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Old 07-18-2015, 08:09 PM   #1
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Need Input - XPS Insulation


I am working on my house build (first house I am building). I have had the idea of using 2" of XPS rigid foam over the exterior OSB in the back of my mind for quite some time. I want to be energy efficient and not have as expensive energy bills.

I am building with 2x6 walls and planning on dense fiberglass or mineral wool insulation. I have my truss package designed for a 12" energy (raised) heel to allow proper insulation at the attic knee walls.

I am wondering if my 2" of XPS is a good idea, in zone 5B. If so, should I be worried about moisture or just use a decent house wrap over the foam and not worry much?
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Old 07-19-2015, 12:57 AM   #2
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Building Science Corporation has some great articles on foam exterior walls. They work great when done right.
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Old 07-19-2015, 09:18 AM   #3
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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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Old 07-19-2015, 09:20 AM   #4
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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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Old 07-19-2015, 09:28 AM   #5
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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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Old 07-19-2015, 12:33 PM   #6
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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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Old 07-19-2015, 01:34 PM   #7
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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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Old 07-23-2015, 11:53 AM   #8
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Quote:
Originally Posted by CTSNicholas View Post
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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Old 07-23-2015, 01:15 PM   #9
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Quote:
Originally Posted by CTSNicholas View Post
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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Old 07-23-2015, 10:14 PM   #10
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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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Old 07-25-2015, 11:47 PM   #11
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Quote:
Originally Posted by Windows on Wash View Post
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
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Old 07-26-2015, 06:47 AM   #12
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Quote:
Originally Posted by CTSNicholas View Post
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
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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Old 07-26-2015, 07:06 AM   #13
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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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Old 07-26-2015, 08:15 AM   #14
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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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Old 07-26-2015, 09:41 AM   #15
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Quote:
Originally Posted by Windows on Wash View Post
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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