I have andersen 400-series windows throughout the house. I'm renovating the attic so the trim and wallboard are off revealing this situation:
The window is not attached to the rough opening on the sides or top. It just sits on that stack of 2x4s at the bottom and the flange is nailed to the sheathing on the outside. I checked Andersen install instructions and they don't specify that you have to nail to the rough opening but they do say to shim them; no shims anywhere (the thing on the left is a cut strap). If I push on the window frame, the whole thing moves a bit as the flange flexes. The stack of 2x4s that it's sitting on seems to be nailed together top to bottom but not toe-nailed to the sides of the rough opening so that rocks slightly too.
Fiberglass insulation tucked in on the sides but not the top. This window has maybe 1/2" space there but the other one (other side of house, same situation) has about 1-1/2" of empty gap. I see some sunlight through the gap and flange but I don't see any signs of leakage. I had some of the other windows replaced and I don't think there's flashing tape over the flanges on the outside. No flashing over sill. Wimpy looking header, and those are collar ties above, not joists, so there's only a few feet between the top of that window and the ridge of the roof.
All the windows were installed at once around 2000 under previous owners so it survived like this for nearly 20 years. My guess all the windows are installed about the same. I'll toe-nail that stack of 2x's to the trimmers on the sides and foam the gap around the windows. Should I be concerned or is this ok? Does the window need to be attach the inside of the rough opening somehow? Does there need to be a proper header above the window?
Edit: The house was built in the early 50s. These windows were installed around 2000. The dark brown wood (full studs and top header board) are original construction, old growth douglas fir lumber, generally very solid. The lighter wood inside that was added during the window replacement. Reasonably certain the frames originally held larger windows. They built up the sides of the rough opening to hold smaller flanged replacement units. The outside was wood shakes originally and vinyl siding was installed over that at the same time as these windows went in.