# Need Input - XPS Insulation



## Windows on Wash (Aug 30, 2011)

Building Science Corporation has some great articles on foam exterior walls. They work great when done right.


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## AlphaPilot (Aug 6, 2012)

I found these documents from GBA about proper flashing when installing windows. Looks like the #1 method is to use 2" x 2" pieces of wood to bump the window opening out to the same level as the foam and fasten the window flanges to that. Will that 2" lip be enough for window trim also, or would a wider piece be advisable?


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## Windows on Wash (Aug 30, 2011)

Depends on the trim detail to be honest. 

Most times we have used stacked 5/4 by 4" boards. Gives you a larger nailing surface.


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## AlphaPilot (Aug 6, 2012)

That's what I was wondering about. So if I have 2" of foam + 3/4" vertical firing strips for siding to install to, I actually need to install the window 2 3/4" out instead of just 2" like most articles explain?


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## Windows on Wash (Aug 30, 2011)

If you want siding to be flush with the outside edge of the window, yes.

Most windows are usually a bit of what we refer to as "innie".


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## AlphaPilot (Aug 6, 2012)

If I do that, what do you use to fill that 3/4" gap between siding and the window flange itself?

I don't want to use vinyl but in terms of cost I think I will have to, so I guess I should look into vinyl trim products. I'd like to use LP Smart Side or Stucco if stucco were DIY friendly.


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## ddsrph (Nov 23, 2013)

CTSNicholas said:


> If I do that, what do you use to fill that 3/4" gap between siding and the window flange itself?
> 
> I don't want to use vinyl but in terms of cost I think I will have to, so I guess I should look into vinyl trim products. I'd like to use LP Smart Side or Stucco if stucco were DIY friendly.



Nick
The pros may disagree but when I do any vinyl DIY projects I use sheet rock screws with # 10 washers under the heads. To me it gives two big advantages. The screws aren't going to loosen and you can tighten just right to allow the vinyl to move as needed. I can't imagine how those stable guns can achieve good results. I am building a new house now with the front wall and all soffit vinyl. It s been on almost a year and still looks perfect. 
JM


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## Windows on Wash (Aug 30, 2011)

CTSNicholas said:


> If I do that, what do you use to fill that 3/4" gap between siding and the window flange itself?
> 
> I don't want to use vinyl but in terms of cost I think I will have to, so I guess I should look into vinyl trim products. I'd like to use LP Smart Side or Stucco if stucco were DIY friendly.


Depends totally on how and with what you are trimming the window out of. 

If you are doing some flat stock around it, you can rabbet out the backside.


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## AlphaPilot (Aug 6, 2012)

Right now I am thinking I will need 2" x 2" blocking to fasten the window flange on that - BUT I don't know if that would be enough material to fasten a simple window trim onto? I am thinking 5/4 stock or some type of vinyl for the window trim?


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## AlphaPilot (Aug 6, 2012)

Windows on Wash said:


> Depends totally on how and with what you are trimming the window out of.
> 
> If you are doing some flat stock around it, you can rabbet out the backside.


What do you mean by rabbet out the back of it...? If my window is installed on the bumped out window opening, then my desired trim would need to fasten to some solid material and not into the window flange - at least that is what I assume :whistling2:


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## Windows on Wash (Aug 30, 2011)

CTSNicholas said:


> What do you mean by rabbet out the back of it...? If my window is installed on the bumped out window opening, then my desired trim would need to fasten to some solid material and not into the window flange - at least that is what I assume :whistling2:


If you have the 5/4 board and the window was still sticking slightly because of the flange, you could still cover it with a trim board that was cut to fit.


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## ddsrph (Nov 23, 2013)

Would it be better to put the XPS over the bare studs then put the OSB on top and held to studs with 3 1/2 or 4 inch screws? Bump out the windows and doors by a flat two by four with a ripped piece of 1/2 inch OSB cut to width of two by four to make the total of two inches. The XPS could be glued and lightly nailed in place to hold before covering with the OSB.


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## Windows on Wash (Aug 30, 2011)

I have never been a fan of foam over studs and then plywood. I just don't like it from the standpoint that the foam isn't structural, it can bow, and you don't get any of the wall bracing effect with foam. 

I would always prefer to go with solid sheathing, seal it up tight, then foam, then strapping, then cladding.


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## ddsrph (Nov 23, 2013)

Windows on Wash said:


> I have never been a fan of foam over studs and then plywood. I just don't like it from the standpoint that the foam isn't structural, it can bow, and you don't get any of the wall bracing effect with foam.
> 
> I would always prefer to go with solid sheathing, seal it up tight, then foam, then strapping, then cladding.


That's a good point especially with something two inches thick. I would still use the two by four plus equal width 1/2 plywood to frame around windows doors.

JM


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## AlphaPilot (Aug 6, 2012)

Ah, I see now with the rabbet you meant for the window flange. I am thinking a 2x2 (true 2" x 2") would not be sufficient for installing window trim in the future (thinking of 5/4 fiber cement board as my choice).

Interesting idea on the 2x4 + plywood to get me 2" thickness. I suppose 3.5" wide wood surface would be just right to install window flange onto and also window trim. Not sure if I want the window to be installed same plane as foam to make flashing with house wrap easier or if I should bump out the window surface an additional 3/4" (rainscreen gap) to match the same plane as siding.


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## Windows on Wash (Aug 30, 2011)

Best bet for you is probably going to do a mock up on one small window and iron out all the trim details and get your materials list together after you decide application. 

Drip caps and water drainage details (i.e. flashings) are a must. 

Probably the best flashing right up you will read on windows is right here. 

http://www.albertsroofing.com/Window%20Flashing.htm


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## Gary in WA (Mar 11, 2009)

Structural sheathing over any foam is not acceptable- defeats the idea, industry standard. Why 2" (code/physics require 1-1/2) foam, and are you dead-set with 2x6 walls?

http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

Gary
PS where in Nebraska?


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