When installing PVC windows, regardless of the brand, is it necessary to completely fill the cavity between the rough opening and the window from the exterior sheathing to the interior wall with foam in order to properly insulate the window?

I am told if this is not done cold can conduct through the frame and cause severe icing on the interior surface.

 

Consult your mfg installation instructions for any recommendations they make first. (for installation warranty reasons) If they don't provide any guidance online or otherwise, what I have always heard and done is try to foam down the middle of the window depth, while trying to leave a small gap behind the nailing fin. (the foam expands both ways so the middle is the best place for it.) Some window mfg's say to leave a gap between the nailing fin and the insulation as a drainage plane for any moisture that might get in or need to dry out.

Window and door flashing is also often called for in the installation instructions as an air and weather resistive barrier, to incorporate the window fin in with the housewrap.

Every window is slightly different, so some might need to be insulated better farther to the exterior, because of the depth where the glass is glazed in the frames. But generally only insulating the interior edge of the window frame is BAD... such as maybe the first inch of the window, leaving a 2" gap from the insulation to the nail fin. This is bad because it allows cold air to circulate around the outer part of the frame, making the rest of the frame cold as well.

You can fill it 100% if you want to but the space does not generally need to be 100% filled. The first layer of foam air seals the window perimeter. If you try to get it 100% full that usually results in getting foam all over the window.

As for the rough opening in FRONT of the window up to the drywall (behind the jamb extensions) I suppose if you live in a northern climate insulating that might help some... but in most climates it probably isn't always necessary. Walls do sometimes leak air from the gap where the drywall meets the studs and some houses are more drafty than others. Foaming the extension jambs is usually a good way to cause them to bow and twist, so fiberglass might be a better choice if you want to insulate them.

Additionally, in houses where the drywall returns back to the window, the jambs are never insulated... only the window perimeter. So I don't know that there is one cut-and-dry answer to your question.

 

Short answer, yes fill it, fiberglass or LOW expansion foam!

 

Thanks for the replies.
re. 'But generally only insulating the interior edge of the window frame is BAD... such as maybe the first inch of the window, leaving a 2" gap from the insulation to the nail fin. This is bad because it allows cold air to circulate around the outer part of the frame, making the rest of the frame cold as well'

My window frames are icing up very badly, including the winder and locking handles. The above statement from Xsleeper is exactly what the window manufacturer is claiming. However, I know that the installed foam extends a good distance into the cavity but likely does not extend out to the nailing flange. The manufacturer is suggesting that because it does not cold is penetrating/conducting through the window frame components causing the interior components to become very cold and ice up. I thought a good window was constructed with a somewhat complex arrangement of dead air spaces and seals so as to prevent cold from penetrating in to the interior surfaces resulting in heat loss and icing. I am having a difficult time believing that every window in the house is not sealed correctly.

 

I would suggest you remove some trim and use a drywall keyhole saw to cut into the insulation and find out. The best window, when poorly installed, will perform poorly.

On a windy day you should also take a smoking incense stick and move it around the frame. Air infiltration (moving air) is usually what makes frames the coldest, due to the cold exterior air coming in faster than the interior air can warm the frame. Every window is rated to have a certain acceptable level of air infiltration when tested at X km/hr, but low quality windows will often perform poorly.

Casement windows are built differently, but generally there will only be 1 or 2 weatherstripping preventing cold air from getting to the hardware. Additionally they could blame it on high humidity in the home. Not saying it's right to do so. The amount of drapes you have over the window can also be a factor. Windows that are cut off from the source of heat will just get colder and colder.

You probably need to have someone from the company come inspect this... you should insist this is done at no charge. Every company "should" have a factory rep, who does that sort of thing. Unfortunately, they are often professional slick talkers.

 

re. poor install will defeat even the best window. You have a good point Xsleeper That was my plan to try and get some kind of core sample. I have also reviewed some of the photos I have of the house construction and have found one that suggests that the windows might not have been installed strictly to the manufacturers instructions. While 'Red Zone' butyl tape was used around the window openings it appears that the final cover over the nailing flanges might not have been applied. if this is the case, I guess this would re-enforce the lesson I have learned that you cannot always rely on a contractor to do a good job. This is not the first time. I payed a lot of money for continuous spray foam insulation from rim joist up to roof ridge and years later discovered a problem. The same goes with my steel roof install and now maybe my 'Cadillac' windows. Like they say 'good help is hard to find'. Again, thanks for the input.

 

One additional thought... If the gap in the rough opening around a window is too tight, (for example, anything less than 1/4") that often prevents the window from being insulated well.

No tape over the fins on the exterior side would certainly not help with the air infiltration around the frame.

 

I appreciate the help so far. I have talked with the manufactures engineer and he has suggested my problem is due to improper installation but I am having a hard time accepting this as it doesn't make sense to me. My problem is as shown:

The cause is apparently cold air leaking in from the RO at the point where the sill plate meets the window frame. It was my understanding that good windows were self sealed and sealing between the rough opening and the window was to prevent cold air that may penetrate under the nailing flange from entering the house, not from entering the window structure. Any comments? Thanks

 

Number one, are the windows locked? And does there seem to be any frost in the rough opening behind the jamb? And when you stick a keyhole saw back behind the jamb, how wide of a gap is there... how much insulation is in that space... how thick is it, and where is it located as far as its depth in the wall?

 

XSleeper, the windows are locked. When the foam was inserted it was laid pretty much on the center of the 2x6 studs so the foam likely covers the middle 3" of the stud and completely fills the cavity from the stud to the window frame. A probe thermometer extended 3 1/4" into the RO indicates a temperature of 52F. I will try and cut a core sample out of the worst window tomorrow to try and see exactly how much foam is in the cavity.