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Musings of an Energy Nerd

Best Construction Details for Deep-Energy Retrofits

A group of energy-efficiency experts get together to share techniques for whittling home energy use to the bone

The devil is in the details. Any idiot can screw thick foam insulation to a wall, but only an experienced builder is capable of getting the details right — so that the wall sheds rain, dries quickly, avoids vapor transmission problems, and reduces heat flow.
Image Credit: Building Science Corportation

A collection of experts working on deep-energy retrofits recently attended a brainstorming session to share design tips and propose topics for further research.

The conference, formally titled the “Expert Meeting for Details for Deep Energy Retrofits,” was held in Boston on March 12. The meeting was funded by the Department of Energy’s Building America program and hosted by the Building Science Corporation.

Several experts — including two principals of the Building Science Corporation, John Straube and Joe Lstiburek — gave presentations. Straube discussed retrofit options for walls; Lstiburek covered roofs; and their colleague Kohta Ueno discussed basements. Paul Eldrenkamp, a remodeler from Newton, Mass., shared his experience with several deep-energy retrofit projects, while energy consultant Marc Rosenbaum shared questions designed to stimulate new approaches to reducing residential energy use.

I’ve mined the published report of the proceedings for the following tips and pithy quotes.

If you’re adding rigid foam to your wall, put it on the exterior

Dr. Straube noted the advantages of exterior over interior wall foam. Thick walls without exterior foam usually have cold OSB sheathing. That’s bad: cold OSB is a potential condensing surface — and damp OSB can rot fast. (By encouraging drying, a ventilated air gap between the OSB and the siding can go a long ways toward reducing the risks associated with cold OSB. But adding exterior foam is the best way to eliminate the risk of condensation.)

Among Dr. Straube’s points:

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  1. Garth Sproule | | #1

    Thanks for another great blog entry. Can you please explain the detail in the photo labeled "cladding vent between furring strips at window head"? What is its purpose and what is it made of?

  2. GBA Editor
    Martin Holladay | | #2

    Screening material between furring strips
    The purpose of the material is to provide screening to limit the entry of vermin and large critters into the rainscreen gap. A lot of builders cut up Cobra Vent (ridge vent material) to insert between the furring strips at the bottom of each rainscreen gap.

  3. Daniel Ernst | | #3

    More Information Needed
    Martin - Thanks for the article and link. As I was reading through the BSC summary I found one of Marc Rosenbaum's questions interesting:

    "Do we need more or less cooling with superinsulation?"

    Based on existing "superinsulated" projects in mixed-humid climates, we know that in proportion to the heating load, the cooling load increases.

    But does the cooling load itself increase?

    Interesting questions and comments!

  4. Jesse Thompson | | #4

    The worst wall
    One of the moments from that day that sunk in for me was a conversation about the worst thing to do with exterior retrofits in a cold climate: install a thin sheet of exterior foam instead of a thick one.

    Attaching a layer of 3/8" fan-fold sheet foam (for example, common practice in New England these days) underneath new siding adds a moisture impermeable layer to a house that had drying capacity before, but doesn't keep the existing sheathing warm enough to keep condensation from being a problem. 2" foam in a cold climate is much less risky than 3/4" foam or thinner.

    Go big or leave the wall alone...

    Jesse Thompson
    Kaplan Thompson Architects

  5. John Brooks | | #5

    Concerning the detail above
    I still do not understand why you guys always show the metal flashing on the face of the furring strips?
    Would it not be better to drain the cavity that includes the furring strips?
    And if you really must do it that way (as in the drawing)should you not also show weep holes?

  6. GBA Editor
    Martin Holladay | | #6

    Uninsulated slabs reduce the cooling load in hot climates
    To the best of my knowledge, a hot-climate house with an uninsulated slab will have a lower cooling load than a hot-climate house with an insulated slab.

    Trade offs, trade offs ....

  7. GBA Editor
    Martin Holladay | | #7

    Metal flashing question
    Good question. The detail comes from Building Science Corp., not GBA. I'll pass your question along to the powers that be and try to get an answer.

  8. John Brooks | | #8

    Hot climate house
    my thinking is that a hot climate house would benefit from slab edge and slab perimeter insulation...but not interior slab insulation
    I think the superinsulated slabs up north may get the unintended consequence of a cooling expense that may have not existed without the insulation.

  9. John Brooks | | #9

    GBA details also
    I believe the GBA details show the same situation.
    I have brought this question up to GBA and BSC before and they do not seem to recognize it as a problem.
    Is it just me?

  10. GBA Editor
    Martin Holladay | | #10

    Metal flashing answer
    I just got off the phone with Betsy Pettit from Building Science Corp. She points out that the metal Z-flashing has only one purpose: to protect the window head trim board. You'll notice that it's too high to be used to flash the bottom of the rainscreen gap.

    Although the illustration doesn't show it, the window is set in a frame that is wrapped in peel-and-stick. The peel-and-stick acts as the flashing which prevents any water in the rainscreen gap from reaching the window.

    One final point: very little liquid water ends up trickling down the rainscreen gap. While water does get behind the siding, it usually just dampens the back of the siding. The gap speeds evoporation; it is rare for liquid water to run down the gap.

  11. Stephen Thwaites | | #11

    Head Flashing with Strapping
    While i might agree w/ BSC on the answer to the Head Flashing Question for this particular detail, in general i would agree with John Brooks - a head flashing should always start behind the strapping.
    While it may be that " it is rare for liquid water to run down the gap", rare is still more frquesnt than never.
    I"ve seen too much heartache from shallow or missing head flashings to worry that in some particular case (like the one shown above) that a 'full depth' head flashing might be redundant. (Note that in my case heartache includes the emotional scars that accompany the erroneous blaming of the window maker)

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  12. John Brooks | | #12

    Drainage Plane
    Which surface would you call the "drainage plane" in the example drawing?
    here is a quote from Building Science-dot-com:

    "Drainage Plane
    Drainage planes are water repellent materials (building paper, housewrap, foam insulation, etc.) which are designed and constructed to drain water. They are interconnected with flashings, window and door openings, and other penetrations of the building enclosure to provide drainage of water to the exterior of the building. The materials that form the drainage plane overlap each other shingle fashion or are sealed so that water flow is downward and outward."

    I assumed the drainage plane was the face of the polyiso.
    Are you saying that it is the back side of the siding?
    Why have a drainage plane if draining it to the outside is not important?

  13. John Brooks | | #13

    yes, The window seems protected
    I see the labeled drainage plane.(the membrane)
    I can see how the window is protected...
    It is the head trim extension box that looks to me like it could trap or puddle water..

  14. GBA Editor
    Martin Holladay | | #14

    One more explanation
    John and Stephen,
    As I explained before, the detail comes from the Building Science Corporation, not GBA. The flashing at the window head is the peel-and-stick flashing, not the metal Z-flashing. The peel-and-stick flashing does indeed extend behind the strapping, so it meets Stephen's requirement.

    I have added another illustration from BSC in hopes that it helps clarify the detail.

  15. homedesign | | #15

    Not enough Detail
    If you were given the 2 illustrations above could you construct or duplicate the assembly?
    Is the extension box head trim meant to be "sacrificial"?
    Is there some kind of peel and stick flashing that extends over the extension box head trim?
    Are there weep holes?
    How does air flow into the vented space above the extension box head trim?
    How does water flow "downward and outward"?

    I am not trying to be a gadfly.
    I am only trying to understand.
    BSC & GBA are my heroes

  16. GBA Editor
    Martin Holladay | | #16

    I'll see if Betsy or Joe can come online and post an answer
    I didn't draw or build this detail, but here's my understanding: the projecting 3/4-in. plywood box is flashed back to the housewrap with peel-and-stick flashing.

    Clearly, the exterior window casing is not intended to be "sacrificial." The metal Z-flashing is specifically provided to keep water off the head trim and thereby extend its life.

    I'm not sure about air entry into the rainscreen gap. But personally, I don't believe that ventilation is always necessary (although ventilation has been shown to speed drying). Even unventilated rainscreens provide a tremendous benefit by encouraging drying.

  17. Dan Kolbert | | #17

    I agree w/ Martin's interpretation. And also that not enough water gets in there to worry about it.

  18. GBA Editor
    Martin Holladay | | #18

    Response from Joe Lstiburek
    [Dr. Lstiburek just e-mailed me; here is his response]

    Good questions/points/issues. There should have been more detail on the detail. We use PVC trim (such as AZEK) for our box head trim. We expect the horizontal piece to get wet and not care. As such there is no peel-and-stick flashing or rigid flashing. A gap is left at both the front and back of the box head trim for drainage. The function of the head flashing shown on the detail is to protect the head trim not to drain the cavity.

    If PVC trim is not used an additional head flashing has to be added to the detail to protect the horizontal piece. This additional head flashing would run behind the rigid insulation and drain behind the vertical head trim — extending over the top of the vertical jamb trim sections on both sides of the opening. There would now be two flashings — an upper one (currently shown) and a lower one that I have just described.

    In terms of your second point: siding is not airtight — even without ventilation gaps, air change is quite significant — several air changes per hour in the ventilation cavity.
    Joseph Lstiburek

  19. homedesign | | #19

    not enough water to worry about?
    It seems to me that the zone around our window and door openings is where we should worry the most.
    Why worry about it in the field of the wall if we are not worried around the windows?
    In the old days the details included weep holes....
    I am not seeing that in these details.
    the weep holes may be there... If they are they are not indicated.

  20. homedesign | | #20

    thanks joe
    my last post was before I saw your post.

  21. GBA Editor
    Martin Holladay | | #21

    Two different answers
    Betsy said the key is to use peel-and-stick flashing to protect the projecting window box.

    Joe said the key is to use Azek for the exterior jamb extensions and head extension.

    Two different answers; take your pick.

    There are many ways to do it. If you have the time and money, you can install a custom-made flashing of soldered 16-oz. copper for each window head, complete with end dams (as long as you can figure out a way to make end dams that don't interfere with the siding installation...).

  22. EJ Palma | | #22

    Excellent entry Martin as
    Excellent entry Martin as usual. Great questions from everyone. This is a very informative blog entry from all sides. I concur with John and Steven in regards to head flashing and weep holes. Dr. Lstibureks explanation provides a better understanding of the 2 piece counter flashing detail needed to overcome the problem. Is that drawing detail in the architectural details section? Peel and stick flashing is a great invention but I still like the idea of a custom formed 2 piece hard flashing system for the head trim in order to provide durability and protection. On another note when has Azek Trim become a sustainable product. It is made of virgin PVC which doesn't seem to fit the cradle to cradle approach. Besides the pollution in manufacturing, the toxic dust and residue from milling on the job presents a great problem in the disposal aspect. Does Azek use any recycled material in their product lines? GBA is a great tool for all of us that subscribe. Thanks to all that participate and present their questions and opinions. Edward J. Palma

  23. homedesign | | #23

    An Order of Magnitude?
    Ok, I know ... I take this stuff way too seriously
    The Devil IS in the details and Especially at The Door and Window Head
    I am still scratching my head ;-)

    As I take out my imaginary high pressure testing aparatus (water hose)
    I try to imagine how much liquid water is getting to each of the multiple drainage planes.

    After the outer face of the siding...
    Is not the "furring/strapping cavity" the space that will see the most liquid water?
    If we could only drain this cavity first... how much water will really get to the 4th drainage surface?
    Since I can only guess .. I will guess
    10% or less(of what Martin describes as very little liquid water)?
    An order of magnitude?
    See image 2 on this blog (a new construction detail .. in progress)

    What if we want to replace our trim and or windows in 50 more years?
    Will our details allow?

    From the Martin/Betsy Post above

    ".....head flashing.....You'll notice that it's too high to be used to flash the bottom of the rainscreen gap."

    Why not move the "bottom of the rainsreen gap" up 4 inches and drain it quickly to the outside?

    As Joe points out the siding comes default with a horizontal ventilation network that becomes a 3 dimensional network as it overlaps the vertical ventilation gap.
    Perhaps the insects will not notice ;-)

    Am I the only one who finds these things difficult to visualize?

  24. GBA Editor
    Martin Holladay | | #24

    John, no one's arguing with you
    Go right ahead — move the "bottom of the rainscreen gap" flashing up 4 inches and drain it to the outside. Sounds good to me.

    There are a lot of ways to flash this detail. I never advocated either Betsy's detail or Joe's detail as the best way. You're thinking like a raindrop. That's good.

  25. Tristan Roberts | | #25

    Larsen truss and condensation worries
    Lots of discussion here about flashing details, but I have a question about John Straube's note that in a Larsen truss assembly the OSB sheathing could see condensation and subsequent rot. He advocates rigid foam on the exterior of the sheathing.

    I like the low embodied energy and global warming potential of cellulose insulation, as well as its low toxicity and ability to biodegrade eventually. I would like to use dense-pack cellulose in a 12-inch thick Larsen truss assembly for new construction. I was going to sheath the outside with either plywood or OSB and have been debating about whether, with good overhangs and in a climate with 30–40 inches of rain I can do without a rainscreen (I'm also concerned about fire protection).

    I want to avoid the extra step of exterior foam insulation, and just use cellulose for the reasons already given. Martin, or anyone, do you have thoughts on how concerned I should be about condensation on the sheathing, and how to mitigate it? Thanks in advance!

  26. GBA Editor
    Martin Holladay | | #26

    John Straube is a smart guy
    I think Straube is right that cold OSB is risky. It tends to get wet and stay wet. Wet OSB rots. Why risk it?

    Your wall will be much more robust if you:

    1. Sheathe it with plywood, not OSB.

    2. Include a ventilated rainscreen gap to speed drying. (At the end of the winter, the plywood and the cellulose behind it will tend to be somewhat damp — so anything you can do to encourage drying is good.)

  27. Tristan Roberts | | #27

    agreed on all points
    I agree with you on items #1 and #2, plus John is a very smart guy! So are you—thanks for another great post and your Q&A engagement. I'm reassured that I don't have to throw out the Larsen truss approach entirely.

  28. jpaige | | #28

    rigid insulation over osb/cedar shake siding
    I am working on a project that has cedar shake siding. I would like to apply rigid foam over osb sheathing. any suggestions! do not want to strap every 7"

  29. Richard Ugarte | | #29

    Code question
    My county abides by the 2006 International Residential Code (2006 IRC). I read in the "Building Energy Codes Resource Center" that, in this code, if I am to have an unvented attic, I am required to have air-impermeable insulation in direct contact with the underside of the roof deck. Is this wrong, or is what is being said here against the 2006 code?:

    Lstiburek’s response: “No. For example, with insulation exterior to the roof deck, virtually any insulation works underneath the deck (that is, insulation below the roof deck need not be air impermeable or offer significant vapor diffusion control in this configuration)." (That's right — even fiberglass batts can be used, as long as you have enough rigid foam on top of the roof sheathing.)

  30. GBA Editor
    Martin Holladay | | #30

    Cedar shingles over foam
    J Paige,
    As far as I can tell, you have two choices when installing cedar shingles over rigid foam wall sheathing:

    1. Install horizontal strapping every 5" to 7" (this is what I did when I built my own house).

    2. Install plywood or OSB on top of the foam (this is a method Jesse Thompson endorses; read more here: ).

  31. GBA Editor
    Martin Holladay | | #31

    2006 IRC requirements for unvented attic assemblies
    You should read the code again. You are referring to section R806.4. The section includes an exception that covers insulation exterior to the roof deck. Here is the language: "Unvented conditioned attic assemblies ... are permitted under the following conditions:... 4. In Zones 3 through 8 as defined in Section N1101.2, sufficient insulation is installed to maintain the monthly average temperature of the condensing surface above 45°F. The condensing surface is defined as either the structural roof deck or the interior surface of an air-impermeable insulation applied in direct contact with the underside/interior of the structural roof deck."

    I'll admit that the code could have been written better. (What else is new?) But what the code is saying is, "If you install enough rigid foam insulation on top of the roof deck to keep the roof sheathing warm enough to avoid condensation, you're good to go."

  32. Richard | | #32

    2006 IRC Code

    Thanks. That clears it up nicely.

  33. Ronald Sauve | | #33

    Cedar Shingles Over Foam
    Why not use a product like Homeslicker by Benjamin Obdyke? I've used this product quite successfully. It provides the necessary airspace behind the shingles. This will prevent cupping of the shingles, and also gives you a true rainscreen which the horizontal strapping will not. The strapping provides the air space but falls short in terms of the rainscreen.

  34. GBA Editor
    Martin Holladay | | #34

    Homeslicker over foam?
    Personally, I wouldn't do Homeslicker over foam sheathing — too squishy, and the installation requires very long nails. And if you're using 4 or 6 inches of foam, I can't imagine what kind of nail you could use.

  35. Michael T Heffron | | #35

    How much foam is enough?
    First of all this is a great blog. There have been all sorts of questions addresssed that I have been wondering about. My questions stems from Jesse Thompsons comment above about how too little foam on the exterior of the wall can actually make things worse. Does any one know of an easy way to figure this out? I live and work in the Seattle area so zone 4 marine climate and was thinking of using 1/2" polyiso on the exterior of a 2x6 wall but now am having second thoughts. I tried running a test on the WUFI program but am having a hard time understanding the output. Thanks for any feed back.

  36. GBA Editor
    Martin Holladay | | #36

    Foam sheathing thickness
    The minimum foam sheathing thickness necessary to keep wall sheathing above the dew point is specified in the 2007 Supplement to the International Residential Code in Table N1102.5.1.

    In Seattle, you are in Climate Zone 4 C (marine). For a 2x6 wall, your climate requires a minimum foam R-value of R-3.75. I would rate 1/2 inch of polyiso at only R-3.3, so you are cutting it too close. Why not spring for 3/4 inch or 1 inch polyiso?

  37. Matt Berges | | #37

    Retrofit window flashing detail
    The detail provided above is very close to what I am doing, but I see one potential flaw.
    Your drainage plane leads to a water trap at the head trim/extension jambs: "Head of Trim Extension Box".
    Instead, I have decided to run this head flashing behind my furring strips, and behind a second layer of tyvek ThermaWrap (my foam is not foil faced).
    And while all seams are overlapped/staggered (on my iso board), I also run the 4x8 foam board on the tall, to minimize horizontal tape joints that are exposed to drainage plane. In other words, behind my outer layer of Tyvek, there are horizontal seams every 8', rather than every 4', and all of the vertical seams are covered and made tight by the furring strps. Would you agree?

  38. GBA Editor
    Martin Holladay | | #38

    Response to Matt
    Your details sound fine, although I'm not a fan of Tyvek ThermaWrap. The marketing materials for the housewrap imply that it improves the thermal performance of the wall, but any improvement is very slight — on the order of R-1 or less. Of course, even that R-1 improvement will only occur if the ThermaWrap is facing an air space.

  39. Dayton | | #39

    Can you explain this to me?

    Can you explain this to me?
    From the published report of the proceedings "Sealing between a basement slab and foundation wall has been demonstrated to have a noticeable effect on overall building air leakage. Below grade air barriers are important"
    Does this refer to sealing cracks at slab wall interface? At what depth of basement slab is this true?

  40. GBA Editor
    Martin Holladay | | #40

    Response to David
    1. Yes, I think the sentence is referring to the need to seal the crack around the perimeter of a basement slab.

    2. Even at normal basement slab depths -- about 7 feet below grade -- it's important for a basement slab to be airtight. Here's why: the stack effect pressurizes the ceiling at the top of a house, and depressurizes the basement. The soil around the house is like a chocolate cake, with connected pores. As air is drawn into the basement through the slab perimeter crack, the air is replaced by air from an adjacent bit of soil -- all the way up to grade.

    3. Of course, this phenomenon is lessened by deep frost and in soils with a high clay content. But it has been measured in many locations.

    4. Because of this phenomenon, it's important to seal the crack. If the basement has a sump, it's also important that the sump has an airtight lid.

    5. There's a side benefit to this crack-sealing effort: less radon.

  41. gred | | #41

    I suppose the best fasteners for a 4" foam retrofit would be 6" big head screws (or 7 if you're going through the rain screen, too). The drawing says "minimal fasteners". So, the lack of shear in the foam to resist gravity on the whole outer assembly (including windows?) makes me wonder about this detail. What do you think will give the best long-term stability?

  42. GBA Editor
    Martin Holladay | | #42

    Response to Gred
    Check out Thorsten Chlupp's article: "Installing Exterior Insulation in Cold Climates."

    Chlupp wrote, "We provide nailing for siding by installing 1x4 furring or strips of 3/4-inch plywood over the foam, fastening through to the studs with long screws. We space the screws about 12 inches on-center and make sure we penetrate at least 11/2 inches into the framing. These long fasteners have to be mail-ordered; we use Wind-Lock W-SIP screws (800/872-5625, and FastenMaster HeadLok and OlyLog screws (800/518-3569,"

  43. Lesley | | #43

    Deck ledger detail
    Thanks for the excellent information in the post and the follow-up discussion! Any advice on how to attach a deck ledger when using 4" of rigid foam on the exterior of a sheathed 2"x4" wall? I have seen one design (CCHRC Remote) that spaces blocks along the rim joist, but in a retrofit situation this may be difficult to shore up sufficiently. Would a continuous built-up ledger be preferable? If so, where should the face of the ledger be in relation to the drainage plane created by the foam?

  44. GBA Editor
    Martin Holladay | | #44

    Deck ledgers over foam
    While it is always possible to attach a deck ledger to an existing building -- even one sheathed with 4 inches of foam -- it isn't always easy. I'm a fan of independent footings and piers for decks -- that's a much better solution than an attached ledger.

  45. Lesley | | #45

    Deck ledger detail
    If I had the space and budget to build a free-standing deck, I definitely would. But unfortunately I don't have either, so I have to work out the details of reattaching the existing deck. Can you point me to any resources that show how to do this safely and securely? The thermal envelope of the house would be severely compromised if the rim joist tore off (!).

  46. GBA Editor
    Martin Holladay | | #46

    Response to Lesley
    You can always hire an engineer to draw up a detail for you. I'm not an engineer.

    Here's one way to do it: remove all of the foam back to the OSB, plywood, or rim joist. Install solid pressure-treated blocking and then your pressure-treated deck ledger. Install carriage bolts (not lags) as required by code. Flash the ledger.

    Once you've tightened the nuts on your carriage bolts from the interior side of your rim joist -- and once your local building official has inspected and approved your attachment method -- use closed-cell spray foam to insulate the rim joist from the interior. The spray foam will mitigate the thermal bridge you've created by removing the exterior foam from the rim joist area.

  47. Lesley | | #47

    Deck ledger detail
    Thanks for your suggestions, Martin. My engineer is great with the load calcs but I'm looking for input on how deep the ledger should be relative to the face of the foam to enable proper flashing. The contractor suggested bringing the front of the ledger flush with the face of the foam. With that configuration, we could apply membrane flashing vertically across the beam and foam to keep the drainage plain continuous. Would this work?

    Perhaps I should throw this question out on your Home Page Q&A to see what other folks have done?

  48. adkjac | | #48

    Condensation and green... sustainability confusion oh my
    Great discussion... Question... I see lots of rot concerns to do with no use of foam. The only huge rot I have found and replaced was with the use of exterior foam 1.5' foil faced over plywood. The plywood and studs were gone totally... only the showing materials looked OK and were OK.

    And open cell is supposed to have issues. I just did an open cell job with no venting, sprayed to roofdeck with trusses, 5" plus icynene with vapor paint sprayed on foam, no drywall over it just drywall ceilings for fire code approval. I find no problems with moisture,.

    I also have never found issues with cellulose.

    I am planning on using cells to get to R 40-60 in future envelopes. I like the idea of a real light frame double wall or I joist wall. I have asked the makers of I joist about the OSB in I joists and they have told me that they are made more like Advantech in durability. Not sure I trust that (documentation?)... but... cells are supposed to keep the moisture off the wood not push it to the wood like glass.

    Anyway.... may stay away from all manufactured woods someday in the future and go with locally milled wood to get to the idea of local is a more sustainable better for our local community of workers and better for the planet ... may have to go back to diagonal board underlayment... why not.

    The one area I see that really has changed for the better is to stop the moisture coming into homes from basements and concrete.

    So how do I not use poly under a slab? Is anyone making forever lasting soy based plastic for under say the new concrete that is not made from portland? Do we just go to borate treated wood instead of concrete?

    Lots of ideas... dreams... but am getting there... net zero... and no taxes and all green... and.... gardens full of food and some flowers for fun.

    Not to keep going all over the place... but... with this last home and the no vent roof... I really felt like it worked amazingly well... I worked in the home through a winter and noticed the furnace just didn't run during the day and this is in Northern NY. Every home around here in the winter shows heat going through the roof via the snow melt patterns... Old homes are wicked bad.... every cathedral ceiling even in any new construction melts terribly with glass and venting. But not with the open cell spray no vent roof.... the roof looks like an unheated camp roof.... the snow is all there and there just isn't any patterns of melt to see. And no icicles.... no ice dams.

    just some thoughts... wish this blog wasn't confusing what I thought was starting to be some good strategies to stay with and head toward.


  49. Melissa Arminio | | #49

    Polyisocyanurate Insulation
    It is my understanding that you never want to use foil faced polyisocyanurate in an application that is exposed to moisture. It is great to use under a roofing membrane, but not in a rainscreen cavity or under a slab on grade. It has a very high R-value per inch, but when it is exposed to moisture the R-value drops considerably.

    We tend to use a mineral fiber cavity insulation whose R-value does not decrease with exposure to moisture and, among many other benefits, does not pose a fire hazard like many other rigid insulations.


  50. GBA Editor
    Martin Holladay | | #50

    Reply to Melissa
    Polyiso should not be used in contact with the soil or below grade on the exterior of a foundation.

    However, polyiso is routinely used on the exterior of above-grade walls as part of a rainscreen application. It works very well there. Not much water gets past the siding -- and any water that does get past the siding should drain quickly and dry quickly if the rainscreen is properly designed and installed.

  51. Rob Dickinson | | #51

    OSB or plywood inside of a thick exterior foam wall
    I've seen a number of comments both in this thread and in a number of other GBA forums suggesting that OSB is a risky material to use inside of a superinsulated wall assembly, largely due to its vulnerability to moisture.

    I understand that the most significant case for this vulnerability is when too little foam is used and the OSB is still cold, leading to condensation risk. But my question is more on the general case, especially when all of the other details are done right or at least to a high level of theoretical "safeness".

    My assembly will be sheathing (OSB or plywood or Zip panels) over existing 1" lap board siding. Sheathing taped and sealed. Drainwrap over the sheathing. 4" of polyiso in two courses of 2" foil-faced boards with staggered seams. Seams taped. 1"x4" vertical furring strips holding the foam down with fiber-cement lap siding attached to the furring strips.

    Climate is Eugene, OR, with a relatively mild (but wet!) climate and roughly 4500 heating degree days.

    Any improvements you want to suggest to this assembly?

    And given this should be a pretty "safe" assembly, would OSB be fine? Or would plywood be the best sheathing to use in any case where ultimate durability is the goal.

    The house was built in 1900 and is still sound and rot-free after 110 years. I would appreciate any advice for ensuring that my retrofit helps it to be equally sound in another 110+ years.

  52. GBA Editor
    Martin Holladay | | #52

    Response to Rob Dickinson
    Your suggested assembly is relatively safe. The OSB is on the warm side of the insulation, which is good.

    However, a few building scientists, notably Bill Rose, have pointed out that we don't really know what OSB is going to look like in 50 years. We do know that plywood holds up very well.

    The incremental cost of plywood over OSB is relatively small. For my own home, I would choose plywood. But if you want to install OSB, your method is the best way to proceed.

  53. jambo13 | | #53

    2" Rigid
    Thank you for providing this resource.

    Would the window installation and resulting flashing system change if you were to only have 2" of rigid on the exterior of the house instead of 4"? Would you ever install the window flange on the outside with the window flange to the outside of the rigid insulation?

  54. GBA Editor
    Martin Holladay | | #54

    Response to James Magner
    Of course flashing details are highly dependent on foam thickness.

    If you are installing a flanged window over exterior foam, it's possible to screw the flange through the foam to the underlying wood components as long as your foam is 1.5 inch thick or less. If your foam is 2 inches thick or more, you'll need to install a "picture frame" of strapping material on top of the foam, around your window opening, and attach the flanges to the picture frame, or else you'll need to buy windows with metal strapping designed for use in masonry walls. This metal strapping provides a structural attachment of the window to the house framing. (Another possibility: install a sturdy window buck of framing lumber that projects outward, proud of the wall framing.)

    Flashing details are difficult to convey with text alone. It's certainly possible to successfully flash and "outie" window in a foam sheathed wall. For more information, see ‘Innie’ Windows or ‘Outie’ Windows?

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