GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Deep energy retrofit — Envelope upgrade questions

drewman141 | Posted in Green Building Techniques on

We are well into the process of a deep energy retrofit for our 2,700 sq. ft. house in NYC suburbs, zone 4/5A. The house is a typical 1988 contemporary with 2×6″ wood frame construction, builder grade, sloppy subs, code minimum and 25 years of neglect. Long story short, we are currently replacing the roof, windows, doors, siding and HVAC. Before we started renovations, the blower door test result of just under 11 ACH50, proved that the envelope could be improved, so that is now our focus.

So far, all of the roof work is complete. The entire roof deck has closed cell SPF of R-38 in the joist bays and attic gable walls. The attic is now sealed (A/C & ERV ductwork, mechanicals are in attic).

We are now gearing up for the windows, doors, siding, envelope upgrades and I’m getting lost in my own head trying to figure out the wall details. The windows have already been ordered. They are VEKA finless Euro style tilt & turn or fixed, uPVC window.

The current wall is:
1/2″ drywall
2×6 framing
R-19 kraft-faced fiberglass
1/2″ plywood
Tar paper
3/4″ cedar vertical shiplap

The upgraded wall will be:
1/2″ drywall
2×6 framing
R-19 kraft-faced fiberglass (walls where drywall needs replacing will be upgraded to R-21 Roxul)
1/2″ plywood
WRB – Blueskin VP 100
1.5″ XPS (max thickness I can use due to overhangs)
Horizontal vinyl siding

The things I’m getting hung up on:
The position and type of the WRB as it pertains to air sealing and weather proofing
The taping of seams – plywood, WRB, foam
The window install detail

My current idea is to picture frame the ROs with 2×3, install the WRB wrapping over the 2x into the ROs. Then installing the 1.5″ foam, flashing from the foam into the ROs with membrane tapes (Extoseal, Wigluv, Vana, etc.). The windows would be installed flush to the exterior. Gap sealed with expanding foam tape, interior and exterior sealed to membrane flashing with tape (Corvum, Profil). Siding trim would cover the exterior tape and window/trim joints would be caulked.

My questions:
Is my wall detail acceptable? Is there a better way to do it?
Should I tape the plywood or foam seams for airsealing?
Should I use a different WRB? I was thinking something like Hydrogap might be a better drainage plane behind the XPS.

Any other suggestions or criticisms are very much welcome. It seems to me, the more I read and research, the more confusing it gets. Getting the best performance while trying to stay within a reasonable budget seems to be a pretty difficult balancing act.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. LucyF | | #1

    My amateur opinion is to tape everything - plywood and foam. I think you need redundancy in the air barrier in case there is a failure at one level, you've covered your bases. If gaps open up in the foam (less likely with XPS, I think), the plywood layer is still airtight.

  2. STEPHEN SHEEHY | | #2

    I agree with Lucy. I used a ton of 3M 8067 flashing tape on the sheathing, house wrap, etc. It sticks to everything and costs about half the European tapes, although I did use Siga tapes with the Siga membrane.

    Check out recycled xps. You'll save a bundle, as well as help save the planet.

  3. iLikeDirt | | #3

    Position the WRB on the same plane that you install the windows on. If you're going to install the windows flush with the exterior (i.e. flush with the outer layer of the XPS foam), then you should apply the WRB over the foam. That's the easiest and sanest approach.

    With only 1.5" of exterior foam, and with the foam being XPS, you can easily install the window flanges right over the foam; no need to build out the window rough openings. Just make sure you use 3" screws so you're fastening the windows firmly into the rough openings. That should simplify the window detail for you.

    I second Lucy's advice to tape everything. It never hurts. As for air-sealing, you should decide which layer is going to be your air barrier according to what's most airtight, easiest to seal, and least likely to be penetrated either during construction or in the future. You've chosen a high-quality WRB so using it as the air barrier is more feasible than with Tyvek or something. But if you choose the WRB as the air barrier and the vinyl siding is fastened right over it, every one of those nail holes becomes a hole in the air barrier. If you attach the vinyl siding to vertical wood furring strips, then it's probably safe to use your WRB as the air barrier. Otherwise, there's a lot to recommend using the plywood sheathing as the air barrier in this scenario--it's be protected within the wall sandwich and is easily sealed at the seams. If you make sure to only fasten the XPS through the plywood and into the studs, then the fasteners won't be poking open holes in it. But yeah, it certainly doesn't hurt to seal the seams of the XPS foam and the WRB as well.

  4. drewman141 | | #4

    Thanks for the suggestions. The more I think about it, I do believe taping the plywood will be the most effective and easiest way to air seal the envelope.

    Nathaniel: The windows are fin-less so no nailing flange. They are installed with clips mounted in the RO or with through frame screws. That is reasoning for the picture framing with 2x3's. We are going with the "outtie" install b/c the ROs are too small to allow trimming with anything other than aluminum cap flashing which I am staunchly against. Because of their design (dual seal, weep holes, all upvc construction) I felt it was best to keep them in the drainage plane. That way if any of the sealing layers failed, water wouldn't get into the structure.

    I know caulking around the inside of the trim/window will inevitably fail. I just can't come up with a better detail for a sill design. The attached pics show what the bottom of the windows look like. The outer groove is only 6mm deep. There would be no easy way to attach a sill. The clearance to the weep hole cover is 1". That's why I figured taping the window perimeter with something like wigluv would be an easier and more durable option. Then the overlapping window trim 3/4" would cover the tape.

    Would using something like Primur sandwiched between the window frame and overlapping trim be better than using caulk on the outside of the joint? Or would you use caulk in addition? Kind of like a belt and suspenders approach. Or is this all overkill and overthinking it? If the joint between the trim and window leaks, the water will just run behind the vinyl siding, on top of the foam and out the bottom of the wall assembly.

    The siding will be mounted with wafer head screws into the studs through the foam. Unfortunately there is just not enough room for furring beyond the 1.5" foam. I am open to any suggestions on the WRB material. Perhaps using the BlueskinVP which is SSA may not be cost effective. Does using HydroGap on top of the foam make more sense? Would one of the European WRB's be a better fit for this application?

  5. GBA Editor
    Martin Holladay | | #5

    Before installing the new windows in your rough opening, you have to flash the rough opening in such a way as to ensure that the rough opening is waterproof. That means that your rough sill should either be sloped to the exterior (by installing tapered clapboard as a shim) or should have an interior dam, and that this rough sill should be flashed with a waterproof peel-and-stick membrane that directs water to your WRB. Then the jambs of the rough opening are flashed, overlapping the sill flashing.

    Before finalizing your plan to install vinyl siding through 1 1/2 inch of rigid foam with no furring strips, talk to a representative of the vinyl siding manufacturer, or review the vinyl siding installation instructions. I'm not sure that every vinyl siding manufacturer allows installation over foam that is that thick.

  6. drewman141 | | #6

    Thank you everyone! I think I may have it figured out now.

    I'm going to air seal the plywood layer. Then install the XPS as described above. Then install the BlueskinVP over the foam. Since this wrb is self stick, there's really no need to tape the seams of the foam and there's no worries on how to attach the wrb to foam. This will also make flashing into the ROs much easier.

    Martin: the ROs will either use membrane flashing and a 1/2" plywood back dam or a prefab sill pan like Suresill. Should the membrane flashing wrap all the way from the exterior foam to the interior Sheetrock?
    As far as the vinyl siding, Royal follows the VSI guidelines. If you have 1.25" of fastener penetration into framing, you're good. Something like a 3-3/8" HeadLOK screw would be more than enough to support the shear load. The s4s pic trim will need to be installed over 2x's. Cortex fasteners are only 2-3/4".

  7. Expert Member
    Dana Dorsett | | #7

    "The entire roof deck has closed cell SPF of R-38 in the joist bays..."

    That means you have about twice the thermal bridging through timbers that you would have if you'd gone with a lower R/inch product, since there's now only 6" of wood (R7-ish) rather than 9.5" (R11ish, for 2x10 rafter) or 11' (R13-ish for 2x12s) for the framing fraction. Even at a 15% framing fraction that's significantly lower performance. (Also note for clarity purposes, if they're supporting the roof deck they are called "rafters", not joists.)

    Hopefully it was installed in three lifts of 2", with sufficient cooling time between lifts?

    Whatever the rafter timber size, it's worth filling it completely with either batts or blown fiber under the ccSPF to reduce the thermal bridging, even if you don't go the extra step of installing rigid insulation over the rafter edges as a thermal break. If it's 2x10 rafter you should be able to install R13-R15 batts in there (even if it's a compression fit), which would bring you up to IRC 2012 code-min (R49). If they're 2 x 12s you can install R23 rock wool in there. With R38 on the exterior of R23 you will never have a condensation issue at the foam/fiber boundary in your climate, even if you don't install an interior side air-barrier (though an interior side air barrier is still a good idea.

  8. drewman141 | | #8

    Thanks for the info Dana. Unfortunately the roof work was completed in the fall. The rafters are only 2x8s and the installer covered the sides of the 2x8s with the foam leaving about a 1" depression in the center of the bay. The only wood exposed is the 1.5" edge. There a few things I would have done differently but it is what it is. Just sealing up the roof this way has cut our winter heating bill pretty much in half from last year.

    We have faced quite a few curveballs with this house in only 2 years of ownership. If were up to me and we were rich, I would have built a passive haus from scratch. But we have to be realistic with the labor and products we can afford. Sometimes you get bad info from reputable companies, the roof was one I missed. That's why I want to focus on getting this wall detail right before the work is started.

  9. Expert Member
    Dana Dorsett | | #9


    On the walls where it will remain R19 fiberglass, it's worth drilling from the exterior to pack cellulose in there (even at lower density) to reduce air leakage and boost the R. The R-value of PERFECTLY installed R19s when compressed to 5.5" is only R18. Most batt installations done by "...sloppy subs..." in 1988 would be far less than perfect, with many gaps and compressions.

    But when you pack cellulose in, compressing the fiberglass to a higher density the end result is R20-R21. That's not a huge performance boost on labeled R-value, but by eliminating the gaps and making it much more air-retardent, it is going to reduce or eliminate a potential thermal bypass. Air leakage behind your new exterior foam into a low-density fiberglass insulated cavity could undercut the wall performance by quite a bit.

  10. drewman141 | | #10

    We were originally planning to dense pack cellulose into the walls. The payback period for increasing the r value from 15 to 20, an overly generous guesstimate, would take over 60 years to break even. For a house we only plan to spend 30 years in, it doesn't make financial sense. Especially when there are so many other areas that need attention. That's why I felt preventing the air from getting into the existing fiberglass and putting as much foam outside as possible would be money better spent. The interior drywall for the most part is staying intact. At this point, instead of pretty good house, we are downgrading to pretty decent house. We will still most likely be more energy efficient than 80% of the homes in Westchester county.

    I should have posted in here 9 months ago when we started planning. Everyone's input has been very helpful.

  11. Expert Member
    Dana Dorsett | | #11

    Before writing off wall insulation upgrades completely, do some infra-red imaging (with or without blower doors to exaggerate it) to find the leaks, gaps and thin spots. It's pretty common to find entire stud-bays where the mice & voles have moved half the insulation out, or sections where it was never insulated at all. When the siding comes off it's an opportunity moment to fix those, one that won't come around again for decades.

    It doesn't have to be dense-packed to make a difference- drilling a 1-1/4" hole every 4' and blowing with a nozzle from the exterior until insulation is flying out the adjacent holes & or the blower stalls will get you to 2.5lbs density or better, and you will have filled all of the voids & compressions, and the air driven fiber will clog all of the framing/sheathing seams. It's not quite as air-tight as caulking the framing to the sheathing at every stud bay from the interior side, but it's pretty close. Air sealing the sheathing to the stud plates & studs from the exterior isn't always possible.

  12. drewman141 | | #12

    Well I thought we had the window install detail figured out, but now I just stumbled upon this Q&A spotlight -

    The install in that article is almost exactly what we were planning to do. We were doing a better job on flashing in the sill pan w/ back dam. But the exterior taping and picture frame trim is what we were planning to do (not covering up the weep holes). After properly flashing the RO to the WRB, installing the window with plastic shims/packers, and then taping all sides of the frame to the WRB, I was planning to use a gasket material between the trim and the window frame, beveling the top "sill" edge of the bottom trim board and caulking the frame to trim seams. Is this assembly destined to fail? All of the mulled units are factory mulled. Tapes and flashing membranes from Proclima.

    The PVC trim boards we are using will be 5/4" and have a 3/4"x7/8" rabbet on the backside to conceal the cut siding. The 1/4"x7/8" lip would act as a drip edge preventing surface tension from pulling the water behind the siding. The trim will be installed with Shaker style butt joints (no miters). Should I install an aluminum drip flashing behind the head casing the entire width of the head?

    The majority of the windows are on the north and south walls and the head casing is very close to or touching the roof overhang so I'm not too worried about bulk water coming from above. My only concern is the windows on the east gable wall that take the most punishment from the weather, have zero roof overhang and are vertically inline. Pretty much worst case scenario and the reason the existing Andersen windows have failed in pretty much every way.

  13. GBA Editor
    Martin Holladay | | #13

    Picture-frame trim makes no sense for exterior window casing.

    These days, windows don't necessarily need exterior casing. But if you are going to install exterior casing, you have to follow a few water-management rules.

    Here in New England, I was taught to install windows the old-fashioned way. The sill is proud of the plane of the siding, so that any water that strikes the sill drips clear of the siding. You can install horizontal trim under the siding if you want -- either narrow trim or trim as wide as your jamb casing -- but that trim mustn't be proud of the sill.

    The sill is sloped, of course. The underside of the sill has a kerf -- the kerf is parallel to the length of the sill, and is about 1/4" back from the drip-edge of the sill. The purpose of this kerf is to interrupt capillarity that can allow water to curl under the sill.

    The length of the sill (left to right) needs to be long enough to catch all of the drips coming down off the jamb casing. This is done by making sure that the sill has generous "ears."

    The casing dies into the sill. The bottom of the jamb casing should be beveled, to match the slope of the sill.

    Here is a photo I grabbed off the Internet to show you what traditional exterior window trim looks like.


  14. drewman141 | | #14

    Thank you Martin. I completely understand the design and function of a traditional sill and it's very easy to do with a standard NC window with nailing fins. The existing windows don't have trim, but that's not a look we care for. We were not intending on having a traditional sill detail. Now the windows are going to be delivered next week and we do not have enough room to install a traditional 5/4" sill under them. It's possible we could make something onsite to work with 3/4" flat stock. But I guess what I'm trying to say is with all we're planning to do to make the WRB air and watertight with high performance materials and keeping the entire window assemblies in the drainage plane, is this really something we should worry about? If there are no organic materials outside of the WRB and a vented drain at the bottom of the wall, would water running between the siding and WRB ever cause an issue?

    If we didn't use casing at all, we'd still need to use J-channel to terminate the siding. Since the face of the window is flush with the wall, what would need to be done for a sill? The typical hack job around here is to bend some aluminum cap flashing, stick it under the window and caulk the seams. I think it looks terrible and I'm sure they all leak within days of being installed.

    All of the euro tape manufacturers show the exterior tape being installed around the entire perimeter of the window. A traditional window install you would only seal 3 sides leaving the bottom open to allow any water to drain. Is the euro practice any better or worse than our traditional install method? In the euro method, if the window frame fails and water makes it into the sill pan, wouldn't it just fill up the pan until it over flowed, most likely into the room? What if both interior and exterior of the window are taped to the WRB and water makes it through the frame, where does it go?

  15. GBA Editor
    Martin Holladay | | #15

    You'll need to post a drawing of your planned assembly if you want me to assess whether it makes sense.

    In general, the sill pan you install on your rough sill should direct water to the exterior. In most cases, that means that it conveys the water either (a) to flashing that directs the water to the exterior side of the siding, or (b) to flashing that directs the water to the exterior side of the WRB, which ideally faces a rainscreen gap.

    I don't advocate the use of flashing tape on all four sides of the exterior of a window. Nor do I advocate the installation of "picture frame" trim on the exterior of a window. But you had better post an illustration so that we can all understand what you want to do.

  16. drewman141 | | #16

    Hi Martin - we installed the first "test" window today to get the contractor familiar with all of the steps and procedures. I think we have a good sense of how everything will work. The plywood is the air barrier. The RO is completely waterproofed and flashed to the BlueskinVP WRB layer. We are using a strip of BlueskinVP under the sill as a bib to direct any water that makes it into the RO out through the siding.

    Here is what we did and the order they were installed:

    Taped all seams and nail holes with 3M Flashing
    Taped Plywood/Foundation with Protecto Super Stick and Protecto Primer
    Installed XPS
    Installed BlueskinVP
    Installed sill back dam
    Used a strip of BlueskinVP as a bib under the sill (this will wrap over the nailing flange of the last full piece of siding)
    Installed Protecto 100% Butyl Sill Pan Flash in the sill and over the back dam. The bottom lip goes over the BlueskinVP bib.
    Installed Protecto Super Stick flashing on jambs from BlueskinVP into RO
    Installed window
    Installed Protecto Super Stick flashing on exterior of window jambs and head, leaving the sill open.

    We are going to proceed with the picture frame trim detail (there just doesn't seem to be a better way). The head casing is going to be AZEK with an integrated drip edge with the nailing flange taped to the BlueskinVP. The sill casing will have a beveled top edge. Durasil sealant will be applied to the window frame behind the trim.

    It seems like these details will direct the majority of bulk water out from behind the siding and anything that does make it behind will be blocked by the waterproof layer so it shouldn't be much of an issue. Let me know if we're missing anything here.

  17. GBA Editor
    Martin Holladay | | #17

    You should be OK, especially since that window is protected by the overhang above.

    But I'm still not a fan of picture-frame trim. It's always better to have a real window sill that is proud of the siding.

    One possible way to handle the dreaded picture-frame trim problem is to install a series of vertical kerfs on the back side of the sill trim, to provide paths allowing the water that dribbles down the window to drain.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |