mnzinnia, I'm a bit baffled at Marvin making a window smaller than what you needed for your opening. Marvin has always been really good at making windows to the specs we require. The only time they can't meet these needs is when we push the over-sized units. Otherwise they've always come within a fraction of an inch of what we need. In your case the person that measured told you the "vertical style", or the proper term being the "Center Mullion or MULL", would be spread to fit. This is a very common practice in the industry used everywhere. We call it a "Spread Mull" when we order a product needing a special sizing. We use it when ordering custom sized doors with side-lights all the time.

If your windows didn't come with the spread mull done properly your installer should have sent them back to be done to the right size. We would have never installed these windows if they were to small unless the customer gave us the go ahead to downsize the opening. One thing I've seen lately are the outstate contractors still do things the old school way. Their continuing education on the changes in our installation practices always seems to be light years behind. One thing the installer has to be aware of is the new building codes from 2006 have specifically stated that the manufacturers installation instructions for windows and doors become code. This means every installer must know and understand the application instructions of each product they install or they will void the manufacturers warranty. In your case there's no problem because Marvin's insructions only state that "If You Desire" you can use a low-expansion foam sealant around their window next to the nailing fin. In todays practice we are using a low-expansion foam all the time during our installations. We have also been using the stuff in fiberglass combined with the low expansion foam. We spray the foam in first against the nail fin and then use the stuff in fiberglass behind the foam. The main reason being it stops the expanding foam from oozing out of the opening and making a mess. The use of low-expansion foam properly will eliminate any "Air Infiltration" from around the window. Fiberglass insulation is really made to be most effective at it's expanded 3-1/2" or more. When the fiberglass is compacted around a window in that tiny gap it's air infiltration impedance is nothing. So any air leaking in past the nail fin will find it's way into the home. The low expansion foam forms a barrier that stops air infiltration from entering the home. It will fill the little voids and cracks that the stuff in fiberglass won't. The positive feature of low-expansion foam is it stops water from entering the cavity. If water happens to get past the exterior barriers the foam will act as a watershed and keep the water from penetrating. The stuffed in fiberglass will soak the water up like a sponge and start to break down or sink in the cavity it's suppose to protect. When this happens you get an opening to possible mold & mildew issues that I'm not even going to think about getting into.

Just to make you feel a little better about things and I don't send you into a panic attack. The worst case scenario of leaking water and air threatening your lake home is very small. Since it sounds like your completely overhauling the exterior of the lake home you have a couple barriers that will more than likely stop anything before it gets to the window cavity. You've already told me the home was wrapped with Tyvek and flashing was done around the outside of the windows. Hopefully the installers used a nail-fin application that was back caulked before anchoring. If all these items were done properly and drip caps were installed above each window with the Hardiplank siding finishing it off. Your probably going to be just fine. Just make sure all the windows are caulked and sealed to the Hardiplank to form that first coat of armor. In other words I think you'll be just fine and dandy down the road. Sounds to me like your only problem will be that sheetrock dust and touch-up painting. I wish you well in the final moments of your project. Then you can sit back and enjoy the great Summer Minnesota has for us every year!! Good Luck!!!