STC question

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Magnolia
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STC question

#1 Post by Magnolia »

Hi. My window configuration is two windows with a fixed picture window between them.

It occurs to me that if the STC of the side windows is different from that of the picture window...
will the overall STC be the lower of the two numbers?

Thank you!

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toddinmn
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Re: STC question

#2 Post by toddinmn »

No.

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Windows on Washington
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Re: STC question

#3 Post by Windows on Washington »

Todd is correct...and succinct as well.

That said, sound flows like water and it will invariably find the path of least resistance. So while not as much sound will move through the assembly as if it were 3 of the lower performing windows, the reality is that the one higher performance window in the middle won't feel like it is doing as much of a job with the two lesser flankers.

Magnolia
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Re: STC question

#4 Post by Magnolia »

Thank you for your responses.

I'm not sure I understand completely: let's say the two windows on the sides have an STC of 40,and the picture window is 41. Would the overall STC be around 40-1/2?

If the picture window has a an STC of 40 and the flankers are 42, would the overall STC be about 41?

Thank you!

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toddinmn
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Re: STC question

#5 Post by toddinmn »

Size of windows would come into play and other things I don’t understand. These questions would be perhaps Be answered in a physics forum. Ideally it would make sense To get all 3 Close to the same level. Perhaps it would be easier if The real questioned was asked.

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Windows on Washington
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Re: STC question

#6 Post by Windows on Washington »

While there is a difference between 41 and 40, as it pertains to STC, the reality is that your ear will be hard pressed to tell the difference. I supposed you could covert the square footage and apply some general math calculations to that and arrive at some increment between 40 and 41, depending on the square footage of the window, but that is probably not something your ear will pick up.

More likely that the flanking window, i.e. operating units, will leak a bit more noise than their STC truly indicates, over time as operating units do leak air. Air = Sound.

That all said, an STC in the 40's is good. Far better than the off the shelf stuff.

Oberon
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Re: STC question

#7 Post by Oberon »

Consider two separate rooms identical in all ways other than one has a window with an STC 40 and the other a window with an STC 30. Imagine an outside noise source that is loud enough to be heard in the room with with the STC 30 but not in the room with the STC 40.

Now place those two windows in the same room and what you have is two windows, one an STC 40 and one an STC 30, and you are hearing outside sounds through one window but not the other. Doesn't matter if you upgrade the better window to STC 100, you are still hearing the outside noise at the level allowed by the lower STC window.

Generically you could make the argument that the STC for a wall of windows is equal to the level of the worst performer in the group, but that argument discounts the higher STC level performers and other factors such as comparative size of the windows and so on.

Consider WoW's water analogy applied to the two windows: one will block wind-blown rain at 40mph and one will block wind-blown rain at 30mph. Put them in the same room on the same wall and that doesn't mean that the windows now average 35mph for blocking wind-blown rain, it does mean that you now have two separate windows that have different levels of performance that happen to be on the same wall in the same room. Doesn't matter if they are in separate spaces or next to one another, what matters is individual performance level.

Magnolia
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Re: STC question

#8 Post by Magnolia »

I appreciate all of your responses! You've made it very clear.

WOW: which is more likely to leak air over time, casements or sliders?

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TheWindowNerd
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Re: STC question

#9 Post by TheWindowNerd »

Sliders due to design allow more air infiltration than casements and DH.

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HomeSealed
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Re: STC question

#10 Post by HomeSealed »

+1.
Out of the box, in order of "air-tightness":

Casement > DH > Slider.

The only thing that I like to caution on casements, is that they do sag over time and that can cause increase air leakage. Not as much of a concern for better quality units, not to mention that they have adjusters built in now to compensate for that when needed, but still something to consider.

Magnolia
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Re: STC question

#11 Post by Magnolia »

Thank you!

HomeSealed, how long might it be before I notice the casements sagging? Is that called "sash drag?"
I looked it up and it sounds like something a handyman could fix.

Can I assume that air leakage with sliders doesn't increase over time?

Thanks!

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toddinmn
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Re: STC question

#12 Post by toddinmn »

I would disagree on what type of windows are more airtight. It will depend on the brand that decides the order. For example, Anderson 400 series casement is 0.05, the double hung is 0.03 and the slider is 0.11.
Okna has double hungs and casements listed at 0.01.
Geeking has casements at 0.03 and sliders and double hungs at 0.05.
It would make sense that any window would loose some performance over time but there is not much for data on this.

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HomeSealed
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Re: STC question

#13 Post by HomeSealed »

toddinmn wrote: Fri Jan 15, 2021 1:28 pm I would disagree on what type of windows are more airtight. It will depend on the brand that decides the order. For example, Anderson 400 series casement is 0.05, the double hung is 0.03 and the slider is 0.11.
Okna has double hungs and casements listed at 0.01.
Geeking has casements at 0.03 and sliders and double hungs at 0.05.
It would make sense that any window would loose some performance over time but there is not much for data on this.
There are exceptions to the general rule Todd. Do you disagree that the majority of window lines follow the lines of CA>DH>SL in order of AI?

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toddinmn
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Re: STC question

#14 Post by toddinmn »

As a betting man I would agree since 51% would make the majority but that was not the way the question was answered.

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HomeSealed
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Re: STC question

#15 Post by HomeSealed »

lol, fair enough

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