Wrap & Strap - Windows

[jcaimbridge]

Hello, I'm new to the forum.  I'm planning on building a very small timber frame house and I'm hoping to get some assistance with the design.  In this thread I'd like to get some opinions on how to approach the windows when using the "wrap and strap" method.  I plan to create a much more comprehensive design thread later on.

Basically my current confusion is with the bearing of the window itself.  With the wrap and strap method, if I use interior mounted windows (see here http://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows ) there would be no issue as the window is bearing on the frame itself.  However, while inset windows provide better thermal performance, they are much more complicated and replacing an inset window would be a nightmare (among other problems).



Therefore I want to use normal "exterior mounted" windows (that is, windows relatively flush with the face of the building, see the attached picture for a rough sketch of the cross section of my wall).  But my issue here is that I'm not confident that the jamb extension and the underlying strapping and insulation would bear the weight of the window all that well.  This is exacerbated by the fact that my current designs have the exterior siding beyond the edge of the concrete slab insulation and 1" of the wall's rigid foam insulation meeting up with it (so there is no bearing directly on the concrete slab). 


So is this actually a problem?  And regardless, are there any creative solutions that don't result in excessive complexity or interior mounted windows?

Also, I've been reading up on the "REMOTE wall system" (see here http://www.cchrc.org/new/manual-remote-walls ) which describes a wall system that is relatively similar to the wrap and strap method and provides instructions on how to implement both interior and exterior mounted windows.  Unfortunately, it does not seem to address my concerns.

[Magicman]

I'll learn with you and Welcome to the Forestry Forum, jcaimbridge.   

[Dave Shepard]

Welcome to the Forum! I look forward to the discussion. I don't know much about wrap and strap, but I like what I've heard so far.

@timberwrestler

[jueston]

in some windows the nailing flange holds the window up, and in some windows the nailing flange holds it in place but the weight is bearing down.

do you know which type of window you will be using? if all of the weight is bearing directly down on what is basically a cantalivered board, then you will need some kind of support under it.

if you can rely on the nailing flange to support a large portion of the window weight on the side jamb extentions, which are much more sound then the top and bottom, then i think your fine....

i'm not an engineer, but i have installed thousands and thousands of windows... and i have seen the shims removed, which means no direct bearing on the sill, and if the nailing flange in correctly nailed in, it will hold fine.

[Brian_Weekley]

I'm interested in this conversation too since I'm doing a wrap and strap for my barn and have the same question regarding the best way to attach windows.  However, my windows will not be too big or heavy.  Right now, I've only got the 1st layer of vertical boards on the frame.  I'll be boxing around the window openings on the outside, attaching them through the siding intothe girts and studs on the inside of the frame.  The thickness of the boxing will be whatever is needed to account for the insulation and strapping.  This illustrates the basic ideal of what I'm planning:

Inside:


Outside:

[timberwrestler]

Wow Dave, I'm impressed on your twitter-like nomenclature abilities to tag me. 

I've done a bunch of wrap and strap, and it's good that you're asking the right questions--the windows and doors are tricky.  The answer partly depends on what kind of window you're putting in.  If it's just a sash, then what you drew will work well.  You'll need a sill, so making your own extra deep sill works well. 

But it's a window unit with jambs and sill, then you'd have a double sill.  Which could work, but it's a little goofy.  If the windows are pretty light (not insulated glass), then you can get away with sitting it on the foam and strapping.  i wouldn't rely on a nailing flange.  I always shim and nail through the side jambs (as the manufacturers recommend).  If they're heavier, then you may want to add blocking.

This ties in with how to flash the windows (and doors).  You have the paper under the foam, which is fine, but that means that the window needs to be flashed back to paper.  With 1-2" of foam, I would put solid blocking around the RO of the window, and have the flashing tape wrap from the window back to the paper.  With thicker foam, the blocking gets kind of ridiculous.  There are a lot people using thicker foam, as deep energy retrofits (google that for more).  So they build a sub-jamb, usually of plywood that slides between the framing (don't forget to add that width to the RO).  It's supports the window, the foam butts up to it, it gives a good nailing surface (whether the windows are innie or outie), and has minimal thermal bridging.

[Dave Shepard]

You were the first one I thought of when this topic came up. Besides, I was itching to play with the Forum's new toy.

[venice]

I´ve seen a lot of installers relying on the nailing fin only. But blocking under the jamb to carry the load is better(and technical correct). One blocking in each corner and depending on the width of your window, one ore more equally spaced.

One thing i´d recommend would be a sill that is extending over your siding. Some builders are using a sillpan as well but this depends on your overall construction.

[Ljohnsaw]

Timely thread. I'll be following it.

[venice]

OK, there was a little confusion with your drawing. After having a second look i see two possible options here.

1. put your window on the inside and use extension jambs/ casing on the outside, this way you got the support of the horizontal stud to carry the weight of the window.

2. if you want the wíndow flush with your siding, screw 90° brackets to the outside of the horizontal stud and place your window sill onto.

Option 1 is the better one in terms of insulating, weatherproofing and carrying the weight of the window.

[jcaimbridge]

Thanks a ton for the help everyone.  I've noted everyone's recommendations and modified my plans.  I've left out the solid blocking for the large window, but in my pictures I'm assuming these would just be 2"x4"s cut out of the 4" foam board insulation and extending from the slab to the 2"x10" jamb extension, right?

I've also left out the flashing, foam, caulking and felt paper details, but when I find time I will come back and add them to the plans.  I'm thinking of doing a combination of what's detailed here (more details here ) and what's outlined in the REMOTE Wall System manual mentioned in my original post.

The pictures assume the window unit does not have its own jambs and sill (the one detailed in the REMOTE manual seems to have its own, which would be convenient but I can't assume whatever deal I come across will have them)

Because the window flange is so difficult to see I've hosted higher resolution images elsewhere. (EDIT: At least I tried to.  Sorry about the resolution guys.  Not sure what I can do about that.  EDIT2: Added a close up, hopefully it's sufficient)

lower half of window cross section


lower half of window cross section close up


upper half of window cross section


lower half of window perspective projection



EDIT3: Forgot to ask: Would it make sense to add a layer of house wrap of some sort between the insulation and the strapping?  Most rainscreens I've seen online do this, but what about in addition to the felt paper? (and I want that felt paper there for sure since there may be shrinkage with the 1" tongue-in-groove)

[venice]

Did you decide on an particular window system yet? Wood or vinyl? Most wooden systems offer an extending exterior sill as an option.

Your sill detail is not optimal yet. If you build it like drafted, it will be the coldest point on your window and the result will be condensation. There will be also a problem weather proofing it. Is the building storage or living space?

1. You should try to create a "step down" from the inside to the outside to prevent incoming water. 
2. The insulation between the window(including all component like sills/jamb extensions) should project all the way through, from the inside to the outside of the wall.

Do the flashing wall/ window as shown in your first link. Start at the sill and work your way up.

With styro-foam as insulation it would be sufficent to tape the seems. With insulation made from stonewool or any other fibrous material you need the additional layer of building paper.

Could we see an horizontal cross section and exterior view?

[venice]

This is a schematic drawing how i install windows that are sitting in the insulation layer with no structural support through the wall - adopted to your construction.

Advantages:

1. weight and windload are carried by 4x4 wall
2. interior vapor barrier at the same level
3. max. insulation between window and framing with no gaps
4. flush exterior flashing at the same level

The drip cap and the exterior casing drawn are things that i like to do for clean termination of siding. But there are many ways to skin the cat.

Sill detail:



 

[venice]

Header detail:



 

[venice]

Side detail - horizontal cross section:



 

[jcaimbridge]

This is a schematic drawing how i install windows that are sitting in the insulation layer with no structural support through the wall - adopted to your construction.

...

This is great venice--extremely helpful!  It'll take me a bit to digest all of this and consider how to incorporate it.  However, do you have any recommendations for what product in particular to use for the steel brackets and at what frequency along the vertical and horizontal sections of the window?  I only ask because it seems like they could get expensive quick.

And to answer your questions:
Have yet to decide on wood or vinyl windows--it really depends on what (if any) deals I come across, and it is a living space

[timberwrestler]

The 'framing' (as opposed to finish) extension jambs used on DERs are almost always scrap 1/2" ply as opposed to 2x.  It's all you need.  Windows are hung through the jambs.  The plywood is nailed off to the framing. 

And I would certainly pick your windows before getting too far into the details.  If you're putting 4" of foam on the walls, I'm going to assume that you're using modern insulated windows.  Other than the uber-fancy European tilt-turns, they're all going to have integral sills. 

[venice]

You´re welcome.

I agree with timberwrestler, choose a window system before going deeper into the details.

The brackets for the sill should be strong enough to carry the weight of the window without bending. There are uber-fancy ones  :D with adjustment screws but a simple piece of galvanized steel will do. Spacing depends on weight and width. For the side and the header you can go with lighter material.