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To vent or not to vent, that is the question

Natur Haus | Posted in Energy Efficiency and Durability on

O.K., I don’t want to be controversial, but in lieu of a recent high priced truss bid, I am thinking of constructing a roof with TJI rafters and am thinking of not venting it. The ceiling will of course be cathedral, the joist rafters will be 16″ tall, and I had planned on using plastic accuvent baffles continuos up the roof to create the venting channel, dense pack cellulose below. I do plan on a vapor barrier, ADA approach, or at least very fastidious air sealing…..Any thoughts, please share.

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  1. user-723121 | | #1


    I would create a vent channel by installing fiberboard sheathing to the bottom of the top flange. If you feel the dense pack will create a lot of pressure use plywood instead. I would vent the eave and ridge, each rafter has it's own vent space. I have used this system countless times with great results. The plastic vents will get crushed by the blown insulation.

  2. Natur Haus | | #2

    I am considering cavity fill, which will supposedly be a little less dense am may not crush accuvent baffles. When I handled the accuvents, I felt like the corrugations could resist 3.5 lbs/ cuft, but I could be wrong. Any experience or thoughts on the "cavity" fill?

  3. 9KwbwJssVn | | #3

    You could also use 1.5" or 2" foam in lieu of fiberboard or plywood since that will resist dense pack and add to the r-value as well. Have done so successfully before. Otherwise consider using laminated insulation/sheathing that has the vent channels built in, place it on top of your I-joists and maybe you can even cut back on I-joist depth, or take advantage of the thicker insulation and get a full thermal break.

  4. Natur Haus | | #4

    Thanks for the responses, I am also wondering about the plastic vent channel creating a double vapor barrier between it and the vapor tight drywall(sandwiching cellulose), and between it and the metal roof(sandwiching plywood). Should I really be concerned about this?

  5. user-940291 | | #5

    I think Doug and Abe nailed the vent channel built from the top-chord of the I-joist with foam.

    Am I the only one surprised that an I-joist roof is being considered as the less expensive alternative? Taking into consideration mandatory structural ridge beams, hangers, added posts and footings, web stiffeners, rafter tails, labor, labor, and more labor, I'd think that trusses are going to come out ahead on costs. I can't say say for certain without seeing the design or perhaps we're only talking about the cathedral ceiling area and not the whole roof but I'm surprised.

  6. Natur Haus | | #6

    The pertinent rake walls and footings below are definite considerations, but with the I- joists, hangers (webs not required) and needed gluelams, I am still around $4,000 vs. $7,000 for the parallel chord trusses. Also, the 18" parallel chord trusses had much more deflection potential than even the smallest series I-joist @16", so I thought this was important to consider as well. I like the foam idea, but it is definitely cost prohibitive, even tough it is adding insulation. The rafter tails were going to implemented even with the trusses. My other option was to use scissor trusses, but that severely lowered both inside and outside pitches, this due to the fact that minimum allowed slope for blowing in cellulose is 2 in 12 pitch. I hear you though Matt, maybe I am choosing to ignore or am not thinking thoroughly through how much labor this process will end up taking. It is counterintuitive that trusses would be out priced by any cut roof, but it seems so in this case. I think the devil could be lurking in the details of the rake wall construction and the miscellaneous added details you mentioned Matt . I appreciate your Questioning of this, potentially saving my ass.......

  7. user-940291 | | #7

    That's what a community is for; that's why we post. Glad to be of help.

  8. Natur Haus | | #8

    The devil is apparently lurking in the construction bank loan appraisal process, where green features are assigned no added value. This project is being held up by one person who represents a system that does not comprehend the tangible values of saving energy and conserving resources. Please share, I need some camaraderie right about now....

  9. gusfhb | | #9

    What type of bank are you using?

    Small local banks tend to have more flexibility than the monster banks. They may like having their sign in front of a green project
    [edit] and shopping for a friendly appraiser is not unheard of,. done it myself.....

  10. Natur Haus | | #10

    Keith, the bank the clients chose is a small local bank, and as I understand the new appraisal laws, banks cannot choose the appraiser, the process has to be a random rotation. Someone told me we could have not gotten a worse appraiser. I hope we can get this through?!??!?...........

  11. gusfhb | | #11

    I don't think they ever could[well 20 years say] I had a different issue a few years back and was able to have a different appraiser re do it. Now appraisers are pretty paranoid now. but I would go so far as to say that if you found an appraiser who specialized in green houses or at least had a good working knowledge that you might be able to get them on the banks list[again, not something that BOA would do] which my bank did for attorneys for me. This may no longer be possible, but I think if you found such a person[do they exist?] the conversation would be worthwhile

    Another issue might be if the bank intends to sell the loan or not. If the bank intends to keep the loan in house it may affect what they are willing to do. My bank keeps certain loans in house[15 year fixed and errr something else] I would think most construction loans would be kept in house.

    [I looked up the new rules, and mostly it sounds similar to rules in place here in the People's Republic of Mass and it does not look like there would be anything preventing a buyer from selecting an appraiser, only the realtors and loan brokers are verboten]

    At the end of the day a 3 BR 2BA house on 1 acre in zip xxxxx is going to be worth whatever the 3 comps the appraiser finds are.

    I do not believe any appraiser is a 'god' in a transaction, but you may have the best luck comparing your proposed house vs other comps in the area and figuring out the best way to move it up. IE 4 BR are worth a ton more than three, maybe that loft becomes a bedroom, or maybe that powder room gets a shower, or maybe a screen porch is worth more than a deck or ,.......

  12. BobHr | | #12

    Mathew and Keith

    The problem may not be with the appraiser. Many MLS systems provide no information on energy efficiency. The appraisers job is to reflect value placed on real estate by the market. If there is a lack of data the appraiser will be unable to support value added. A lot of good appraisers have left the residential lending field leaving crappy appraisers willing t o work for peanut.

    What you need to do is see if there are any appraisers that specialize in energy efficienct homes. Check with appraisal organizations such as the appraisal institute. Hopefully a specialized appraiser would know what lenders can make the loan.

    You may also want to go to Post their seeking thier advice.

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