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

Community and Q&A

Pros and Cons of Additional Insulation

teandrle | Posted in Energy Efficiency and Durability on

I am preparing my house built in 2004 to be re-sided due to storm damage.  I have 2×6 walls with Batt insulation in them.  I am considering going with an R5 foil faced insulation under the siding from Johns Manville.  It will cost me about $3k to add the insulation and trying to figure out if it is worth that investment? They will be adding tyvek over the top to completely seal the structure and leave the existing tyvek on the building.  This is 3/4 inch and the contractor does not want to go any thicker due to the exterior outlets he has to work around will become more challenging.  I am in climate Zone 5 (Iowa).  Any thoughts or suggestions you have would be very welcome.


GBA Prime

Join the leading community of building science experts

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


  1. Expert Member


    I tend to think of these things in terms of opportunities first, cost second (unless the cost is way outside your budget).

    I'm not sure of your exact location, but in your climate zone, as you well know, there is more time spent heating than there is cooling. I found these plots from the city of Ames (, which shows the cooling and heating degree days.

    It will reduce the amount of energy required to heat and cool your home, with a few caveats. Air leakage can reduce insulation efficiency quite substantially if there are a lot of leaks. Large thermal 'holes' can make the addition of insulation futile, as all the energy will sill flow out of them. Having said that I think it's a good idea to do, since you'll only have this chance once (hopefully) for the next few decades. Often times there are incentives for energy upgrades that help soften the costs.

    Regarding the exterior outlets, that should be an easily solved problem for your contractor. There are many different types of weatherproof receptacle extensions that can bring the surface back outside your siding, or mounting blocks that accomplish the same thing. I've attached a picture of one from Lowes just to show what they look like.

  2. teandrle | | #2

    Thanks for the reply I will look at that outlet extension. The biggest issues are the faucets since I spray foamed my sill plate the 2 outdoor faucets will not move unless I do some cutting unless you have other ideas. I could go to an inch on most of the house but I would only go to R6 compared to the 3/4 R5. I have a stone edge to make sure I do not cover too much of the reveal on one side of the house so that would be a max of 3/4. All windows are being replaced so that is not an issue. I have 2 fireplace chimneys out the side of the house all the electrical and cable etc... to deal with and dryer vents. Any suggestions on trimming things out. I am near Cedar Rapids about the same N/S as Ames. One issue I am trying to solve and I think it is a combination of things is that one room if the door is closed will get down to about 57 degrees in the winter while an adjacent room will be about 69. New windows will help and I think proper sealing with also help. I had a thermal image camera and could see all the studs in the wall which is why I am planning to use the rigid foam on the exterior to minimize the thermal bridge. We are also adding insulation in the attic going form about R28 to about R60ish. I am also planning to seal all my can lights and vents in the attic.

    Any other feedback on the exterior foam board would be most helpful.. maybe I need to plunge cut around my faucet so I can move it and re-seal the area?

  3. Expert Member

    There are quite a few ways to do get around it. I hope others will chime in too.

    If moving the faucets represents a hurdle, you could always buck out around them. The wall area that they enclose is pretty small, and given that it's a metal pipe extending into the wall anyways, I don't think it's much of an energy loss to do so. If it were me I'd just bite the bullet and move them if that's the route you plan on going.

    If you only go up to 1", the trim should work mostly like normal. You can install windows on the outside of the insulation, or create a nailing section like in the video below. Most other things can mount in their own mounting block on top of the foam, creating a plane that is mostly like osb sheathing.

  4. walta100 | | #4

    Continuous insulation worth it?

    On the surface your question sounds so simple it should have an easy answer.

    The problem is there are several ways to look at your question.

    From a Financial point of view the question becomes will this insulation reduce the amount of fuel I will purchase enough to recover its cost before I sell this home?

    From a Green point of view, Will covering my house in foam reduce my carbon foot print?

    From an Ascetic point of view will the added insulation distort all the trim details of my home in a negative way and make it unappealing.

    None of the three questions you really asked has a clear answer.

    Financially there are hundred or so variables in your question.
    In what year will you sell the house?
    What fuel do you use to heat this house?
    What is you guess at what that fuel will cost in the future?
    What would be the rate of return you are missing if you buy the insulation?
    What is your local weather?
    The only way I know to answer to the Financial question is to build a computer model of your home and run the numbers with BEopt program.

    The Green answer is equally complex but with its own set of variables.

    The Ascetic answer is about your choice of contractors and how you chouse to motivate him.


  5. teandrle | | #5

    Well Clearly I do not know how to operate that BEopt tool, I am sure it is useful but I would need to spend a little time learning how to use it first. Although I am concerned about Green I am more concerned about interior comfort of the home, specifically in one of the rooms that can get down to 57 degrees while others are 10 degrees warmer. We do not plan to move any time soon, going on 18 years already in this home. Natural Gas is used along with central iar in the summer. Iowa can get pretty muggy in the summer and cold in the wither. I think one of the previous answers showed a fairly accurate climate chart.

  6. Expert Member
    BILL WICHERS | | #6

    If you have a basement or crawlspace, you can probably access the interior side of those faucets relatively easily. You can then replace them with frost proof assemblies of whatever length you need to get through the new insulation.

    Electrical is easy -- use extensions as previously mentioned, splicing wires to be longer if needed. Another solution is to put a cover plate with a knockout over the original box, then use that to connect to a new box right in front. The inner box is still "accessible" this way, so code is satisfied, since you can remove the top box to get to the back box. Splice some wires in the old box to reach to the new box. If you have the siding off, a more elegant way to extend the electrical is to pull the wire back one stud bay, then reinstall the receptacle there and use the ~16" of slack you gain by pulling back one stud bay's worth. That's what I would do unless other considerations limited the placement of the outlet.


    1. teandrle | | #11

      They are sheetrocked in... Not easy access.

  7. Expert Member
    Akos | | #7

    If this was 2x4 construction the answer would definitely be to add the extra iso.

    The R5 doesn't make all that much difference on a 2x6 wall, assuming R19 cavity insulation, it brings your assembly R value up from ~R15 to R20. Better but not a whole lot. If you can bump up the rigid to R10 that would be a bigger change that would be worth the extra labour. With 1.5" rigid you can still most siding up directly through the foam.

    The thing that will make a big difference in your case in terms of comfort is air sealing the house. The simplest is taping the seams on the sheathing which is easy now but you would have to remove the original house wrap. Make sure to seal the sheathing to your foundation as well while there as this tends to be a big leak spot.

    Most likely your room with temperature issues has a lot of air leaks. If you hunt these down and seal them, it should make it much better. If the room has three outside walls or canti floors, putting thicker exterior rigid just there would also help to improve comfort.

    1. teandrle | | #13

      Can't really go much thicker. It would cover the stone, and make it challenging around all the utility ingress. With being a retrofit I am weighing the value of adding this to the walls for $3200. Perhaps the money is better spent elsewhere and I seal it. One thought I had was to just do it on the North wall and the East wall of that room to help. Like I indicated once I put plastic on the window I realized about a 5 -6 degree increase in that room. So air leak is a huge issue her and the windows are all being replaced.

    2. Expert Member
      Josh Salinger | | #14

      I think Akos hit it on the head. If you are replacing the siding which only happens once every few decades it is a huge opportunity to address the air leakage. I would remove the existing Tyvek, it will be redundant anyways since you are considering a bulk water control layer to the exterior of the foam anyways. Taping the seams and sealing the sheathing to the foundation and detailing the punched openings will have a much bigger effect on energy usage and comfort (not to mention air quality and durability) than adding the insulation.

      I live in an expensive market so my perspective is skewed, but adding $3K for exterior insulation is a screaming bargain. For that price I would go for it. Especially if you plan on living in the home indefinitely.

      1. teandrle | | #20

        $3K is a number but is there enough value in 3/4 inch R4 or R5 board to justify is what I am trying to figure out, or is sealing it just better and spend the cash on something else.

        1. Expert Member
          Akos | | #21

          Say you have about 2000 sqft of exterior walls. In zone 5 you are probably looking at 5000 heating degree days.

          So the losses through the existing R15 assembly is roughly:

          5000 HDD * 24 h * 2000 sqft / R15=160 Therms per year.

          Bumping that up to R20:

          5000 *24 *2000 /20=120 Therms per year

          You can look at your heating costs to calculate the ROI. Unfortunately, most likely it will take a very long time.

          One more benefit of more insulation is it will also reduce your overall heating and cooling load so if you ever need to replace your heating equipment or convert to a heat pump, you can select one that is smaller/cheaper. Might not pay for the entire $3k but probably 1/3 of it.

          1. teandrle | | #22

            Thanks for that detail, what I see you say in there is that it might save 40 therms per year with the average being .95 per therm. This is just under $40 per year on heating savings. It would not account for the other half of the year air conditioning which is probably more costly but even if it were the same saving $80 per year is about a 40 year pay off... It would really come down to the value in comfort I am looking for and maybe proper sealing is a better option.

          2. Expert Member
            Akos | | #25

            Getting proper air sealing is the most important detail. This is the one you should spend money and effort at. While the siding is off, it might even be worth while to get a blower test done after the initial sealing to see if all the leaks have been taken care of especially in the room that is too cold. Any leaks that are found during the blower test can be sealed up before the new house wrap and siding goes on.

            After that, adding the rigid is a bonus. How much that extra bit of comfort is worth is hard to say. $3k for R5 rigid install is a pretty good deal.

  8. walta100 | | #8

    It seems like the current wall would be about an R20 and with the foam R 25 that amount of change is unlikely to make a noticeable change in your gas bill.

    With one room being so much cooler than the rest of the house there must be some big problem in or around that room.

    Sounds like a patio changed to a screened porch converted to a three season room that was upgraded to a family room. Wild guess is that room has a much higher percentage of windows to walls than the other rooms and was an addition to the house.


    1. teandrle | | #9

      Same windows has 1 North and 1 East wall with a 18 inch return on the east wall facing south. It is a bedroom not a converted wall.

      1. walta100 | | #10

        The ten degree temp difference says something is clearly very different about this room.

        It seems my psychic abilities have failed in this case so you will need to tell us what is so very different about this room.


        1. teandrle | | #12

          Other than 2 walls and a short return wall if 18 inches so 2.25 exterior walls not much. I do know the window leaks and it is being replaced. We put plastic on this winter temporarily and it helped some maybe gained 5 or 6 degrees with just that so it is a start. I have a plan to spray that sill plate under that room in the north wall. The east wall has blown in on the sill plate and might be hard to get to. We will also be adding ceiling insulation soon will get up to about r50 or higher up there.

  9. paul_wiedefeld | | #15

    On the financial side, remember that not all return on investments are created equal. Often we see 2% vs 7% and think the 7% is better but it’s a meaningless comparison without knowing how much each varies year to year. A positive point in insulations favor is that heating and cooling needs are pretty consistent year to year.

  10. teandrle | | #16

    Reading through all the comments makes me think that a sealed taped tyvek might do more good than 3/4 inch foam? we pulled a small piece of siding off to run a color check and found the tyvek was up about 9 inches from the bottom of the sheeting meaning the sill plate is not covered all the way and it is not taped down just stapled on. My contractor said he will tape all seams, and windows etc.. creating that air barrier. Maybe that is better money spent? That and we are putting in all new windows "single hung" with less moving parts vs our current double hungs that get opened once or twice a year.. at most.

  11. Expert Member
    Josh Salinger | | #17

    I would do both, but if it came down to choosing between the insulation and the air barrier, I would choose the air barrier.

    Taping the seams on the mechanically attached (and apparently poorly installed) membrane doesn't make for a great continuous air barrier. It's better than not taping the membrane, but I would remove the existing Tyvek and tape the seams of the sheathing. This will make for a robust air barrier at the wall sheathing. Again, I would tape the sheathing to the foundation with a good quality tape, I highly recommend Siga Fentrim for this purpose. After the sheathing seams are taped, go ahead and install the mechanically attached Tyvek for bulk water control. There would be no need to tape the seams of the membrane at this point.

    1. teandrle | | #18

      so you’re saying that taping the seams of the new membrane will not create an air barrier? I thought that was the purpose is to stop air and water to infiltrate the wood structure, assuming it was installed correctly. If I did the insulation at R4/R5 and taped it that also should create the air barrier?

  12. Expert Member
    Josh Salinger | | #19

    Both taping the sheathing and taping the membrane could be the air barrier. You could tape the rigid insulation, too and that could be your air barrier. It's a matter of durability, effectiveness and how it becomes part of the AB system as a whole. Mechanically attached membranes have thousands of tiny holes (the staples) and can be affected by wind, stack effect, construction damage, UV, etc. By taping the seams of the sheathing you are putting the air barrier in a more protected place in the assembly and the materials (plywood or OSB, depending what you have on the home) are more durable-- especially if they are protected by rigid foam board. It's really a good, better, best situation with the best being a taped sheathing protected by rigid foam board and then a membrane for bulk water control.

    It should be noted that an air barrier is a system and not a material. The tapes, membranes or sheathing are all parts of the air barrier system. Connecting it to the foundation makes the concrete one of those parts. Whatever is used to connect the top of the wall to the ceiling or roof plane is another. When deciding on which materials to use for the AB, be it the membrane, sheathing or foam one must think about how durable each solution is, how well protected it is, and how it connects to the subsequent parts of the AB system.

  13. Expert Member
    Dana Dorsett | | #23

    Do NOT install R5 foil faced insulation- it's a code violation and puts the structural sheathing at risk!

    For 2x6 construction in IECC climate zone 5 the IRC calls out R7.5 MINIMUM for exterior insulation, as dew-point control at the sheathing layer. Aside from it being a code violation it's possible (especially with interior side vapor retarders or vapor retardent latex paint on the interior) to get away with somewhat less exterior R, air leaks from the interior side can become a serious moisture accumulation risk in the sheathing, especially on shaded or north facing walls.

    Foil facers are true vapor barriers, so installing foil faced polyiso on the exterior creates a moisture trap, severely limiting the ability of the sheathing to dry toward the exterior. If you're going to cheat the R-value due to the depth limitation, going with unfaced 3/4" EPS or HFO-blown XPS (pink, green, blue, just not shiny silver or plastic film facers) is more moisture safe. At 3/4" all EPS and most XPS is still a Class-III vapor retarder, allowing at least a modest drying rate toward the exterior. Going with Ambic's SilveRboard Graphite XS (SBGXS) delivering R3 @ 0.635" at 2.74 US perms vapor permeance (about the same as 2-3 coats of latex interior paint on gypsum wallboard) would probably be the best thermal/hygric performance compromise.

    1. Expert Member
      Akos | | #24

      R 7.5 is for walls with class III vapor retarder (painted drywall). With faced batts/interior poly/vapor barrier primer (class I or II) the exterior insulation ratio can be pushed without problems. R5+2x6+poly walls have been built around me (zone 5) for a long time and they work just fine. It is even passes code in Zone 7 according to the link bellow.

      Going for a permeable rigid foam is a good idea as it does add additional drying capacity which makes the assembly more robust.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |