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Window replacment detail question

VapourOpen | Posted in Green Building Techniques on

I I am installing new basement windows, most likely marvin integrity. I can’t afford any better windows at the moment, and may even have to settle for vinyl. The basement windows are are clerestory, averaging about 10sq feet each, there are 6 on a 1000 sq foot plate

This is a 50 year old house that has 10″ concrete block walls. Currently the old windows were face sealed, an “outie” window with a concrete brick sill. Some sills have one small crack, that i planned on fixing next year. I plan on installing new 2″ XPS on the interior of the basement for insulation, and would like to line up the thermal plane of the window. There is NO WRB. It is just concrete brick whythe followed by a concrete block. Near the window location, since the basement sits high, I will install extra XPS in the studs on the upper half of the foundation wall

My installer recommends doing a retrofit install, leaving the existing frame. I would like to bring the windows inwards to make them an “innie”. Do new window bucks need to be installed? The plan for the installer was just to use aluminum flashing and sealant. Not even any blueskin (i have extra in my garage).
Also is a drip edge necessary with the current type of sill i have. My gut also says yes.

SHould i move the windows inwards, or keep them close to faced sealed.The big concern i have is that there is no WRB, and my gut tells me not to bring the windows too far in. I have some details of my basement wall i could attach.


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  1. GBA Editor
    Martin Holladay | | #1

    My own preference for windows installed in a concrete block basement wall would be for outie windows. That way the foundation stays dryer, and you don't have to invent and install a complicated metal flashing to cover the exposed concrete sill.

    If you haven't ordered the windows yet, here is some advice: plan to install rigid insulation on the jambs and rough sills of the window rough openings. This will make your windows smaller, but will improve the thermal performance of your insulated wall.

  2. VapourOpen | | #2

    This sounds like a good idea.
    I have attached a drawing. When you are talking about extending the XPS, are you talking about the drawing on the right? or the middle?

    Does the detail look correct?

    Currently the sill is concrete brick (not clay). Should i install a drip edge on top of this type of sill? my concern is the mortar joints (future cracks) Should i install blueskin on top of XPS to Brick?

  3. GBA Editor
    Martin Holladay | | #3

    The option on the right is the best option. It would probably require a plywood buck, screwed through the foam to the concrete blocks, to be secure.

    If you decided to install exterior flashing on the sill, to convey rain away from the concrete sill, the flashing should begin under the window frame, and should terminate in a drip leg beyond the foundation wall plane.

  4. VapourOpen | | #4

    Do you think it is a good idea to create a drip edge in this type of setup when, the concrete brick is already a drip edge, albeit probably not a good one.

    i take it that creating a new buck leaves the old frame in place then.
    where should the termination of the blueskin on the exterior be?

  5. GBA Editor
    Martin Holladay | | #5

    If the existing concrete sill is in good shape, and if it slopes to the exterior, you don't need to install metal flashing over it. Only you can assess the condition of the concrete sill.

    I don't know why you would want to keep the old window frames if you are installing new windows. Why not rip them out?

  6. VapourOpen | | #6

    I wasn't sure if that was necessary. I was being quoted on window retrofit prices, and assumed from what they told me that i should keep them in place. It shouldn't be that hard to create new window frames by myself, but my greatest concern is the metal flashing (if i need to install it), i feel like that would be out of my skillset.

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