I have a question regarding the installation of windows in ICF applications.

 

I’m building an ICF home for my family.  All ICF work is done, and the framing is almost done (1^st floor ICF; 2^nd floor stick built).  We plan on siding the house with Alaskan yellow cedar using a 5” to 5-1/2” exposure. 

 

Based on conversations with the Cedar and Shingle Bureau, they strongly recommend furring out the shingles ¾” from the EPS (using cedar furring strips 2½”-3” wide).  This ¾” furring, when combined with the 1-1/4” shingle thickness of the shingles (18” shingles at a 5” to 5-1/2” exposure yield a 1-3/16” thickness), create a total protrusion from the house of 2”. 

 

If the Azek trim I use is ¾” thick, it appears I need to fir out the windows 1-1/2” – so I plan on using 2 courses of the cedar furring around the windows.  The 2 questions I have are:

 

1.         Is there any way to avoid the need to make all the inside extension jambs, since the factory-made Andersen window extension jambs will now be 1-1/2” to short?

2.         Is there a better way to install the window on the outside?

3.         What’s the best way to flash the window to make sure there are no later water problems?

 

Thanks for any advice, tips and suggestions.

 

Regards,

 

Jeff

Mystic, CT      

I'm been wrestling with the window installation issue also. I've got mine figured out and I'm also using Andersen windows.

First question before we can start figuring out how to do yours. How did you buck out the window openings?

I installed pressure treated 2x8 (ripped to 6") in the 6" ICF space between the EPS, and then added KD 2x12 spruce (ripped to 11") across the entire width of the wall, so that I'd have a good nailer for the windows. The inside of the KD 2x12 spruce is the rough opening specified by Andersen.

I'll let you know what I suggest late tomorrow or Friday. I've got funeral related activities from now until late tomorrow. A brother passed away last week.

In the mean time I'll be interested in seeing what others recommend.

I'm watching this space, too. Seems like windows will be slightly different on every ICF job. It's good to hear how other people solve the issues.

Sorry to hear the bad news Mac.

Jbrown, are you using cedar lap with 5"-5.5" exposure or cedar shingles?

Mac, sorry to hear of your loss. On another note that BI I was talking about purchased GRACE vycor plus for his window flashing and said it stuck to the ICF just fine.

Dave

Why not attach your windows directly to the wood buck-on the plane of the ICF- then use a wider-thicker- window trim (like in log siding, where one uses a 3 by 6 on edge attached with 8 inch screws, possibly a rough hewn side facing out) Then your strapping and buildout for the shingles is no problem, your windows will be "inset" slightly but will look great.

Thoughout about composite siding shingles instead of wood?

Kevin

I'm using shingles with a 5"-5-1/2" exposure. I looked at composite shingles, just didn't look quite as good (too uniform).

Jeff

Do you have Andersen's window installation instruction? It's available from their web site, somewhere. You will probably need to go the builders' area of the site to find it. I downloaded the one for casement windows so I don't have the link now.

For what it's worth here's my suggestion, shown in the attached PDF. I'm addressing the window in the rough opening aspect only. I'll let you figure out the trim part! I've come up with this approach based on a lot of comments and debates in these forums, and particularly in the JLC Online forums, and other building science reference materials.

A basic principle for window flashing is recognizing you cannot make the window installation water proof, only water resistant. The idea is to minimize the possibility of water getting behind the window flanges, and creating an exit path for water that just might get in.

What I've got drawn up in the attached PDF pretty much follows the Andersen guideline with a several differences. Where they show house wrap wrapping around into the rough opening, I'm showing peel & stick flashing. This is to create a continuous moisture barrier from the surface of the ICF around into the rough opening. On the sill you should use Dupont Flexwrap or something like Vycor flashing in conjunction with the plastic corner pieces, Vycorners.

I also show a moisture dam on the sill. You should also slope the rough sill. The slope and dam help to prevent wind blown water below the window from traveling to the house interior. This isn't shown in the Andersen instruction.

Now, some other differences. Andersen says fasten all the way around with nails/screws every 6" to 8". Don't do this on the sill flange. Put a fastener only about every 18" to 24" of window width. The reason for this is to allow the flange to have some gap between it and the sill flashing so if water does collect on the rough sill it will be free to drain down the wall surface. You will notice also that flashing is placed over the the side and header flanges only, not over the sill flange. You don't want to trap water on the rough sill. Same thing with caulk. Caulk the flanges on the sides and header only. You won't find agreement as to which is the best caulk to use, but I don't think you can go wrong with pure silicone. Messy, but good, and should last longer than the windows!

Now here's where I have a radical departure from what Andersen, and Dupont and others, say to do. That is with regard to sealing on the interior side of the window. I have details for both the Andersen way and my way in the attached PDF.

Andersen and others show putting a bead of caulk or foam between the window jamb and the rough opening just inside the interior edge of the jamb. I think this invites moisture problems. Combined with the caulk under the flange this seal at the interior edge of the jamb creates a closed volume all around the window, a space that has a passageway to the outdoor atmosphere via the gaps in the sill area. Any closed volume that has a restricted opening to the atmosphere becomes a water vapor pump, so to speak. As the volume heats up and cools down in response to the daily temperature cycle, especially when the sun shines on it, air moves in and out. Warm air will hold more moisture than does cool air. As the air moves in and out of the closed space the space does not exhale vapor nearly as effectively as it inhales it. The result is the space eventually draws in enough moisture to become saturated. When it cools off at night that vapor will condense. I am convinced the space that is created between the two caulk lines around the window will become saturated with water vapor and we end up with exactly the situation we're trying to avoid, water around the window jamb.

That is why you will see in the detail labeled Mac's Way I show placing foam in the space between the window jamb and the rough opening all the way up front against the flange. I think if one is careful in placing the foam, using of course blue can non-pressure expanding foam, you can fill up the entire space between the window and the rough opening by using multiple passes. This eliminates any volume for air to breathe in and out bringing with it water vapor, and gives as high an insulation value as possible.

Just in case anyone is skeptical about the idea of a closed volume sucking in vapor let me assure this is based on real world problems. When I worked at the Strategic Petroleum Reserve in Louisiana we had a serious problem with the interiors of objects like spare pipeline valves that were stored outside corroding. In some cases the corrosion seriously impacted the serviceability of the valves. The valves had all openings closed, but not air tight. An Army corrosion specialist called in as a consultant clued us in. Since it was impractical to totally prevent the problem we switched to sealing all openings with cosmoline and fabric and put desicant inside.

You can also find discussions of this problem on the JLC Online board in the Electrical forum. There're several discussions about moisture build up inside closed conduits. At the SPR vertical runs of conduits have moisture release valves at the bottom of the vertical run.

Hope this helps.

Sure wish the weather would warm up and the ground thaw out so I can get going on my house. It's frustrating having construction loan money available and not being able to use it!!

I should have said I'm making my own extension jambs from 1 x 12 MDF. They will be installed after the windows are in place and sealed up. The extension jambs will be rabbeted to fit into the window groove and fastened to the rough opening. There's no good way to fasten them to the window jamb other than glue.

I'm following along with Kevin on this one or Place the windows anywhere you want in the opening, waterproof, run jamb extensions (Same as the inside) out to the face edge of the strapping, trim the outside with a 1x4 or something similair, trims out the window nicely. I like the rough sawn face idea, will look authentic against the cedar and it can be painted any color you like.

I'm planning on using Azek trim on the outside. It seems that my choice is either to fur out the window before installing it, and thus needing to make custom extension jambs for every window, or installing the windows flush, and building out the window casing so that the shingles don't protrude past it (I'll put a 3/8" -3/4" plastic batten furring strip behind the shingles, plus the 1-1/8" thickness of the shingles at 5" exposure for 18" R and R Certigrade #1s). Any other ideas?

I go along with Kevin about using a thicker & wider trim, although I would probably use a finished size on the order of something like 2 1/4" thick by 4 or 4 1/2" wide. Let the window frame sit slightly behind the trim. I think it would look plenty OK. My concern with furring out the window is you complicate the flashing for the waterproofing, especially at the top where you need to overlap the header flashing over the side flashing. You also have the potential for a lot more air paths in the window-to-wall region making air sealing a lot tougher. You may have gaps between the furring strips and between the furring strip and foam. Of course you could caulk the hell out of everything when you put it up, but that's a lot of work and caulk.

I've attached a revised drawing showing how I would do the trim. On the sill you would want to slope the top side of the trim to facilitate water draining off. At the header you'll want to put a metal flashing under the shingles and over the trim to keep water from running behind the trim.

If you are dead set on using Azek trim, then I would layer the Azek on top of cedar 1 x's in place of the 2 1/2" thick trim I show. On the sill you could put metal flashing on the top side of the trim to dress it up a little, or use a thin Azek piece flat on the top of the trim.

Out of curiosity, if you're using cedar shingles why not use cedar for the trim? Or do you plan to let the shingles weather but want the trim to be colored?

Did you special order extension jambs from Andersen to fit the thickness of the ICF wall? I don't see any jambs in their product catalog that would work for a 6" ICF wall. Or are you using a regular extension jamb and doing wrap around with the drywall?

Can try a Trex type product which is 1 1/2 thick for trim, or even double it up- a 2by4 on a 2 by 6- the 2 by six acts as your nailer for the furred out shingles. We have done this also. Prime it properly and it holds paint great.

Kevin

If you used 1-1/2" thick trim, though, how do you account for the profile of the Andersen window, which only protrudes 1-5/16"? Thanks

That's what Kevin and I are saying, don't worry about it. Let the window profile sit behind the trim by 1/2" to 1", or whatever. In my sketch I show butting the trim fairly close to the window profile and then caulking in the corner between the two. There's nothing in the rule book that says the window profile must extend beyond the trim.

Another option would be to bevel the front side of the trim so that the trim thickness against the window matches the window profile. More work, and might preclude the use of Azek, but something to think about. Or maybe do some sort of stepped trim, match the window profile on one side and the shingle thickness on the other. How large a wall space do you have? If the windows are proportionally small with respect to the wall area you could probably use wider trim and make beveling or stepping easier. Pick a width that strikes a good visual balance between the window size and wall size.

What you could do is loosely insert one window in its opening and then experiment with different trim options to get a better idea what they may look like. Go with the one that looks best to you.

As as is always the case, the window details are critical. Being careful with wareproofing and caulking methods during installation will insure no problems. I am familiar with Hydrostop or Sealoflex as waterproofing materials on our ICF projects here in FL. They are used to bridge the wood/foam door and window openings.

So I have 2 situations:

On the 1st floor, the walls are 11" thick ICF, and I'll need to make extension jambs. I saw an article in Fine Homebuilding where the regular 2x6 extension jambs were used, and another piece was rabbited to it (with a small reveal) to extend the extension jamb to the required length. Does this make sense, or should I just make the whole extension jamb? Article at: Aug/Sept 2007 Fine Homebuilding (TRIMMING A BASEMENT WINDOW by Chris Whalen)

On the 2nd floor, what you're saying is to install the window flush against the wall, and use 3/4" or so of cedar under 1" or so of Azek. I need at least 1-3/4" of thickness, as a 1/2" vented plastic furring strip (http://dciproducts.com/html/cedarvent.htm) could be used as discussed in a good article in Coastal Contractor magazine (http://www.coastalcontractor.net/pdf/2007/0703/0703best.pdf). This 1/2" furring strip, along a shingle thickness of 1-3/16" (18” shingles at a 5” exposure yield a 1-3/16” thickness) necessitates the 1-3/4" plus. And I'll let it run past the Andersen flange.

Any other thoughts?

Jeff

One change to my last comment. On the 1st floor, because the ICF wall only has vertical nailers every 6", I need to install a piece of cedar that is 6"+ wide. This piece of cedar needs to be the SAME thickness as the furring strip, since the shingles need to extend from the furring strip over the cedar, and butt up against the 3-4" wide trim. Right? I could use 3/4" thick furring strips and 3/4" cedar under the trim, and then use Azek ATM trim, which is a full 1-1/4" thick. Does this work?

Jeff

Is the second floor frame or ICF? I would install the windows the same in both cases, the only difference being that if it's frame you will have house wrap wrapping around into the rough opening instead of P&S like on the ICF. The P&S over the flange will be the same for both. Is there a difference in total thickness of shingle and furring on the second floor and first floor?

I think the wide cedar under the trim and shingles as you're proposing would be a good approach.

As far as extension jambs do whichever method you're more comfortable with. I plan to make extension jambs out of 1 x 12 MDF the full depth. Main reason is, I save the cost of Andersen jambs, and the rabbet that is in the Andersen frame will be easy to fit the extension into. I can also have some mismatch in the depth of the jamb I make without having a visible gap between the frame and extension jamb. See the attached PDF.

Maybe you're aware of this, but the reason finish carpenters use reveals, like in the Fine Homebuilding article, is that a reveal won't show slight mismatches between the alignment of the joining surfaces like you would see if you make them flush! Sneaky carpenter trick I learned on JLC Online Forums.

Hey, if anyone has any suggestions to improve or correct my suggestions please jump in. I sure don't want to be doing something screwy, and having Jeff follow me!!!