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What is the best option for an existing home in Baton Rouge, LA with regard to sealing the ventilated attic or installing a radiant barrier?

Wynn White | Posted in Energy Efficiency and Durability on

The attic is now ventilated.

And if sealing the attic is the recommendation, which foam (open or closed cell)?

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Replies

  1. Andy Ault, CLC | | #1

    1) Do you have any mechanical equipment in the space?
    2) What is your roof type and age?
    3) What is the roof framing material / dimensions?

  2. Wynn White | | #2

    2 water heaters (gas) and duct work. I will change out water heaters if need be.

    The roof is asphalt shingle 2 years old.

    The roof is 8 on 12 pitch rafter construction. The house is about 3400 sf heated (and cooled).

  3. Andy Ault, CLC | | #3

    If the water heaters and duct work have stay in the attic, then making that space part of the conditioned envelope would be your best bet. However, if it’s possible to move the heaters (as you suggested) and ALSO move the duct work, then that would be the most desirable option. The question is which of the two is the most budget friendly and aesthetically acceptable for you?

    If everything stays where it is, then you’re looking at making the attic part of the conditioned envelope by sealing it off from the exterior climate. As you alluded to in your question, spray foams (SPFs) are the most often referred to method for doing this. As I’ve written here and elsewhere, I am NOT a fan of SPF’s in general, but especially when it comes to existing occupied homes where the owners will be back inside with days, or even hours, of the application. Instead, we’ll use polyiso board adhered to the underside of the roof deck and then air-sealed at the edges with either can foam or preferably Tremco sealant. This achieves similar insulating values without mixing and spraying highly-potent chemicals in your home.

    The downside to this approach is the inability to detect roof leaks, but since your roof is only 2 years old, this may not be a concern in your mind. The other possible downside is the release of pretty nasty chemicals in the event of a fire, but then again, if you’re roof is on fire perhaps you’ve got bigger concerns (assuming the fumes from the dioxins and cyanides don’t overtake you first).

    So with all of those concerns (chemicals, fires, water leaks, etc.) you can see why I’d first say to move the water heaters and duct work instead. You’ve already said the water heaters are an option, so I’ll assume you’ve got the part figured out. The tougher part many people have trouble wrapping their brain around is how to move the duct work inside. It’s actually not too difficult with a little planning.

    You can use a high-velocity, small diameter duct system from a company like Unico Systems Then you can plan the duct layout to follow the perimeter of the home which allows you to easily conceal it within small bulkheads in the top corners of the ceiling/wall intersection. These may seem complicated or expensive at first, but a little new drywall and a bit of ductwork are often an even trade, and sometimes actually cost less, when compared to the cost of foam.

    This approach will have you ductwork and your water heaters performing far more efficiently; will allow you to avoid all of the “sealed attic” concerns; you can ventilate your roof normally; and you can use non-toxic insulation in your attic. It really is a win-win all around if you can make it happen.

    The last option would be to move the heaters, bury the duct work under piles of non-toxic loose-fill insulation (after doing proper air-sealing at the ceiling plane) and seal the duct work really well from the interior with an aerosol sealing system like Aero Seal ... but only after having them professionally cleaned by a company which is a member of the National Air Duct Cleaners Association (i.e. not some hack with a shop vac and some duct tape offering to do the whole house for $79!).

  4. Wynn White | | #4

    Andy,

    I'll leave the water heaters in the attic--and the ductwork. So sealing the bottom of the roof deck is the option I'm looking at.

    With the polyiso board, how do you seal the attic at the eaves? And, does it come with a fire/smoke barrier?

  5. GBA Editor
    Martin Holladay | | #5

    Wynn White,
    Most insulation contractors would recommend closed-cell spray polyurethane foam, not rigid polyisocyanurate, to insulate your roof sheathing from below.

    Spray foam should be protected by a thermal barrier; gypsum wallboard is best, although some building inspectors accept intumescent coatings, which are controversial.

    Regardless of which type of insulation you choose, you need to perform air sealing work at the perimeter of your attic near the eaves. This can be done with rigid foam board and caulk; however, spray foam makes the job much easier.

    If you insulate under your roof sheathing, you'll have to decided whether to install ventilation chutes between your rafters before insulating. If you want ventilation along with spray foam, be sure to build sturdy site-built ventilation chutes that can resist the pressure of expanding foam.

  6. Michael Blasnik | | #6

    I'd like to go against grain here. I don't think you should try to make it a conditioned attic -- it would be very costly and could easily result in greater energy use than simply sealing and insulating the existing ceiling, sealing and insulating the duct work and, perhaps installing a radiant barrier on the underside of the roof.

    You have two water heaters in that space which, if you make it conditioned, will add to cooling loads from their standby losses while also performing at lower efficiency making hot water. You will also need to pay extra attention to properly venting them.

    Instead, you can reduce duct losses to a reasonable level by sealing and insulating them and air sealing and insulating at the ceiling plane can perform at least as well as what you could do on the roof deck and at just a fraction of the cost. I would guess that this latter option will cost maybe one quarter as much as the conditioned attic option and save more energy as well.

    I think conditioned attics have become far too trendy -- especially for retrofits -- given their high cost and questionable performance.

  7. Scott Van Kerkhove, TheEnergyGroup.org | | #7

    Wynn...Hope all is well with you. This topic is one that seems to bring a bevy of "opinions" to the surface. With all due respect to some of the answers that you may get, experience and energy modeling tell us that an unvented attic is most efficient in hot/humid climates. In ABSOLUTELY NO cases have I seen, nor do I believe that it is possible (from a thermodynamic standpoint) for a vented attic to perform more efficiently than an unvented attic when ducts are located in the attic.

    That being said, there are cost and safety considerations involved, and those must be evaluated in each case. The best method of evaluating the feasibility of these scenarios is to have a certified energy auditor take a look at your existing structure and model some scenarios in black and white.

    Please keep in mind that Louisiana has a cash rebate program in place to help pay for these types of energy efficiency upgrades. You have my contact info from the seminar that we hosted last year. Call me and I'd be happy to assist.

  8. GBA Editor
    Martin Holladay | | #8

    Wynn,
    In case you missed it, here's an article with a list of questions that you have to consider when contemplating a conditioned attic: Creating a Conditioned Attic.

  9. Wynn White | | #9

    Martin, Thanks for pointing out the article--I missed it.

    I'm still on the fence on what I'll do. If it were new construction, there is no doubt--I'd enclose the attic.

    Scott, thanks for the email. I'll call you before I move forward.

    Wynn

  10. John Brooks | | #10

    I think
    Michael Blasnik's comments make good sense

  11. Danny Kelly | | #11

    I too agree with Michael Blasnik.
    I would like to see an auditor show a quicker or better ROI or payback period than a good air sealing job and added insulation vs. a properly installed unvented roof. The key here is properly installed - enough foam to meet the required R-Value and has the proper ignition barrier. There are too many foam companies out there merely installing 4"-5" of foam and not installing the required ignition barrier to keep their price down.
    If your HVAC is in the attic, a radiant barrier will help in a hot climate but only AFTER air sealing and adding insulation at the attic level.

  12. Michael Blasnik | | #12

    Scott Van Kerkhove said

    In ABSOLUTELY NO cases have I seen, nor do I believe that it is possible (from a thermodynamic standpoint) for a vented attic to perform more efficiently than an unvented attic when ducts are located in the attic.

    I guess I must have been out sick the day they explained this in my thermodynamics class (although I think heat transfer was actually the subject that would cover this).

    I think it's pretty easy to create a scenario where the conditioned attic is less efficient. The unconditioned attic could have well sealed R-8 ducts (you go even higher than R-8 given the money saved by not using foam) with an R38 ceiling that is very tight and a radiant barrier on the underside of the roof. That attic would be pretty efficient and the duct losses would likely be maybe 10% of the load. The conditioned attic would typically involve insulating the roof and gable ends with maybe R-20 or R-25 foam with an area maybe about 40% larger than the ceiling area -- providing an assembly UA more than doubel that of the R38 eiling. In addition, the insulated roof deck will experience a larger dT than the ceiling insulation would in a vented attic with a radiant barrier. The net result would be somewhere around 3x the heat gain for the conditioned attic than the unconditioned one. Depending on the efficiency of the rest of the home, that added heat gain could easily offset the duct losses. I haven't even added in the extra heat gain from the 2 gas water heaters that should be included in the conditioned attic case.

    There is certainly no law of thermodynamics that will make it impossible for the unconditioned attic to be more efficient. When you consider the cost differential, you could go to great lengths to improve the existing duct system and ceiling plane and still have a few thousand dollars left over compared to a conditioned attic retrofit.

    By the way, it sounds disingenuous at best -- or perhaps just sarcastic - to say "with all due respect" right after you write about other peoples opinions and place quotes around the word "opinions" .

  13. Riversong | | #13

    I also agree with Michael Blasnik on this, and Wynn's case is a perfect example of when the conventional "wisdom" (intentionally put in quotes) becomes counterproductive.

    And Danny Kelly's quite right about the relative ROI of a hot roof retrofit vs improving existing air sealing, duct sealing and ceiling insulation (plus, as Micheal points out, adding a radiant barrier).

    It's disappointing that Scott Van Kerkhove, of The Energy Group - a firm that claims to be "a leading systems integrator in sustainable, renewable & energy efficiency solutions...helping clients to prioritize...and implement a broad range of energy projects that reduce dependency on fossil fuels... and enables clients to maximize ROI today!" - would not understand that there is not a one-size-fits-all solution to warm climate retrofits, and that themodynamics operates in two directions (gains and losses).

    Return on investment (ROI) requires that both sides of the equation be optimized - minimal investment for maximum savings - and that "savings" include environmental externalities as well as financial cost.

  14. Wynn White | | #14

    Everyone, thanks for your input. The question generated a great discussion.

    I'm leaning towards getting two proposals (from different contractors). A and B (outlined below)

    A. Seal & insulate my ducts better
    Seal the ceiling (air)
    Duct my returns (through the wall to the ahus)
    Blow more fiberglass in attic (make sure I have adequate thickness and coverage)
    And tack foil to the underside of the rafters

    B. foam (with fire/smoke barrier applied) roof line (and soffits/gables)
    Replace water heaters (and furnaces) with sealed combustion units.

    What makes this super important to me is that it is my house!

  15. Riversong | | #15

    Wynn,

    My only suggested alteration to your proposal is to blow cellulose over the existing fiberglass.

    Loose-fill fiberglass is notoriously vulnerable to convection (as well as rodents), and even a few inches of cellulose on top of fiberglass has been demonstrated to dramatically reduce air flow.

  16. John Brooks | | #16

    ...

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