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Incrementally Increasing Insulation in PNW Walls

MALCOLM TAYLOR | Posted in Energy Efficiency and Durability on

I am trying to develop a method of increasing the insulation in walls built in the PNW which does not interfere with the normative sequence or techniques of building used in the area. So to be clear this is a “pretty good” approach which doesn’t require special trades or materials and can be used in typical houses without significantly increasing the cost, not boutique projects.
For better or worse the material of choice here is FG batts used in 2″x6″ walls yielding a nominal R20. Can anyone suggest a wall assembly using those existing materials, that is wood and batt insulation, that would increase the walls efficiency, or comment on the results of using double walls, or other framing variants usually associated with blown in insulation?

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  1. albertrooks | | #1


    I think you'd be better starting off fresh. Take a look at some of the work by local builders like Martha Rose. She consistently builds a "better wall" while remaining "developer" competitive.

    The first increment is to dump the batts and air seal below 3ach50. Then add a layer of exterior insulation. Foam is the cheapest material to buy but keep looking past it at mineral wool and cork. If you want an effective and thermal bridge free R40 in a single frame, try framing with 2x8's with 2" Agepan THD woodfibre insulation board exterior with an OSB or plywood at the interior as the air and vapor control layer. Place an interior service cavity on that.

    If your trying to improve your assembly, drop the dead weight of batts, thermal bridging and air infiltration as soon as you can.

  2. Expert Member

    Albert, plenty of people are starting off fresh and good luck to them. Hopefully from their work a series of better assemblies will emerge and be integrated into mainstream building.
    That ignores where I and the huge majority of designers and builders responsible for 99.9% of residential construction are now. We are part of a well established industry and economic system with methods of construction and financing that preclude large scale innovation. If I want to successfully make incremental changes in the way I design and build I have to respect those realities, and as I said in my first post that precludes new materials or methods outside of contractor's experience. I really don't see how your suggestions respond to my situation.

  3. albertrooks | | #3


    I guess I don't fully understand what your suggesting. I thought you were trying for an improvement. You wrote: "That ignores where I and the huge majority of designers and builders responsible for 99.9% of residential construction are now. We are part of a well established industry and economic system with methods of construction and financing that preclude large scale innovation."

    My suggestion was 1, better quality insulation, 2, better air sealing, 3, adding exterior insulation. That's pretty consistent with the Northwest Energy Star program. I see that as very mainstream incremental step. Take a look at it from Earth Advantage. It's a well thought out and proven program:

    Since your original,post asked about double frame experience, I quoted our diffusion open R40 wall.

    The 99.9% are not 99.9% code minimum anymore. There are plenty of the mainstream builders and developers doing better. In Seattle look at Dwell Development and then, as I suggested Marth Rose: I know that to you, I'm happily in the fringes and reside in "boutique land", but these folks are razor sharp business people producing a product that competes in the market and returns a profit.

    Best of luck.

  4. Expert Member

    Let me put my (common) situation in real terms. My business has two parts. I design custom homes and I build speculative housing.
    The clients that come to me for designs do so because they appreciate the way my work fits the site and its west Coast vernacular style. I am not practicing in the niche market catering to those interested in Green homes so my experience is that when I suggest spending on either enhanced seismic or insulation it is balanced against other features they would like. If I am going to make a case for a better envelope it has to be meet the criteria I have outlined.
    My spec. building is sold in direct competition with other builders, and the market is very price sensitive. To be blunt no one particularly cares how well their walls perform, so If I increase the efficiency of the house it is being done for reasons other than market demand. In other words there is a good chance any additional costs will come out of my own pocket.
    These are the dilemmas facing the vast majority of builders. Perhaps this helps seeing why my question was posed the way it is.
    Edit: All power to Martha Rose and her ilk, but it is one thing to build that way in hipster havens like Seattle and quite another in logging communities on Vancouver Island.

  5. user-757117 | | #5

    If I understand your situation correctly...

    As a first increment, I would focus on improving the "air-tightness" of the enclosures - ie, the "red line" should be continuous all the way around the enclosure, from foundation to wall to ceiling.
    Further attention may be required to develop details for various wall penetrations and duct layout...
    As well as some effort to steer clients away from things like recessed ceiling cans.
    There are a number of approaches you could take towards improving air tightness...
    But a conventional 2x6 wall with FG batts will benefit from increased "air-tightness" in terms of both energy efficiency and durabilty.

    As a second increment, I would work at ensuring quality installation of the FG batts.

  6. Expert Member

    Thanks Lucas. Fortunately the BC code and inspectors do a very good job of insuring air sealing, and because it has been the dominant type of insulation here for so long batt insulation installers do very good work. I have had inspections fail for not having two wires through interior top plates sealed.
    The first incremental improvement here occurred in the early '90s when practice changed from 2"x4"@16" to 2'x6"@24" wall construction and sealed vapour barriers were introduced. That is the sort of shift I am talking about. It meant very little changed in the building process as the costs and new techniques were minimal, but the effect on wall performance was profound. I was simply wondering if anyone here had thoughts on what might constitute a similar shift? Considering the effect that change had on virtually the entire housing production of BC, it seems logical that any small change that is easily adopted will have a much more profound results than perfecting the "high performance" house so commonly discussed here.

  7. homedesign | | #7

    Malcolm, are you doing blower door testing?
    If so ...what is a typical ACH-50?

  8. Expert Member

    No John, no blower door tests. Nor do I see doing them in the foreseeable future. I'm sorry, it's probably my fault but I don't seem to be able to get my point across in this discussion.

    I am designing and building fairly well sealed and insulated houses within the parameters of the profession in my region. I am interested in what minor changes would improve on this standard of construction as I believe that this approach makes more sense than building one off, un-affordable "high performance" houses on green-field sites.

    After seeing he enthusiasm with which posters offered advice on sealing the last few minor penetrations in wildly over-insulated Passivehouses, I thought I might get some common sense thoughts on the merits of perhaps a double wall or interior strapping, and some discussion around the real world economies of such changes, which seems to be entirely absent in the projects commonly brought up here. As I said, the mistake was probably mine. I'm barking up the wrong tree.

  9. GBA Editor
    Martin Holladay | | #9

    It's possible that you already have a firm grasp of the economic realities you face, and that you are already building walls that are about as good as your market will support. If that's the case, then there isn't any advice anyone can provide that will prove useful.

    Here's my answer to your question: to improve the performance of your walls, reduce the air leakage rate and/or address thermal bridging. To help pay for the cost of these improvements, market your homes in a way that highlights the quality improvements that differentiate your homes from others in your market.

    Only you know where the air leaks are on the walls you build, so only you know how to eliminate them. One way to find them is with a blower door -- but if that's not going to happen, you have to find them a different way. You might want to use a fog machine if you don't want to use a blower door. (Here's more information on this approach: Pinpointing Leaks With a Fog Machine.)

    When it comes to addressing thermal bridging, adding a layer of exterior foam is probably the least expensive way to go.

  10. user-626934 | | #10

    "The first incremental improvement here occurred in the early '90s when practice changed from 2"x4"@16" to 2'x6"@24" wall construction and sealed vapour barriers were introduced....the effect on wall performance was profound."

    Yes, indeed. The effect of sealed vapor barriers WAS profound in your climate.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    John, I attended the CMHC seminars that discussed the Leaky Condo Crisis, read their report and the Building Science extract you have posted. How from any of those do you get a causal link between poly vbs and the damage that was overwhelmingly the result of exterior moisture penetrating the walls? Building Science does recommend an air tight drywall approach to air sealing as an alternative that will allow drying to the interior, but I've never heard anyone involved suggest that would have been sufficient to avert the damage we have seen.

  12. Expert Member
    Dana Dorsett | | #12

    Designing wall lengths and window/door placements to take advantage of standardized matererial legnth/widths can reduce framing fractions from ~20-25% of the wall area to something like 15%, at reduced labor cost (fewer framing elements, and fewer cuts, less batt-trimming for oddball widths,etc) and using optimum value engineering framed corner techniques, single rather than doubled studwall headers, are a good start. It all adds up to an easier to build wall with lower material & labor costs, with lower thermal bridging and more reliable insulation installation.

    The net result isn't a huge performance boost, only incremental.

    Double-studwalls or staggered studs on wider plates aren't a huge stretch for most framing contractors to pull off but it's a cost adder. The net boost in wall performance putting staggered-stud 2x4s on 2x8 plates using standard R13-R15 batts for both the exterior and interior sets of studs is pretty good (~R19 after thermal bridging) which is about a 35% improvement in whole-wall R over standard (non- OVE/advanced framing) 24" o.c 2x6 / R20 construction that comes in at about R14 whole-wall after thermal bridging.

    But that same ~35% improvement in thermal performance can be achieved with 1" of foil-faced rigid polyiso or XPS on the exterior of the sheathing (seams staggered with those of the structural sheathing), at which point you can (and should) skip the interior vapor retarders to avoid the issues seen in John Semmelhack's referenced article.

    Whether the 1-inch foam-over is cheaper than the double-2x4 on wide plates depends on your local material & labor rates. The foam-over is usually cheaper, and from a total drying capacity and robustness point of view it's a better wall than going with doubled up stud and interior poly. Air sealing both the foam and the structural sheathing (either tape or goop, as-appropriate for the materials) make for fairly good overall air-tightness for minimal additional cost.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Thank you Dana. Those are exactly the sort of suggestions I was hoping for - with real numbers attached too! Thanks again.

  14. Mike Steffen | | #14


    My understanding of your request is that you’re seeking upgrades that would have minimal impacts on standard trade practices and thus resulting costs. After a considerable amount of study on numerous projects we found that increasing the performance of the exterior enclosure of buildings actually does not involve insulation levels or airtightness. The first thing to do is to boost the thermal performance of the windows from R-3 (code minimum) to R-4 (or R-5). R-4 window performance can usually be achieved by using IGUs with the proper coatings and argon fill. Given a window to wall ratio of 25% plus/minus you will get more thermal improvement overall from using R-4 windows vs. R-3 windows than you would get from using R-40 walls vs. R-20 walls. This is due to the nature of heat loss, with it being much greater through the low thermal resistance of the glazed window areas as compared to the relatively high thermal resistance of the opaque wall areas.

    Once the windows have been upgraded, then it makes sense to focus on incremental improvements to the opaque wall areas. Advanced framing methods, including “integrated” window opening layout within the walls to minimize framing members, should be the first focus. Airtightness is very important so that should be the next focus and significant improvement can be achieved for relatively low cost. Both of these measures have been noted by others in the thread and I would concur.

    Now to the insulation! The first upgrade to consider is going from FG batts to blown FG. The cost increment is relatively low in our area (Portland, OR); perhaps it’d be the same in BC. You get a little more “official” R-value with blown FG compared to batts but there is a significantly increased likelihood of much higher installed R-value. If, as you claim, the FB batts in BC are installed in a quality manner this might not be something to focus on. Next would be to consider the addition of exterior insulation. As Martin said this is the typical approach to improving R-value, and it does mitigate thermal bridging effects. However there is a potential hidden problem with using some foam insulations as they may inhibit the wall’s drying capacity. When thinner layers of exterior insulation are used on walls, the dew point location will often still occur with the wall framing, during colder months at buildings in colder climates. Consider using mineral wool in lieu of the foam as the mineral wool is vapor permeable and thus allows drying to the exterior side of the wall. Another consideration with adding exterior insulation is the complexity of cladding/flashing details that can be created at windows, doors and other penetrations. This can cause significant issues with the trades (and with costs). A layer of exterior insulation up to 1” in thickness should not create major issues but thicker layers of insulation will. But again, adding only 1” of insulation to the exterior side of your walls can create other unintended moisture performance issues, so the importance of understanding those “issues” cannot be stressed enough.

    One last idea that should be put on the table: the 2x6 exterior wall could be revised to 2x8. This very modest incremental change would boost the whole wall r-value from R-17 to R-22. If you used staggered 2x4s in exterior walls with 2x8 plates (assuming loads would allow this) then the R-value increase would be even greater, while possibly keeping framing costs the same as a 2x6 wall.

    We commissioned several studies on improving the thermal performance of exterior walls, specifically looking at walls for buildings in the Pacific Northwest climate zone (4c Marine). Those can be accessed at the Building Science Corporation website if you have interest:

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Mike, Great analysis. I am in your debt. Most of the houses I'm currently involved in are on the coast of Vancouver Island facing south so invariably a high proportion of the envelope is glazed. While the insulators here do a very good job of installing batts that does preclude using double walls, as to fill voids I imagine a third layer of batts has to be installed between the walls and that might prove prohibitively expensive. Do you think that two 2"x4" walls separated by a 1 1/2" gap with one wall containing 3 1/2" batts and the other 5 1/2" would have too much convective movement due to the gaps at the studs? 2"x8" plates and staggered studs as both you and Dana have suggested may be a better solution. I am very wary of adding more complexity to the flashings as exterior insulation would necessitate. we have just managed to get a very good system of protection through our rain screen detailing and I'm not keen to meddle with it.

  16. heinblod | | #16

    To avoid structural damage, health risks and litigations the job should be done on a sound investigation of the status quo and solid planning.

    The dynamic dew point calculation is important.

    See here:

    and also here:

  17. Mike Steffen | | #17


    If the window to wall ratio on these houses is higher than the norm then increasing the thermal performance of the windows becomes that much more imperative. Of course with lots of south facing glazing, solar control is important as well! You mentioned not wanting to get away from the standard rainscreen detailing because it provides excellent protection. I hear you clearly! Another reason to consider the glazing performance upgrade well before upgrading the walls has to do with durability risks. One can look at it this way: adding r-value to the windows (through upgrade of the glazing materials/properties) does not add hardly any durability risk at all. The only thing that changes typically is the glazing, which is a moisture tolerant enclosure component. However, adding r-value to the walls adds considerable durability risk. Adding r-value reduces heat flow. Adding lots of r-value reduces lots of heat flow. Reducing lots of heat flow reduces drying capacity and thus increases the risk of moisture induced damage to sensitive materials such as wood frame wall components. This is just another take on the issue of whether to focus on the walls first or the windows...

    Regarding your double stud wall idea, I think that could work quite well from a constructability and cost standpoint. You'll still have a good degree of thermal brigding at the stud lines because the batts would not fill the 1 1/2" space between the outer and inner studs (I'm assuming the studs are aligned, not staggered). I have a gut feel the staggered stud wall idea (2x4s staggered on 2x8 plate) with blown FG would be as constructable and cost effective and probably yield slightly higher r-value but if you don't have subs in the area that offer the blown FG operation cost effectively than the doubel stud / batts approach may make the most sense.

    One last note: when insulating only in the wall framing cavities (with no exterior insulation) the thicker the wall, the more the r-value, the higher the durability risk. So both of these walls we'
    ve been talking about are at higher risk of moisture damage than the typical 2x6 R-21 batt insulated wall. Air leakage control becomes much more critical at these walls to ensure no damage from interstitial condensation. And of course it becomes even more critical to ensure that exterior water is robustly managed such that it doesn't get into the inner layers of the wall assembly through the water-resistive barrier system. Cheers.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Mike, Having taken the time to read both of the very interesting studies you commissioned I'm coming to the rather gloomy conclusion Martin and you suggest that I may not be in a position to do more than increase my vigilance in air sealing to improve the efficiency of the walls. This (relatively new) understanding of the effects of increased insulation on the sheathing is a game changer.

    We are currently somewhere around 47 days of straight rain here. The primary emphasis in this climate has to be to maximize the walls resilience to moisture damage, even if it means foregoing some efficiency. I am wary of the long term viability of relying on drying to the interior, as it pre-supposes certain behaviours of the future occupants, like never using wall paper or other impermeable wall surfaces, which don't seem realistic to me. (I also have other reservations about foam which are probably best left to another discussion) .

    Given this I'd be interested in your views on increasing the sheathing's ability to dry to the outside. Many houses here continue to be sheathed in 1"x8"s, and if plywood is used it is gapped at the horizontal joints and in "dead" cavities such as those under windows holes are drilled to provide for greater permeability. This runs counter to the current emphasis on air-sealing both the interior and exterior planes of the wall - and when I suggested it here both Martin and John Straub both dismissed it out of hand. I realize that without using the sheathing for air-sealing you are basically foregoing that function on the exterior as house wrap or building paper don't do it at all well, but it does seem to me that, especially in conjunction with a rain screen, you do significantly improve the walls ability to dry to the outside. Any thoughts?

    I would of course welcome any input from Martin, or any one else on these issues.

  19. GBA Editor
    Martin Holladay | | #19

    It's important for you to separate the economic considerations from the building science considerations.

    There are many ways that you can build walls that are more airtight, and better insulated, than the walls you now build -- without any risk of trapping moisture in walls. (Vinyl wallpaper is relatively rare; most wallpaper is vapor-permeable. But if you want to build a wall that tolerates vinyl wallpaper, you can.)

    There's no question that it's quite possible to build a high-R wall that performs well. But only you can decide whether your customers are willing to pay the upcharge for better performance.

    Since you seem to vividly recall the earlier discussion on whether it makes sense to drill lots of holes in your wall sheathing (in order to encourage more air to flow through your walls), I won't bother to repeat my argument here -- except to say that a building practice that deliberately attempts to make walls more air-leaky is counterproductive. That word -- "counterproductive" -- is a diplomatic way of saying what I am thinking.

  20. albertrooks | | #20


    Your question: "Given this I'd be interested in your views on increasing the sheathing's ability to dry to the outside."

    I'll be less diplomatic: To me purposefully adding airleaks into stud bays with loose fill or dense packed insulation is nuts. A very poor idea that short circuits the point of the insulation by wind washing as windspeeds increase and precisely when it is supposed to perform it's thermal isolation duties.

    Don't fix drying problems by creating holes.

    Add enough exterior insulation to make exterior drying a non-issue.
    Increase the exterior layer permanence while decreasing interior layer permeance and add (permanent) interior air sealing.

    The latter can be done in a few ways:
    1, Diagonal boards as exterior structural sheathing with a good membrane like Siga Majvest or equivalent. Apply battens for a rainscreen which will limit the penetrations through the membrane to those holding up the battens.

    2, Reverse the wall: (plywood/OSB/shear at the interior) and use a high perm board like Agepan DWD or THD. They are made for this application and climate. Both are 18 perms or higher. DenseDeck Gold will also work if your not gypsum adverse for sheathing.

    3, Use Plywood and add just enough mineral wool to keep the dew point off the sheathing and design in a good permanent interior air barrier.

    Lastly. I know it's tough to sell these upgrades when the buyer doesn't "think" past the finishes. Oh well... I guess you have chosen a tough challenge in a tough market.

    All I can say is: get better at making the argument. If you want to see how some are making cost arguments on insulation then check out the APHN Dashboard Contest. These guys are climbing a far steeper hill than you are. Perhaps you can you can get the "theme" of the argument: A little bit of insulation doesn't go very far. More is better.

    At 47 days of rain you must be on the west side of the Island. I don't live in what you call the "hipster havens" of Seattle or Portland. I live in a place that gets more rain than both: Olympia. We still make thick airtight walls that dry. They are not that tough make. It's tougher to sell them and we work on it everyday.

    Get better at it or be happy designing thermal poverty.

  21. Expert Member
    MALCOLM TAYLOR | | #21

    Martin and Albert, I appreciate your comments and am not as fixed in my position as you think. If there is no benefit to drying the cavities through allowing some air movement at the sheathing then I'm not going to do it. My movement to practices that most most here enthusiastically adopt may be more circumscribed that you would like. I happily use fixes for problems I see but I am reluctant to abandon working assemblies for experimental ones. Being late to the game can have the advantage of avoiding the false starts and adopting practices the implications of which are not immediately apparent. While my prudence may be irritating in the general atmosphere of this board, it's not unreasonable to take the position that many of the currently used strategies will be quickly superseded and that many may already be obvious non- starters.

  22. albertrooks | | #22


    Your trying to figure out how to improve performance at the least cost to your clients. No worries, I'm happy that's your direction. All of our "collective clients" are best served when we balance cost and quality.

    You're not far away from the point where your material list of framing, sheathing and FG batts are arranged and layered as efficiently as they possibly can be. To increase resistance (or lower conductivity) will require more insulation be added.

    Thicker envelops are not that new. There continue to be many successful paths.

    It's a good discussion in my mind because, while we know how to make envelops with more insulation than code minimums, If we can't convince the value of these improvements to each other as building professionals, how would we hope to convince the clients?

  23. Expert Member
    MALCOLM TAYLOR | | #23

    Very true Albert. I guess one of the answers to that question is that as an architect, and taking a wider cultural perspective, I don't see improvements to the building envelope as having that significant an impact for a couple of reasons.

    The first is the local context in which I am building. The annual cost for heating a 1200 sf not very tight or well insulated house using baseboard units is around $600 here. Given that, improvements in efficiency have to be justified either by looking to future possible energy price increases or suggesting it's the right thing to do for moral reasons. Both arguments are hard to sustain when, as is the case now, the house I am designing has a three bay garage for a full sized pickup, SUV and 22 ft sport fishing boat. The energy needed to move any one of those to the nearest town dwarfs any possible savings brought about in the building process.

    My feeling is that concentrating technically improving the building envelope, while not addressing the nature and type of houses we are building, may actually be counter-productive as it gives us a false sense that the solution lies in slightly better versions of the houses and communities that have already been built. That the conventional suburban box is fine as long as it has high performance envelope.

    So what am I suggesting to those clients rather than a a large incremental increase in their walls efficiency? That we reduce the size of the house and use those resources to provide several large attached covered areas which make outdoor living in the PNW possible. Also that the shape and fenestration of the house take advantage of its south facing site while allowing for passive cooling through natural ventilation during the summer. Would I prefer to include a better insulated envelope? Sure, that's why I asked for some suggestions.

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