew point than the inside air. Windows cause condensation not because they leak air, but because they leak heat -- the inside surface of the window is below the dew point. An air-leaky window is actually less likely to have condensation than an air-tight window with the same interior temperature. The current best practice is to build the house as tight as possible, then add ventilation to control humidity in winter.
In the summer, if you use air conditioning the interior air will be less humid than the exterior air. Condensation will occur when outside air gets in and contacts something cold. If you're running AC, ventilation is not your friend and air leakage is the mortal enemy -- although a tight house will require ventilation to dispel CO2.
In either case you want the house tight.
Deleted
If the windows are 'underspeced' per passive house threshold, condensation issues will be immediate and obvious near the bottom glass edge during peak conditions. This would dry out once the interior relative humidity is brought back down to a reasonable level or it warms up outside. Its unlikely for this incidental condensation to become a durability issue unless the frames and glazing are junk (thermally speaking) and/or interior RH is consistently too high. If mold is your concern, and PH threshold window specs aren't in the budget, focus on excellent installation detailing to manage bulk water and airtightness and use non-moisture sensitive frame and interior sill materials.
I forgot to mention the old-school approaches still work: throw energy at the problem. Have heat sources near the sills or supply registers facing them. Run a dehumidifier. Increase ventilation rate of the HRV/ERV to reduce moisture. All would reduce condensation and mold risk but at a cost.
Interesting point. What would be a less moisture sensitive frame? Is wood good because it doesn't get as cold as aluminum? Where would fiberglass or vinyl fit in?
I'd love to know what the humidity will be compared to what it is now. I'm lucky to get to 20% humidity in my current house in winter, and that's typical around here. I'm not sure what it will be when we lose the leakiness but add ventilation with an ERV. Then again I know that a small amount of humidity becomes a lot more humidity when it touches a cold glass surface, and even an R7 window is going to be a lot colder than the room when it's 10 degrees out.
So it sounds like you generally believe the PH specs should be followed if possible? That it does really make a difference?
Wood is actually the least desirable frame if the case is it doesn't meet passive house spec -- it's the most moisture-sensitive of the frame materials you named. Any of the others can handle more incidental condensation.
The PHIUS website has a window condensation risk calculator that's useful for running 'what-ifs'.
https://www.phius.org/software-resources/wufi-passive-and-other-modeling-tools/calculators-and-protocols
Edit: To answer your last question, yes, find the lowest U factor possible that can be afforded to reduce risk. I'm not a fan of throwing energy at the problem when I know there are better competitive products on the market that address the issue. All this said, I've never seen mold growing on a window sill. It's usually more of an annoyance than a durability issue as long as the owner doesn't neglect it.
Jason S "If the windows are 'underspeced' per passive house threshold, condensation issues will be immediate and obvious near the bottom glass edge during peak conditions. "
Peak conditions are not common. I have moderately priced double pane windows (don't recall the U value), and rarely have condensation.
Brad,
Ditto. And agreed on peak conditions, except if an owner chooses to humidify to a constant 50% RH at 75F during the cold months. I've had more than one client...
Better for the OP to use the calculator for the actual U values, design parameters and climate under consideration than anecdotal accounts.
There are lots of North American window manufacturers who can supply good triple-pane low-e argon windows. That's plenty to avoid condensation.
Note that the outdoor humidity in the middle of winter doesn't really matter. Once you bring the air inside and warm it up, even 100% humidity outside at 0 F becomes only 5% indoor humidity: it's bone dry regardless of that outdoor humidity reading. The humidity sources are all inside the house.
That first statement is part of why this is confusing because most of the NA window manufacturers with triple pain low 3 argon are R3 at best. And they don't have Tilt-Turn which seems key due to the good seals. Nothing close to the R8 that is recommended in my CZ.
> humidity sources are all inside the house
Note that indoor sources are highly variable. So at times indoor humidity will be too high and at others, too low. I suppose the ideal solution is a humidity controlled ERV/HRV to increase airflow when it's too high. Plus a humidifier (yes, this can be done safely) for when it's too low.
Regarding the inside glass surface temperature of dual-pane vs triple-pane windows, a few days ago we had the remnants of a turkey boiling in a large pot on the range, as part of making turkey soup. The humidity in the kitchen was somewhat elevated, despite the HRV being on full-time. The triple pane windows in the kitchen remained clear, while an adjacent dual-pane full height glass door was totally covered in condensation.
On a triple pane window, the interior pane will usually always be identical to the interior ambient air temperature. So if it's 10F outside and 68F inside. The interior pane will be 68F. Which also explains why triple pane windows don't suffer from convection like double pane windows do. When warm air inside comes into contact with a double pane cold window pane, it cools and sinks, creating a convection current of air near the window. So you can sit next to a triple pane window and not feel that "cold breeze", even when it's 10F outside. That's a comfort factor you don't get with double pane windows.
Peter,
The OP isn't considering double glazed based on the "R6 or R7" criteria. Also this statement is slippery and misleading: "...the interior pane will usually always be identical to the interior ambient air temperature."
Windows are the weakest thermal link in an assembly. Even great ones. That's why they still become the de facto condensing dehumidifier during (limited) peak conditions, when the #6 glazing surface near the sill would decidedly be at a temperature below the interior ambient air temperature.
We've got PH certified upvc triple pane windows here in zone 6 Maine. We've literally never had condensation on the windows.
Hi Stephen, Do you know the U value of those windows? PH windows can go all the way from R6 to R11. Thanks!
I don't remember exactly, but it was around U .12. Windows were from Intus. Unfortunately, they no longer offer residential products. We've been very happy with them.
Ah yes, R8 is quite good.
Double pane 0.24 U, zone 6 Montana, slight condensation in bedroom (2 adults one dog - a large one) at bottom corners no window treatment (outdoor less than 15 F and zoned with an air handler). There's also a slider, fiberglass frame, it gets more due to the window treatment (curtain). So, consider window treatment too. It's likely heavy curtains on triple pane may result in condensation. Wood window frames may be more likely to be impacted when compared to other materials.
Also, consider whether windows are set deep with respect to interior, 2x4 framing, 2x6 framing, etc. The deeper they are, there may be more risk of being cool (noting that air flow - throwing energy at it - may not reach the corners of deep windows).
The only window coverings really will be a few sun shades on the west, and blackout curtains in a bedroom. So blackout curtains at night would be a problem? Maybe I should get those windows in an R8.
qofmiwok,
8,000' CZ6B .15U fixed .19 operable triple glazed. Bathrooms of course will condense readily during showers. Bedroom with heavy shade on east window will produce condensation mostly on bottom edge if we forget to hold the shade up a few inches. The master bath being right off bedroom likely raises RH in the one room. Extremely unusual to see any condensation anywhere else. We probably average 30% RH house wide which likely helps prevent condensation.
Windows are all set "middie" so 7" window sill minimum depth. Heat is all electric, so no air mixing (or noise). Sitting next to large windows even at zero or below is unremarkable. After four years I still have no need for heat below windows. The power boxes remain capped. The major portion of the glass area stays above 62 even when zero outside, the edges can drop to low 50's according to my thermistor attachment on my meter. I would never consider dual pane again.
Materials-wise, my windows are fiberglass pultrusion with a fine veneer overlay inside. I did the finishing with wipe-on poly, five thin coats. I do wipe the condensation off the bottoms in the bathroom randomly and so far all is fine. The extreme sunlight here is more relevant as the stains I used have not proven as resistant to sun fade as I would hope.
My only experience with PVC windows has been builder grade ones I simply had to live with. The ones I left behind are about 15 yrs old and have gotten brittle and yellowed. I was warned not to select dark colors in PVC and would have avoided in current windows, but for being married. Turns out both being married and having dark colored pultrusion windows is continuing to work well. The fiberglass expansion seems negligible, or at least less than the foam outsulation, which has moved enough to warrant re-caulking the east and south windows a bit where they set into the stucco.
Thanks. Bathrooms condense even with your fan running? Do you have whole house ventilation? What brand windows are fiberglass with veneer inside? I haven't seen those.
qofmiwok,
The windows are Alpen 525S circa 2015, now improved in the Z6 series. The veneer option still appears with some limitations on the frame profile choices.
The veneer is very thin, so if you are having others do the finishing work, don't let them sand the windows like wood windows. Use 220 grit MAX if anything needs de-fuzzing. I did the between coat nit removal with 320. Some tapes I used to protect the veneer while doing the walls ended up pulling off bits of the veneer. The problem was due to the growth ring wood being weaker than the other parts combined with poor tape choices. Using low tack tape and removing the tape by back pulling on a sharp angle eliminated the risk. Only I can find the few places I messed up.
If you do choose the Alpens, I do not recommend following their tool suggestion for unseating the little strips that hold in the glazing when hiding the capillary tubes. I will spare you the long winded explanation for now. Other than that, I was happy putting the windows in and the performance has been light years ahead of anything I have installed so far.
As to the shower condensation, I don't run the fan until after my shower thanks to the peculiarities of the local dry environment. The TMI answer is: I discovered on earlier trips to the area before we moved here, that stepping out of a shower into the very low humidity air creates a flash evaporation effect that is quite chilling (to me at least). My cure has been to allow the humidity to stay in the shower room, which is separate from the rest of the bath, which makes the room more like Louisiana until I am done drying off. My spouse runs the fan and seems unaffected by the flash drying effect. Your mileage may vary I guess. Still, just making sure you put enough coats of poly on the windows seems to be viable.
The black out shade we have on the east master window is the condensation culprit during the winter. Because we have deep set middie windows and the master bath feeds my shower humidity into it, letting the shade all the way down will cause a problem. I ordered the shade to fit very closely to the opening, so while it blocks the dawn's early light, it also insulates the window glass enough to allow the frame and edges to cool much more than the non-shaded windows in the same room. Simply keeping the shade raised a few inches to allow a bit of warmer room air to reach the bottom edge of the window controls the problem.
We do have shades on the west side where my wife has her hobby room. There is never a condensation issue there, even if left down overnight. That is why I am pretty sure my shower habits make the one window and shade combo risk condensation. I have noted in other threads that my attempts to use "energy saver" honeycomb shades in an earlier home created the same issue. I was keeping the room warmer, but the glass would get much colder and weep. The earlier windows were double pane with aluminum cladding that helped to accentuate the edge losses. We quickly discovered that on the coldest days we had frozen ice wedges along the bottoms of the window that ultimately started rotting the wood underneath the latex paint finish. Worse the latex paint was excellent for harboring and growing mold in abundance.
These particular windows is what set me on the course to find much better options for our current home.
They don't have the "wood" interiors anymore. (They have a few styles but very limited.)
Lots of people build houses to less than passive house levels very few get moldy.
Part of the passive house cool aid is that the thought distortion that any house that will not meet our standards is a pile of garbage and will use too much fuel and will rot and mold.
In zone 6 triple pane windows is likely worth the cost form a comfort point of view if not an economic one.
The way I see it whatever quality of window you select set your indoor humidity level. If you select a single pane window its surface temperature will be lower. The surface temperature of the glass is going to be the coldest thing in the house and this fact sets the maximum dew point that house can have give the outdoor temp. With better glass you can have higher dew points and higher winter time humidity and better comfort.
I do not see condensation on glass linked to mold. Most windows are made to deal with some water and tend to dry quickly. Keeping paint or varnish on wood windows becomes a maintenance issue.
Without a humidifier mold seem unlikely.
I have a mix of windows some fiberglass and others vinyl. On the 5 or 10 days a year in zone 4 we get condensation I mop it up twice a day.
Walta
> how important is it really in a tight home to have super insulated window (esp frames) to avoid mold and condensation?
This more flexible tool will show you when condensation occurs - typically not until indoor humidity is way too high. Also note that running a few degrees warmer interior temp can make double pane windows more comfortable than triple pane.
https://www.payette.com/glazing-and-winter-comfort-tool/
Do not conclude that if you have very good windows, you can safely run a high interior humidity. Walls and cathedral ceilings can mold too.
>Do not conclude that if you have very good windows, you can safely run a high interior humidity. Walls and cathedral ceilings can mold too.
You've put your finger on what I found weird about the original question. Yeah, windows can have condensation. But that's not where mold problems start. If there's condensation on the windows I'd be checking the rim joists and sheathing.
Fairly tight renovated house, 2.2 ach, triple pane windows from Northstar, so relatively cheap vinyl windows. U-0.19. Interior 72°F, 50%RH. Very minor condensation at bottom of windows when outside temp is below freezing, does not evaporate back into the air readily. Mold grows on the vinyl edge of the window when I forget to periodically wipe it down.
I think this is due to something wrong with the window design (crappy frames) and not the actual window U-value. The high interior RH is the other reason. If you plan on keeping a high RH (+30%) and want zero mold risk on your windows, get better ones than I did. I'm happy with them for the price.
> If you plan on keeping a high RH ...
IMO, if this is your plan in a cold climate, change your plan. Or build the entire house (not just windows) with this in mind.