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Community and Q&A

How to manage an existing residence with soffits and ridge vent to convert to closed envelope

darango | Posted in Energy Efficiency and Durability on

Dear Martin

I am in Orlando Florida and just recently bought a house, I have been following you for years and read your articles.

I have some questions that I have not been able to get answers to. I have a one story home with attic space that I am converting into livable space. It is typical florida construction with cement block and the roof is built with 2×12 rafters, The eaves have the material that has perforations that allow circulation along the roof deck to the ridge vent. On the inside the first floor is cement block and there is Celotex on the cement block for insulation. I will be redoing the roof and removing the ridge vent to make a closed system to bring in the mechanicals in the condition space. However I have not been able to find how to manage the eaves.


1. I read your book and there was a situation where Celotex was used and cause a large moisture problem. Should I remove the Celotex and is so what should I replace it with? Note that there is no sign of mold on the celotex so far. .. At the top of the cement block is where the rafters rest. The celotex rises up to the top of the block only.

If I or dont should i extend it up to the roof deck? there is no mold on the existing celotex that I have found yet. I plan to spray foam the ceilings.

3. On the outside at the eaves do I change the material and remove the perforated siding. If so what do I change it to. also once it is sealed off. there is now a dead space between the now sealed off eave and the new barrier yet to be determine on the inside of the building. will that dead space trap moisture.? Or do I not change the perforated material and allow ventilation to reach the new insulation that is yet to be determined. What will happen.

4. I will be spray foam the ceiling however I read that if there is a roof problem and the sheathing needs to be replace that rips out the spray foam. I was going to place a foam board ( which do you suggest XPS, polysio etc) on the undersurface of the roof deck adjacent to it. Then I was going to spray foam on top of the foam board. and then use batts , (Flash and Batt) finally foam board on top of the rafters sealed to prevent thermal bridging. (no can lights)

None of builders know what I am talking about so I have to guide them through this process.

Also you would be very interested that this is will be first house in florida using the LED light with cat 5 wiring instead of copper with brighter and energy efficient LEDs This is a new product that I am using.

Also I am going off the grid using a MAG drive for energy. I really think you would be very interested in this product. See at American Independent

I hope that it is not too confusing, but I your input on this project. I am all alone on this. Thank you for your time

David Arango

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  1. GBA Editor
    Martin Holladay | | #1

    Celotex is the name of a company that was restructured after it had to pay out large sums in an asbestos settlement. It used to make lots of products. I have no idea whether you are talking about Celotex fiberboard (a type of sheathing) or Celotex polyisocyanurate (a type of rigid foam insulation).

    Assuming you can describe the product, it would be interesting to know what it is.

    Is the Celotex material on the interior of the concrete block walls or the exterior of the concrete block walls?

  2. GBA Editor
    Martin Holladay | | #2

    Some of your questions -- for example, how to prevent air from entering at the soffits -- are basic carpentry questions. This raises a few questions:

    1. Will you be doing the work yourself?

    2. Do you have carpentry experience or construction experience?

  3. Expert Member
    Dana Dorsett | | #3

    Celotex fiberboard is sometimes given a bad rap as sheathing on framed walls with fiber insulation in the cavities in air conditioned houses. The high vapor permeance of Celotex allows more moisture diffusion into the wall cavities, and the insulation can make the wallboard substantially cooler, allowing more moisture to build up inside the wall cavity. It gets even worse when vinyl or foil wallpaper is used, which effectively blocks drying toward the interior. The mold risk is in the paper facers of the wallboard, and on the studs, never the Celotex. Interior paint failures are also common with Celtotex sheathed insulated studwalls due to that moisture drive.

    I assume your current stackup is:

    great outdoors| exterior paint | CMU wall | Celotex | furring | wallboard | interior paint | conditioned space air ????

    That stackup works OK since the air-gap between the Celotex & wallboard created by the furring isn't very insulating, and the wallboard stays fairly warm on hot days, and with latex interior paint it can still dry toward the interior. But even 3/4" Celotex is only good for about R2.

    Orlando is US climate zone 2, and CMU has sufficient thermal mass to meet the code definitions of "mass wall". IRC 2015 code minimum for mass walls in zone 2 is R4 continuous insulation, if at least half of the insulation is on the exterior side of the thermal mass, R6 if more than half the R is on the interior side of the thermal mass. That means that with the Celotex left in place you can still hit code min with as little as 3/4" of EPS on the exterior with half-inch Celotex, or 1/2" of exterior EPS if it's 3/4" Celotex. If insulating on the interior it would have to be at least and inch of EPS added to the Celotex, or stripping the Celotex and adding at least 1" of foil-faced polyiso or 1.5" of EPS.

    The better solution is to insulate on the exterior, since that keeps the CMU and cellulose drier too. Putting foam on the interior side reduces drying toward the interior, which raises the moisture content of the CMU making it more prone to exterior side paint failure, and possibly compromising structural wood that is in direct contact with the CMU (such as the sill board where the rafters rest.) There are numerous stucco and faux stucco finishes designed to be applied to an EPS substrate, and I would expect contractors in FL would be familiar with how to apply those. It's key to deal with the window flashing/drainage details when adding foam on the exterior of the CMU. One approach is to use a spray applied weather resistant barrier and polystyrene foam that's designed to drain (usually XPS rather than EPS, which is another subject).

    With 2x12s you have 11.25" of cavity space to work with without impinging on the cathedralized ceiling. That's enough to hit a code-min R38, leaving the roof ventilation in place. The air gap needs to be a minimum of 1", so that leaves 10.25". With strips of 1" cut foam board tacked/glued to the underside of the roof deck to preserve the air gap, and a 1" sheet of foil-faced polyiso (R6) cut'n'cobbled, glued/caulked to the strips, you then have 9.25" of space left for the remaining (R38 - R6 =) R32 needed to meet code. Compressing a low-density R38 (that performs at R37 in an 11.25" 2x12 cavity) down to 9.25 yields R35, and you're there with R3 to spare:

    The foil facer on the cut'n'cobbled polyiso gives it a very real performance boost under a hot roof deck, and another ~R1 even under a cool roof deck. It also is sufficient R-value on the exterior of the R35-ish compressed batt for wintertime dew point control. Unfaced batts would still stay dry. (Do NOT use foil faced batts here. Kraft facers would be fine, but not necessary.)

    If you want to give the roof deck even more ventilation space (I would) 1.5" or 2" spacers can be used, and you would still get the minimum R32 or better out of an R38 compressed to 8.75" or 8.25". But 8.75" & 8 .25" aren't standard lumber dimensions found in the charts, so you'd be forcing the inspector to do some arithmetic. The easy argument is to point out that according to manufacturers' compression charts at 7.25" (2x8 timber) an R38 performs at R30, which is R4.1 per inch, and at 9.25" ( 2x10 timber) it's R35, or R3.8 per inch, so the additional inch to go to 8.25" would be at LEAST another R2 to bring it up to between R32 and R33, even with the 2" vent space allowance.

    Since foam board will warp a bit under the compression, apply the spacer strips next to the rafters on each side but another spacer strip at the mid-way between to keep it from bowing by very much. With a bit of practice, with a 4" or 5" wallboard taping knife sharpened on it's sides it's pretty easy to cut even 2" foil faced polyiso into 1" wide strips. See:

    For ease of installation, cut the foam chute/barrier panels 3/8"-1/2" narrower than the space between the rafters, tack it in place with foam board adhesive, tape the seams between panels with a decent quality temperature rated aluminum tape, then seal the edges of the foam to the rafters with can foam.

  4. darango | | #4

    Dear Martin

    I am not doing the worok myself. THe celoex is a foam board product. I do have an architect but does not know about closed envelope house. Secondarily i have a contractor but also does not know about closed envelope systems. therefore I am the lead on the project instructing them on the project. They can do anything I want but the closed envelope its new to them


  5. GBA Editor
    Martin Holladay | | #5

    I'm not sure what you mean by "closed envelope." Can you define "closed envelope"? What are you trying to achieve?

  6. Expert Member
    Dana Dorsett | | #6

    How thick is the Celotex foam board? Does it have a shiny aluminum facer, or something else?

  7. GBA Editor
    Martin Holladay | | #7

    So it sounds like the product you are calling Celotex is a brand of polyiso insulation. Is it on the interior of your concrete block walls or the exterior?

  8. user-2310254 | | #8

    Maybe Dave means "unvented."


    What is a MAG drive? Is it a kinetic or flywheel arrangement? It is worrisome that the vendor has very little technical detail on its website.

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