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Community and Q&A

Insulation — propane vs. solar powered — electric heat, etc.

Shane_McCafferty | Posted in General Questions on

We recently bought a unique home and we’re planning a major renovation.  This has raised a lot of questions regarding insulation, energy efficiency, heating choices, solar power, etc.   I was hoping we could get some expert advice on a number of these issues.  It would be greatly appreciated.

Let me start with a description of the property.  We bought an old country schoolhouse in the area of Niagara Falls, Ontario, Canada (see pic attached).   The original 2 storey schoolhouse (approx. 700sq ft) was built on a rubble foundation with double-brick wall construction in 1896.   A 2,200sq ft single-storey addition was added to the school in 1957 to bring plumbing, heating and extra classroom space.  This was built slab on grade with block wall construction and brick veneer.   The school closed in the late ‘60s and was later converted to a family home.   The older 2 storey portion of the home was clad with masonite siding in the  ’90s and the windows were upgraded to vinyl.  The home has been heated with numerous radiators from an old oil-fired boiler.  There is no insulation to speak of…and it’s been pretty cold in here this winter!!  In our first month in the home (December) we spent approximately $1,500 on fuel oil!!  This was quite a shock.  We quickly switched to a large propane-fired combi-boiler from Navien which has cut our fuel costs in half.  But this isn’t good enough.

Here’s what we’re planning:

·         Exterior retrofit with rigid insulation and clad with Celect composite siding;

·         Interior 2×4 wall framing with batt insulation and drywall;

·         Spray foam the crawl space from the interior;

·         R50 attic insulation;

·         Rigid insulation and radiant in-floor heating for slab on grade portion of house (2200 sq ft);

·         Forced air heating for old 700sq ft school house and new proposed second storey addition;

·         All new plumbing, electrical, flooring, etc., etc.

We are now looking at the possibility of installing an off-grid solar system as part of the reno.  We’re excited about the idea of reducing our energy bills and realized that we could also reduce our reliance on fossil fuels.  We were originally planning on heating the concrete slab hydronically with the new propane combi boiler…but now we’re thinking of heating it with electric mats powered with solar.  Pros? Cons?  Or perhaps we should switch to an electric combi boiler and stick with the plan to heat the floor with hot water.  We’d like to reduce our propane use as much as possible so any advice would be welcomed.   Could a solar system realistically power the whole house including the heating system…specifically, in-floor heat, forced air heat (whether electric or hydronic), and air conditioning?   Or is this too much demand for a solar system?  Perhaps we could supplement the solar system with a propane fired generator?

Also, do my insulation plans sound ok?  I know I haven’t provided much detail because I’m not yet sure the exact type and thickness of rigid insulation we’ll use, whether or not we need a separate WRB, etc.  But overall, am I on the right track?

Thank you in advance for any help you can provide.

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  1. Expert Member
    Peter Yost | | #1

    I am sure others from the GBA community will weigh in on this one but if you spend some time on this site, you will hear the same mantra: air seal before you insulate; manage bulk water on the outside of your building before you air seal.

    The other thing that jumps out from the photo of your home: quite a bit of glazing in the addition. This bank of windows (and the skylights) likely has a big impact on energy performance and comfort. Consider how much you really need all that glass, particularly is it does not face south.


  2. Expert Member
    Dana Dorsett | | #2

    ">"We were originally planning on heating the concrete slab hydronically with the new propane combi boiler…but now we’re thinking of heating it with electric mats powered with solar. Pros? Cons?"

    Heating DC with mats uses a lot of electricity and isn't very efficient. With a decent sized GRID TIED PV it's possible to heat the slab hydronically with heat pumps or reversible chillers using about 1/3 the amount of electricity. A 5 ton LG Multi V S can probably do it all (including domestic hot water) on a house that size once the insulation and air tightness is improved, using one or possibly two of their low temperature Hydro Kit modules. But there are other solutions.

    A 2x4 studwall up against double-wythe brick is pretty low performance, and would not meet code minimum on the other side of the river. A continuous layer of 3" reclaimed roofing polyiso and furring to mount the wallboard would (barely) make code min, but 2.5-3" of reclaimed roofing foam between the brick and a 2x4/R15 studwall would be financially rational even if going off-grid. Reclaimed and factory seconds foam board is regularly available near you from this outfit:


    I don't know how big of a PITA it is to take a truckload of used foam across the border would be, but they may even be willing to deliver.

    1. Shane_McCafferty | | #4

      Thank you Dana.
      So this LG unit could provide hydronic infloor heating for the slab and DHW using solar energy? So it would replace my Combi boiler?
      Could it also be used to provide hydronic forced air to the new second storey addition?

      1. Expert Member
        Dana Dorsett | | #10

        The Multi V S system has all of those capabilities, but it would need to be grid tied unless you want to spend more money on batteries that you spend on the house. To work well requires a pretty good designer.

        Though there's little technical information, you can get a peek at one of those systems (including grid tied PV) starting at 14:10 on this video:

        Though this house was heated/cooled with air handlers, it could easily have replaced one of the air handlers with a second Hydro Kit to run radiant heating. While the climate in that location is a bit warmer than Niagara Falls Ontario, the systems have fully rated output down to -20C, and continue to operate at temperatures below that (albeit with unspecified capacity.) The 99% outside design condition in your area is about -15C (at the Buffalo airport), so it's possible to design for your loads properly with a Multi V S, whereas it couldn't if your design temp was -25C.

        1. Shane_McCafferty | | #12

          Any recommendations on how to find a good designer? I'll be needing an HVAC designer to review my building plans before submitting the building permit to the city for the renovation. Once the drawings are done (which for the last few weeks I've been told they'll be done "tomorrow"!!!) I intended on using a local guy who is a "friend of a friend"...but we haven't yet spoken about the details of the heating system...including my recent idea about going I don't yet know if he has any experience with this. If not, is there a good resource for finding someone experienced in this area?
          I'm planning on sending an email to LG for details about the Multi V S and any local suppliers near me. Maybe I can find someone this way too.

  3. Shane_McCafferty | | #3

    Thank you for the reply Peter.
    That bank of windows in the front with the skylights will be torn down in the renovation. This is simply a front porch that the previous owner turned into a sunroom. It is part of the exterior and leads to our front door.
    What’s the best way to air seal without breaking the bank?

  4. Expert Member
    BILL WICHERS | | #5

    There are lots of ways to air seal. If you’re going to have all the walls open, you can used canned foam to seal any/all penetrations (wiring, plumbing, etc), and you can install the new drywall to be airtight with acoustic sealant. I think you will need poly in your walls to meet Ontario code. My brother in law is a remodeler in Hamilton, he’s told me about Canadian code requiring poly but I don’t remember the details. Mostly we were “comparing notes” about the mostly minor differences between US and Canadian building codes.

    I would pay particular attention to the interface between the foundation and framing. The larger spray foam kits are great for this.

    You’ll want to air seal the attic prior to insulating. Get some canned foam and some caulk and seal all the penetrations and gaps you find. There are articles on GBA about what to look for.

    If you put in a solar system, make sure you install a gridtie system. Ontario has a VERY generous power buyback program.


    1. Shane_McCafferty | | #6

      Thanks Bill. I had seen some information on GBA about using ice and water shield on the entire exterior...but that’s out of my price range. I’ve also seen it used on key areas on the exterior...which I may consider. I’ll do some more research but spray foam may be a consideration for me too.
      You’re right about using poly on the interior “warm side” of the insulation. I hadn’t even thought about that as the solution for air sealing. My focus was on sealing from the outside. Good point!

      1. Expert Member
        BILL WICHERS | | #7

        The poly is a vapor barrier, not an air barrier. If you’re super careful with detailing, the poly can also act as an air barrier but it’s better to seal the drywall for that purpose.

        Spray foam is good for rim joists, but is unlikely to be your best option for the walls. Batts are probably your best option for the walls, or some form of blown insulation. I’d probably use mineral wool batts myself. The thermal bridging of the studs kills the performance advange spray foam would otherwise have in the walls so it doesn’t make sense to use it there. Rim joists are different because spray foam really helps with air sealing that otherwise difficult to seal area.

        You mention sealing from the “outside”. If you’re going to have the exterior open too, you definitely want to consider putting on a layer of rigid foam. If you use mineral wool batts in the wall cavities and a thick layer of rigid foam on the exterior, you can do a really great job of insulating your exterior walls.

        Ice and water shield is used as a “fully adhered membrane”. It’s great for keeping water out, but expensive. You can get good performance from the more common housewrap materials too (water resistant barriers, WRBs) if you do a good job installing them.


        1. Shane_McCafferty | | #13

          Yes we're planning to insulate the exterior too. The single storey portion of the house (which actually used to be a school) is constructed of brick veneered concrete block. We were thinking of adding a layer of rigid to the exterior brick before cladding with siding. Is there a recommended way of air sealing the exterior brick before putting up the exterior rigid? Or over the rigid (like Tyvek)?

  5. tommay | | #8

    Cute house. Since you are renovating you have many choices. The photo you gave, is that south facing? How much sun do you get in the sunroom? Numerous radiators? Cast iron? Forced hot water?
    Where do you plan on installing your PV panels? Have you considered solar hot water? Adding a double loop of radiant, one for solar HW and one from a boiler can be done or all hooked together and zoned.
    Off grid solar, so hopefully you are thinking low voltage lighting in portions of the house as well as dedicated plugs in case of power outage and year round savings. Propane tankless hot water heaters use very little power, and can be used for your radiant zone, and if used in conjunction with a SHW heater it may save you lots also, if they are both hooked up to the off grid set up to run pumps and controls. Although you already purchased a new combi boiler, there are ways to switch the boiler off when not needed and turned on automatically when needed, if fed with a SHW heater.

    1. Shane_McCafferty | | #15

      It is south facing. Yes there are numerous cast iron radiators throughout that were heated with water from an oil boiler. We bought the place this past December and quickly switched to a new propane combi boiler hoping to improve efficiency and save on fuel.
      We're adding a second storey on the house and were thinking of putting the PV panels on the roof. I hadn't really considered SHW.
      It sounds like there are endless heating options. Where do I turn for the optimal design?

      1. tommay | | #21

        If there were an optimal design, there wouldn't be so many options. If you want to keep the look with the old radiators and learn to like how they heat I would keep them. Once you insulate you may find they work well. Hopefully your plumber purged all of the radiators after installing the new boiler so they operate correctly. Of course adding heat to the rest of the house has to be considered. If you keep the new hot water system, you can either try to find some more cast iron radiators or add baseboards to the new addition and existing sunroom as needed. Cast iron radiators also work well with SHW.
        Where do you plan on this new second story addition? On top of that sunroom?......just saw your reply that the sunroom is coming down. Hopefully you follow suit and take advantage of your orientation.

  6. GBA Editor
    Martin Holladay | | #9

    First of all, can you tell us your name? (I'm Martin.)

    If you have access to grid power, it makes no sense -- neither financial sense nor environmental sense -- to cut the cord and go off-grid. I know, because I've lived in an off-grid house for 44 years. Off-grid power is more expensive than grid power, and requires the use of expensive batteries that need frequent replacement, as well as the use (during the winter) of a generator that depends on fossil fuels.

    For more information, see "How to Design an Off-Grid House."

    1. Shane_McCafferty | | #11

      Sorry Martin...I'm Shane. I've updated my profile now so my screen name shows my actual name.
      I've been talking to a solar provider locally and came to the same doesn't seem to make financial sense to go off-grid.
      I've been doing lots of research to try and educate myself on all of these power, insulation, heating systems, etc. This site has been a great help...and I value the input of the members in helping me answer all these questions.

  7. user-723121 | | #14

    What a project ! The slab on grade portion will have to get rigid insulation dug in around the perimeter to keep the cold out of the slab. This will not raise the temperature of the slab beyond the seasonal ground temp. To get any kind of comfort and efficiency from a floor heating system you will have to insulate on top of the slab with rigid foam or some type of appropriate insulation. In other words, isolate the slab on grade from the conditioned space with adequate insulation.

    1. Shane_McCafferty | | #16

      It's definitely a big undertaking! A friend of mine will be the GC and he recommended rigid on top of the slab and around the interior perimeter and then 4" concrete poured over the pex tubing.

      1. user-723121 | | #18


        I was thinking about rigid insulation around the exterior perimeter to protect from frost. This will be essential if you are going to isolate the slab from the conditioned space. I really think you need to define the energy upgrade portion first, determine the new heating load, then you can consider your space conditioning options.

        1. Shane_McCafferty | | #19

          So does that mean when i'm insulating the exterior with rigid I should excavate around the outside of the house and extend rigid down below grade so the slab is insulated from the outside and inside?

          1. user-723121 | | #20

            Certainly for the slab on grade portion otherwise the frost under the slab will take over. For the excavated portion of the basement you can insulate from the inside. Make sure the footings for that foundation are below the frost line because you will be taking away the heat from those foundation walls when you insulate on the interior.

  8. Shane_McCafferty | | #17

    What should I do about heating the second floor and cooling the whole house? It was recommended that I run a separate system for this. Specifically: use the combi boiler for radiant in floor of the slab and DHW and then run a separate furnace, a/c and duct work for the second storey addition. The duct runs to the second floor could be used to cool the main floor. I assume the furnace will run off hydronic forced air and not a traditional burner...but I'm not sure if this is what was recommended. What are the pros and cons of this system versus a ductless split type system like the LG Multi V S? Or what about hydronic floor heat on the second floor? I could add this to the underside of the subfloor between the joists after the addition goes on. Is this a good way to heat the second floor? If so, I'd still be looking at ductless splits for A/C throughout the house right?
    So many variables!!!

    1. Expert Member
      Akos | | #23


      It very much depends on what your energy cots are. If your propane is around $3/gall and your electricity is $0.11 then there is no point for propane. Even going full resistive heat would be the same fuel costs.

      For example, my electric costs a north of you around $0.14/kWh. If I go with a heat pump based system with a yearly average COP of around 2.5, propane would have to cost less then $1.5/gallon to be worth using it.

      As for the exact setup of heating system, it will come down to your final heat load needs and install costs. Usually the cheapest high efficiency electric heat is either a wall mount or a ducted mini split.

      Hydronic heated floors start making less and less sense as you insulate the house. Basically the floor never gets hot enough to be noticeable. You are spending a lot of money on pluming/install and don't get any of the "warm toes" benefit. Way less money to go with electric heated mats for things like entrance/bath/kitchen.

      Commercial system like the LG unit, only really start to make sense if you need a lot of heat in the summer time for something like heating a pool. Installing something like this just for DHW is silly. You are much better off with just going with an electric tank with about 8 extra solar panels on the roof (half that if you go with a heat pump water heater).

    2. tommay | | #24

      Depends on what your design is. If you take advantage of your orientation and add glazing on the south side, along with other techniques, this may contribute to part or all of your heating needs in that area. Overhangs for shading and cooling. So possibly you would just need a hot water zone off the boiler you already purchased. If you use baseboards, instead of underfloor, the amount of heat can be easily adjusted by adding or removing sections in the future if necessary.
      After you insulate, and if you add exhaust venting ,(you already have the cupola/ bell tower on the school, maybe you can match it on the addition), you may find that you do not need so much cooling, something that can be added later also.

    3. Expert Member
      Dana Dorsett | | #25

      You really need to run the load numbers to end up with the optimal system. A full VRF system like LG Multi VS can manage something like 6 zones (more, with branch boxes), and can simultaneously cool the upstairs while heating the downstairs, moving the heat it's pulling from one zone and putting it into the hot water & hydronic heated zones, if that's how you set it up. Sized properly the air conditioning would provide a large amount of your summertime & shoulder seasoning, moving that heat from indoors into the insulated buffer tank rather than putting it outdoors. In a high-R building the cooling season may be as much as 6 months of the year in your location.

      Since upper floors will almost always have a bigger cooling load in summer (or even during the shoulder seasons), it's reasonable to do the upstairs zone(s) with ductless heads or a small air handler running off a heat pump. Whether that the upstairs zone is running off an LG Multi V S or mini-splits it doesn't really matter, as long as the equipment is sized correctly for the zone loads.

      Without realistically calculated load values there is no way to make the optimal choices here. But if a goal is to have PV provide part or all of the heating energy at some point you HAVE to build something substantially better than a 2x4/R15 wall. The amount of exterior rigid insulation matters- limiting it to the IRC code min R5 would be a mistake. If 3" or more of reclaimed roofing polyiso can go on the exterior side of the brick with a 2x4 batt insulated on the interior side there would be measurable thermal mass benefit provided by the brick wall that will lower both the peak and average loads.

      If there's room for as much R60-R70 blown insulation in the attic rather than a code-min R50, the additional R10-R20 insulation would usually be cost effective on energy use on a lifecycle basis in your climate, even though it barely moves the needle on peak load numbers.

      With a sufficiently lowered heating load it's likely that re-using the radiators (at a much lower water temp) in lieu of a radiant floor would be possible, but if the plumbing would have to move it may be just as cheap to go with a radiant floor.

      Sticking with propane as the heat source doesn't usually make sense, but local energy markets vary. In my area even with fairly high electricity rates it's substantially cheaper to heat with a modulating heat pump than it is to keep burning propane. Over the longer term there is downward price pressure on electricity as renewable sources of energy keep getting cheaper & cheaper, whereas propane prices will remain volatile with large year on year swings up/down based on the international fuel markets, but will most likely rise over time as carbon emissions get priced in.

      1. Shane_McCafferty | | #27

        I've contacted an HVAC designer and sent him my drawings so he can calculate the heat loss numbers, design the system requirements, etc. I mentioned the LG Multi V S system because I really like what you've described (and what I've subsequently read and viewed on the video link you sent me)...but he advised against it. He claims that he's seen these units freeze up in the winter because they can't handle the low temperatures. He says he's seen a large block of ice form on the back side of the unit which needed to be chipped off or melted away before getting the system back up and running - which is less than ideal in the coldest part of winter! Is he exaggerating? Would I really have a difficult time heating my place throughout the winter? Or are there options that I can run propane on super cold days when the unit may struggle to make heat?

        I've got an insulation contractor coming to the house this week to assess options and give me prices. I'm not sure of his experience in building I'm not sure if his recommendations will be in line with what's best for the building...or what's best for his pocket! I suspect he'll push spray foam for just about everywhere. Should I consider spray foam as much as financially possible or is it too cost prohibitive? For example, since it provides air sealing and good R value, should I consider it on the exterior in place of rigid? Or would polyiso on the exterior provide a similar result (r-value per inch) at less cost? Should I consider spray foam on the interior in place of batts...or rigid? Perhaps there's a good trade-off here somewhere from an economical standpoint?

        I really like the idea of reducing my reliance on fossil fuel and leaning more on solar power. If a heat pump is not feasible for winter (as suggested by my HVAC guy...which may or may not be correct), would it be feasible to look at an electric combi boiler for hydronic heating? And then perhaps adding a heat pump for cooling?

        1. Expert Member
          Dana Dorsett | | #28

          >"He claims that he's seen these units freeze up in the winter because they can't handle the low temperatures. He says he's seen a large block of ice form on the back side of the unit which needed to be chipped off or melted away before getting the system back up and running - which is less than ideal in the coldest part of winter! Is he exaggerating?"

          Maybe exaggerating, maybe not. It depends on what he means by "...these units..." Not all air source heat pumps are alike- I'm skeptical that he has actually seen an LG Multi V S freeze up or even seen one at all, given that they've only been available in North America for two winters now. The vendor and model numbers matter. I'd never spec a Mitsubishi GExxNA mini-split for heating in your area even though it's specified operating temperature extends to -20C, but that's more of a capacity & efficiency issue, not an ice-up issue. I'm very comfortable specifying their FExxNA or FHxxNA series for your climate.

          Cold climate mini-splits & multi-splits with the integrated pan heaters work just fine in climates like yours or colder. I've even stayed at a ski condo in norther Vermont heated & cooled with a 2 ton cold climate Mitsubishi multi-split. Fujitsu's xxRLS3H units work just fine at -30C, even though "only" specified down to -25C.

          Mini-splits not designed for colder climates can potentially experience the "...large block of ice form on the back side of the unit which needed to be chipped off or melted away before getting the system back up and running..." symptom, but that would be a symptom of improper or inadequate defrost mode controls. Almost any heat pump with capacity tables that go as low as -20C should have adequate defrost controls for extended operation at that temperature but I can't vouch an LG on that, since they aren't as common in my area. My 99% outside design temperature is identical to yours, and though they don't have anywhere near the market share that Mitsubishi or Fujitsu, they aren't particularly rare, and I haven't heard of chronic icing issues with them.

          The spray foam vs. rigid is largely a cost issue. The installation of rigid foam on the exterior takes more labor than spray foam, but the material cost of rigid foam is less. When using reclaimed rigid foam the material cost can be a LOT less. There are cheaper ways to air seal a brick building, but if you're going to do it with spray foam, insulating the interior side with open cell foam does as good a job of doing it with closed cell foam, at a lower material & environmental cost. But to use 3.5" of open cell foam on the interior without an interior side vapor barrier would need at least 2" of rigid foam on the exterior to keep wintertime moisture issues well controlled.

          >"I really like the idea of reducing my reliance on fossil fuel and leaning more on solar power. If a heat pump is not feasible for winter (as suggested by my HVAC guy...which may or may not be correct), would it be feasible to look at an electric combi boiler for hydronic heating? And then perhaps adding a heat pump for cooling?"

          There are any number of ways of doing it with an electric boiler, but the efficiency compared to a cold climate air source heat pump is atrocious. If you can abandon the idea of heating solely with a radiant floor, an electric boiler operating off a floor thermostat and using a cold climate Fujitsu or Mitsubishi ductless keeping the room up to temp can work reasonably efficiently, as long as the temperature of the floor is kept within a degree of the room temperature. I'd personally be inclined to skip the radiant, putting that money into a higher-R building envelope and do it all with cold climate heat pumps.

          Your HVAC guy doesn't seem inclined to work with you on that though, most likely out of insufficient experience with them. You may find this thread from a few weeks ago encouraging/enlightening:

          (For a quick-reference unit conversions while reading that: +5F = -15C -4F = -20C -13F = -25C -22F = -30C )

          1. Shane_McCafferty | | #29

            Thank you for the excellent info Dana!! This is exactly the type of advice I’ve been looking for!!
            You’re right, my guy doesn’t have experience with the LG unit. His comment about icing was in reference to some unnamed brand and model of heat pump. I’m happy to hear that Mitsubishi and Fujitsu have models that would work in my climate zone! I’ll definitely look into these further.
            Just to clarify...if I insulate the exterior with 3” polyiso and the interior with open cell other air barrier is required on the exterior? Or any form of WRB? Just rigid over brick and then apply siding?
            I like your idea of abandoning the radiant infloor heat and investing this into more R value. What are your thoughts about having numerous ductless splits throughout the house versus running ductwork and an air handler?

  9. user-6185887 | | #22

    I have to ask it sounds like none of the original exterior of the building will remain visible and the entire interior is being replaced. If that is true and the original structure is complicating your goals.

    You may come out better building spending less money and time, if you tear it down and build a tribute to the original. With an old building every time you open a wall there is a surprise and they are rarely pleasant.


    1. Expert Member
      Dana Dorsett | | #26

      >" sounds like none of the original exterior of the building will remain visible and the entire interior is being replaced."

      That would be the case with the plan as described:

      "· Exterior retrofit with rigid insulation and clad with Celect composite siding;

      · Interior 2×4 wall framing with batt insulation and drywall;"

      >"If that is true and the original structure is complicating your goals."

      Not necessarily. A double-wythe brick structure with insulation on both sides can be a fairly high performance building, and can hit a performance mark expensive to hit with a tear-down and replace approach. The thermal mass benefits can be pretty significant when more than half the insulation is on the exterior.

  10. Expert Member
    Dana Dorsett | | #30

    >"Just to clarify...if I insulate the exterior with 3” polyiso and the interior with open cell other air barrier is required on the exterior? Or any form of WRB? Just rigid over brick and then apply siding?"

    Spray foam on brick (on either side) makes for an extremely air tight wall. You'll still want to tape the seams of the exterior foam board and any broadsheet WRB, but the foamed-brick makes a great primary air barrier.

    Where to place the exterior WRB depends on how you are mounting and flashing the windows. If the glass is going to be roughly co-planar with the new siding (aka "outie mounted windows"), the WRB goes on the exterior of the foam, and the window flashing needs to lapped properly. If the glass is going to be roughly co-planar with the exterior of the brick (aka "innie mounted"), using a spray-applied WRB directly on the brick may be the best approach.

    Most brick is sufficiently uneven that there is some amount of gap between brick & foam no matter what, and the mortar lines provide ample drainage for any major bulk water to drain. If the brick & mortar is unusually smooth, when going with innie windows it may be useful to use a thin " XPS drain-mat type layer as the WRB. A drain-mat approach does allow some amount of convection behind the foam for drying, but not enough to become a serious thermal bypass. eg:


    1. Shane_McCafferty | | #31

      I like the look of innie windows and have read that they’re less likely to frost in the winter because of reduced wind washing. So I was intending to leave them where they are. The previous owner replaced all the windows with vinyl double pane units and I was planning to keep them. Can I leave them in place when doing the exterior retrofit? Is there an effective way of flashing the windows without removing them? Or do they have to come out and be reinstalled?

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