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Community and Q&A

Zone 5/6a Vinyl Residing – Double Blueboard, Wrap?

whiteroses | Posted in GBA Pro Help on

As a follow up to:

Dow SIS is not available in this area, so we will insulate the walls as they are:

kraft faced fiberglass insulation (very, very thin)
two 1/2″ layers of fiberboard as sheathing
one 1/2″ layer of blueboard

My thought is to dense pack cellulose in the walls, tape the existing blueboard, add a second 1/2″ layer of taped blueboard, possibly wrap, then side.

Once concern is the age of the fiberboard – the home was built in 1960. Will it last for the life of the vinyl siding?

Does it make sense to tape the existing blueboard?

Any problems adding a second layer? This will add 1/2″ to the exterior. Trim will be redone at the same time.

To wrap or not to wrap?

Many thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    J. Heckmann,
    Q. "Once concern is the age of the fiberboard - the home was built in 1960. Will it last for the life of the vinyl siding?"

    A. The way to assess the condition of your fiberboard sheathing is to remove the siding and inspect it. It may be dry as a bone, and good for several more decades, or it may be turning to mush. You'll have to look at it to determine its condition.

    Q. "Does it make sense to tape the existing blueboard?"

    A. Yes. (I assume that by "blueboard" you mean XPS rigid foam.)

    Q. "Any problems adding a second layer?"

    A.I advise you to make the second layer thicker. It's hardly worth the trouble to install only 1/2 inch of foam, and you also have to consider the minimum required R-value of your exterior foam, which depends on your wall thickness and climate. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. whiteroses | | #2

    Thanks, Martin, looking at the map at the link, the home is in Zone 5A, which for a 2x4 wall would require R-5, which two 1/2" layers of blueboard (at this point none of the existing siding has been removed, all I can see is blue, no printed identification) would provide.

    I see now that the table is giving a MINIMUM R-value, hence your recommendation to add more. Got it.

    Thank you.

  3. Expert Member
    Dana Dorsett | | #3

    Depending on your heating & cooling energy costs/rates it could be will worth adding as much as 3" of fiber faced polyiso or 5" of unfaced EPS to that stackup, adding R18-R20 to the stackup for a total of ~R30.

    The whole-wall R of fiber-insulated 2x4 construction with half inch of XPS or EPS is about R12, and from a lifecycle cost point of view R30 is sort of a middle-ground starting point. If your insulation costs are higher than average and your energy costs lower than average you might adjust downward an inch or so, but if you're in $4/gallon heating-oil & 20 cent/kwh land it's worth going up from there. See table 2, p10 of this document (and read the whole first chapter.):

    If you have access to reclaimed roofing foam the costs per unit-R of the foam can be lower than that of batts, and well worth going a bit higher. Up to about 6" or so the labor cost of adding 2-layers of foam are about the same, and the cost of the longer timber-screw fasteners negligible. Beyond that it's a bit harder to assemble as a foam-over, and you also end up with apparent wall-flatness issues. Commercial roofing and demolition companies will often have some amount of reclaimed goods on hand, and in my neighborhood there are multiple vendors whose primary business is reselling reclaimed foam at about 1/4-1/3 the price of virgin-stock.

    For exterior applications like this, unless you're pulling the batts with the kraft facers you'll want to avoid foil/poly/vinyl facers, so that there is at least SOME outward-drying capacity. (A more expensive option would be to use rigid rock wool panels which are very vapor-permeable, but the cost-effectiveness crossover would happen much lower R.) Similarly, beyond about 2" or so XPS (blue, pink, green, whatever) becomes fairly vapor retardent but still might be a good option at 2" if it's reclaimed goods. If virgin-stock XPS is considerably more damaging to the environment due to the very high global warming potential of the blowing agents used (more than 100x more damaging than virgin-stock EPS or polyiso.)

  4. whiteroses | | #4

    Dana, thank you for that link - I will be reading through it this evening.

    Thanks also for the idea of looking into reclaimed foam. Attached is a craigslist ad for nearby reclaimed polyiso with kraft facing that may be a good fit.

    Energy costs are in the .18 to .19 range for electric (A/C) and $1/therm for natural gas. The home has been air sealed and cellulose blown into the attic, but nothing has been done to the walls.

    Energy Star Home Energy Yardstick score is 9.2 out of 10, 3650 kWh and 410 therms for 2012, 1384 sq ft. However the thermostat was kept pretty low over the winter, sacrificing some comfort.

  5. Expert Member
    Dana Dorsett | | #5

    Some of the black asphalted facers are fairly low permeance, more vapor-tight than the fiberglass or gray paper facers, so bear that in mind in designing the total stackup- assume it's on the stiff side of class-II vapor retardency. That means it's a good idea (but not absolutely necessary) to pull the kraft-facers on the pre-existing fiberglass if you can. But kraft facers on fiberglass batts are variable-permeance and become more vapor open when damp- should you get some bulk-moisture in the stud bays it'll still dry toward the interior at a reasonable rate, and if you're putting R18 or more over the pre-existing blue-board, the wood sheathing will average well above 40F at Rochester-area mid-winter temps, with effectively zero wintertime moisture accumulation via vapor diffusion. That means it would only really need to dry from bulk water leaks, and getting the flashing details right on the windows & doors is always job-1, far more important than diffusion drying rates.

    Take the R9 & R18 claims with a small grain of salt- 2lb roofing iso tends to run a bit shy of R6/inch at a 75F center-foam temp (partly due to the thickness and lower R value of the facers) and at the midwinter averages you see in Rochester NY derate that to ~R5.7/inch for a dew-point calculation point of view (but you'll have HUGE margin, even at R8.5- there's no need to actually calculate it). Taking it as nominally R6/inch the $13/R9 goods works out to about 4.5cents per R-foot, and the $24/R18 goods works out to about 4.2 cents per R-foot, which is well under half the cost of virgin stock roofing iso f.o.b. the distributor's yard. I've seen 3" roofing iso in excellent shape as cheap as $10/sheet for 4x8' locally, and I've never paid more than $20/sheet, so feel free to offer them $12 and see if they won't dicker. The market does vary year to year, location to location.

    But even at 4-4.5 cents per R-foot asking price it's already cheaper than box store pricing on high density batts, and nearly competitive with crummy low density R19s (that are really only R18, when compressed to 5.5" 2x6 stud bays), that usually come in around 3.5-3.8 cents per R-foot (with properly derating to R18 at it's installed-compressed value) and orders of magnitude more air & water tight.

  6. whiteroses | | #6

    I will make sure to discuss the flashing details again with the contractor, in light of your comments.

    Yes, it's always worth making an offer, any additional savings will help. Thank you for the R-value and price comparisons, that's very helpful.

    I read through the first chapter and portions of the rest of the document. What an education I'm getting through this forum!

    I did have a little trouble understanding this paragraph:

    Whole-house Ventilation

    As building airtightness improves, the number of annual hours during which ventilation through accidental openings driven by highly variable natural forces increases to the point that indoor air quality begins to suffer. For these reasons it is assumed that a home with High-R enclosures will require mechanical ventilation.

    I think the general idea is that I should include the cost of a heat recovery ventilator in this project, is that right?

    Still not sure if wrap is needed here or not. I'm thinking not, based on your comments regarding permeance, even though the wrap may be fairly permeable it will restrict airflow further. We would not be able to remove the kraft faced insulation in the walls.

  7. GBA Editor
    Martin Holladay | | #7

    J. Heckman,
    When you write "wrap," I assume that you are talking about plastic housewrap, which is a type of water-resistive barrier (WRB). All walls need a WRB; it is code-required (and a good idea). Although it's possible to use rigid foam as your WRB, that approach has several challenges you should consider before you decide to go that route. For more information on WRBs, see All About Water-Resistive Barriers.

    You raised a question about mechanical ventilation systems. If you end up tightening your home significantly, you probably want to install a mechanical ventilation system. An HRV is not the only option. To learn more, see Designing a Good Ventilation System.

  8. whiteroses | | #8

    Thank you Martin. This is quite a puzzle to put all these pieces together.

    Very interesting reading about ventilation.

    Both bathroom fans are new Panasonics. A 6" kitchen vent will be installed in the roof as part of this siding project. There is a large ceiling fan in the living room. There has been no problem with musty or stale air, so I'm going with no MV for now.

    One oddity of this home is that the basement entry is through the garage. This means the forced-air natural gas furnace and A/C may be drawing some air from outside, through the basement and garage. Once the siding is installed, I do plan to add a storage locker outside for chemicals, which should help improve air quality inside the home.

  9. Expert Member
    Dana Dorsett | | #9

    You definitely can't use polyiso as the WRB, only certain assemblies using XPS with appropriate Z-flashing taping methods fully meet spec as a WRB. I'd personally avoid that approach.

    The crinkle-type spun polyolefin is a better choice than the smooth/flat goods if the WRB is between the foam & sheathing, since it has much lower capillary draw, creating a micro-gap between the foam & sheathing.

  10. GBA Editor
    Martin Holladay | | #10

    Sorry, but you are wrong. Several brands of polyiso have been approved for use as a WRB, including Thermax and Tuff-R. And none of the approved foam WRB systems include the use of Z-flashing; instead, they all depend on tape. Read all about it here: Using Rigid Foam As a Water-Resistive Barrier.

  11. whiteroses | | #11

    Wait, we're not planning to remove the existing blueboard. So there's no opportunity to put a wrap between the fiberboard sheathing and the foam.

    When you say "several brands of polyiso have been approved for use as a WRB", that won't apply in this case if we use reclaimed polyiso. And I wonder, approved by whom? For a retrofit situation like this, or new build?

    I found this, which seems to imply that the tape/flashing is more important than the foam:

  12. GBA Editor
    Martin Holladay | | #12

    J Heckman,
    It's entirely possible that you don't have to follow any building code requirements for this type of work. It's also possible that your local building department will be looking over your shoulder. The only way to determine what requirements you must follow is to call your local building department.

    You asked, "approved by whom?" The building code specifies the use of asphalt felt as a WRB. Any other product must be tested and approved as an alternate material. The most common testing lab performing this type of approval is the ICC Evaluation Service in Whittier, California.

  13. whiteroses | | #13

    One thing I'm still confused about - I see a lot of resources that address flashing of new or replacement windows, where the opening is flashed and then the window is installed. However in this situation, vinyl replacement windows were installed several years ago. We will not be removing the windows.

    Where are the instructions regarding flashing of windows and doors, where exterior foam will increase the exterior dimensions by 1", and the windows are not being replaced?

    I also looked for a manufacturer who might have made the kraft faced polyiso, didn't find anything. Without the manufacturer, I couldn't search ICC's database for information.

  14. GBA Editor
    Martin Holladay | | #14

    J. Heckman,
    Adding exterior foam without removing the existing windows is somewhat risky. You have to evaluate the existing window flashing and the exposure of the wall to rain to decide whether you want to accept the risk.

    It's always preferable to remove the existing windows and properly flash the rough opening.

    If you choose to leave the windows in place, think about how you are going to get the water that reaches the rough sill away from your wall and out toward the exterior of the building.

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