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  1. #16
    Join Date
    Aug 2004
    Posts
    53

    Default Re: Air Sealing Vented Attic in New Construction

    My point exactly. Do the air sealing from below. We use foam sheeting in conjunction with airtight drywall and find it very effective and economical plus it provides a thermal break. In blower door testing of our last house the tech told us that it was the tightest house he ever tested and to his knowledge was the tightest tested in the state under the Energy Star program. No air in, no air out. Keep it simple. I'm always amazed at how things become more and more complicated when the simple solution is at hand. Perhaps some builders are looking for the "magic" solution that doesn't take much work. Good luck.

  2. #17
    Join Date
    Nov 2004
    Location
    Boston, MA
    Posts
    4,612

    Default Re: Air Sealing Vented Attic in New Construction

    Bruce

    What state are you in?
    I will never buy a Nissan Cargo Van.

  3. #18
    Join Date
    Jun 2004
    Location
    St Louis, Mo for the past 25 years
    Posts
    7,327

    Default Re: Air Sealing Vented Attic in New Construction

    Bruce can you elaborate a little more?
    Asking because I had thought about doing just that but ran into other things that concerned me and I have not researched. One had to do with finding electrical boxes that could be used that were that deep or we had to know exactly what we were doing so boxes were set at the right height.
    Also are you then hanging the drywall thru the foam with a longer screw or installing a strapping over the foam to fasten the drywall to.

    I am asking because we often work on what we call flats. These are 100 year old structures with a basic crawl space above the ceiling that usually tapers from 24 inches at the front of the building to about 8 at the back. A hot mop tar roof is installed as the roof of choice. So there is not a lot of room for insulation much less a way to install more. I have often thought that installing some form of insulation would be beneficial but never really figured out a good way to do so.

  4. #19
    Join Date
    Aug 2004
    Posts
    53

    Default Re: Air Sealing Vented Attic in New Construction

    My country is the world and my religion is to do good. Thomas Paine
    Quote Originally Posted by Ted S. View Post
    Bruce

    What state are you in?
    I work in Wisconsin (state of confusion)

  5. #20
    Join Date
    Aug 2004
    Posts
    53

    Default Re: Air Sealing Vented Attic in New Construction

    Beezo:
    When we do this on new const., we know in advance where the boxes need to be located. In remodels we either have the boxes reset or add extensions. Very easy and straightforward. We do use longer screws to hang the drywall. If we use 1" foam we use a 3" screw. Kind of a pain, but no pain, no gain. Really not a big issue. FYI, I build my own house in 2008 using this method. The drywall is still perfect, not even one crack and I can heat the house with my crockpot. Next time I build one for myself, I might go with 1&1/2" foam. We make sure that all the foam is airtight; joints and perimeters. Nothing can compare with doing things on your own house and then being able to monitor how effective they are over an extended period of time. We have used this method on restorations with similar positive results. I really like rigid foam for effective results and economy. We'd use spray foam a lot more, but it's usually outside the budget. Maybe that will change.

  6. #21
    Join Date
    Aug 2013
    Location
    KS
    Posts
    323

    Default Re: Air Sealing Vented Attic in New Construction

    Problems I see with foam sandwiched between drywall, insulation, and wood which shifts the dew point from the center of cavity to the foam. If the attic vents/holes are not balanced properly which is common, or create depressurization, the foam is not ventilated to outside air and trapped it can create a breeding ground for moisture, mold, and mildew, along with OSB/plywood. Foam does not dry as fast as insulation especially high density (2 PCF), so if you trap it in and do not ventilate it won’t sustain and can potentially degrade the insulation at the interface. If you don’t install the insulation tight to blocking there is a potential for thermal bridging and a reduction in whole R-value since HD foam is thermally conductive. Sealing all the plates, hatches and doors can be labor intensive. Rodents and insects can also breach the barrier. On the inside, if the drywall is not sealed a moisture path to the foam, again creates or traps moisture that can make its way back into living space and cause health issues. 2” of foam this hybrid design does not provide enough thermal mass to call it an effective thermal break, prevent freezing, or bridging especially at the lid, vented attic. Also, having to rely on the trades consistently sealing the penetrations and gaps is a turn off to me, time consuming compared to other methods.

    There are coming code issues with spray foam. I don't know how valid the point will be until we have years of experience with the product, but lawyers are lining up for the class action suits coming due to off-gassing, and fire departments are getting involved over the toxicity of the product when burning. Fire retardants and other air born chemicals are an issue as Dick mentioned.

    A blower door test at installation can be misleading if the design does not sustain it, look at the cavity thermodynamics, especially in hot humid or cold wet climate zones.

    See attached, rigid foam 4-6" walls in a REMOTE system, sealing the lid with drywall and a vapor seal makes more sense. The dew point is moved out to the exterior foam wall mass and not allowed in the addict, all the drying to the heated interior. You don't have to add insulation to the wall cavities. The best time to do a blower door test when the foam board, exterior membrane, ceiling vapor barrier, windows and doors are all in place and the ceiling has been sheetrocked. Ideally any plumbing and wiring penetrations will also be in place. The specific locations of air leaks can be easily identified and remedied.

    http://www.cchrc.org/docs/best_pract...OTE_Manual.pdf

    SCIP or ICF wall-roof monolithic makes more sense but I have not got that one figured out yet, working on it.
    Attached Images Attached Images
    Last edited by CASHCOW; 12-22-2013 at 01:33 PM.
    Terry

  7. #22
    Join Date
    Jun 2004
    Location
    St Louis, Mo for the past 25 years
    Posts
    7,327

    Default Re: Air Sealing Vented Attic in New Construction

    Bruce,
    I would be in the remodel area of work. Most folks would prefer to leave what is in place if possible. So for some of my work it would be try to leave the existing plaster or drywall and the existing insulation in the attic. So are remodels are you installing the foam over the existing ceiling or taking it down and then starting over. Have a concern about the additional cost of taking down versus leaving plus it up. Balance that with will it work as well......

  8. #23
    Join Date
    Aug 2004
    Posts
    53

    Default Re: Air Sealing Vented Attic in New Construction

    Hi Beezo,

    We have done remodels using foam. As with you, we try to avoid tearing out the existing ceiling for several reasons:
    1) To keep the existing attic insulation from falling down and filling our orifices with crud. Of course, there are always space suits.
    2) Cleanup and disposal costs to remove plaster/drywall and the aforementioned insulation.
    3) laziness
    4) Trying to make enough money to survive

    What we normally do is to screw 1x4 x3/4" furring strips to the existing ceiling to secure the plaster/drywall (which in many cases has sagged) Then we attach 1" or 3/4" foam with Plasticap nails. On one job, we used 1&1/2" foam at the owners direction. Then we drywall as normal making sure that the perimeter is air sealed. Drywall finish is standard procedure followed by drywall primer and latex finish coat. Texture is optional.

    I know some of the guys on this forum have incredibly detailed technical knowledge about moisture, dew points, condensation, attic ventilation, etc. I'm not that smart, but I know that we have followed this procedure numerous times and NEVER had a problem. One caveat that I will mention: We NEVER do this unless the attic is properly vented. Just a very pragmatic approach based on many jobs.

  9. #24

    Default Re: Air Sealing Vented Attic in New Construction

    My partner and I built The second passive house in Wisconsin as well as the first to make Energy Star level 4. We seal attics by building what amounts to a "floor" framed of 2x6 sheeted with 1/2" osb and sealed with either Siga tape or 3M 8067. This LID (as we call it) is sealed to the wall sheeting on the edges. The trusses are then erected as normal. Now there is a complete and unbroken air barrier on the ceiling. This can be insulated (or not) as you prefer. It has the advantage of also giving you a "floor" to stand on while setting and sheeting the trusses. It also means can lights no longer penetrate the attic seal. Lastly, we do not have a "scuttle hole" any more-we instead create an access door on the gable end. We do this on all new construction now. Go to the website below and click on "current work" for more details. (sorry the site hasn't been updated in a while....

    http://www.kickapoogreen.org/DavidR.html

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