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Old 04-16-2010, 09:49 AM   #1
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Insulation Plan/Idea/Recommendations


Hello,
Doing a major rennovation, ripped out all the old insulation and did a bunch of work. At the point where I am Getting ready to insulate. The work is on the second floor and has a shed dormer on one side, with a Kneewall on the other side with storage eave (attic) space running the length on the other side of the kneewall. Heres my plan: I am having spray foam sprayed on the roof line only down to the kneewall to avoid venting. The attic/eave space will not heated. I am using fiberglass for the walls and the floor in the eave space. The plan is to use foamboard on the floor bays below the knee wall to prevent air and heat loss going between the two spaces. The floor joists are 2X8. Is that suffice for insulation? or what I was thinking was to put stips of wood on top of the floor joists (in the eave space) to allow for more insulation? I plan to put plywood down and use the space for storage and dont want the insulation to compress. How does that sound for a plan? I could get more insualtion with no compression. Also would I get faced or unfaced? Currently everything is out and it is down to the drywall ceiling of the first floor?

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Old 04-16-2010, 12:09 PM   #2
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If I understand you, you are not venting the upper half of the roof assembly, but you are approaching the eave portion of the roof assembly as one would a vented, non-conditioned attic. In theory, you would need to install a venting system for the eave portion. If you did that, then you would install a vapor barrier or retarder (paper faced ins., etc.) on the knee wall.

It might be more sensible to treat the entire roof the same way, applying spray foam along the full length of the rafters, although that will cost you more. Are the existing rafters deep enough to accommodate necessary insulation? When one converts to a vaulted ceiling, it's often necessary to fir out the rafters, just as you are contemplating approaching the floor system.

As for the batt thickness in the 2x8 bays, that probably depends on where you live. Typically, roof/ceiling insulation separating the heated areas of a house from non-conditioned attics is quite a bit thicker than that. You need to decide what R value you want, and then build to it.

Good luck.

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Old 04-17-2010, 06:30 AM   #3
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Hi

Personally I would not use spray foam on the underside of the roof, as moisture/condensation/roof leaks can lead to trapped moisture/saturated timber which can lead to timber to decay - when that happens you have got serious problems and the first you are likely to know about it is when the roof caves in or someone on the roof carrying out repairs falls through the roof.

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Old 04-17-2010, 11:39 AM   #4
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If you go to http://www.buildingscience.com/index_html and search for "roof assembly" you will see a range of general and climate specific research on how to design and build a healthy, long lasting and efficient roof.

There are several articles that specifically address vented vs non vented roofs.

Good luck,
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Old 04-17-2010, 11:50 AM   #5
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Thanks for the input, the cost was significant higher to do the whole roof so thats why I opted to do what I mentioned. I am planning on treating the eave portion as you mention. The eave already has vents on each end (gable vents maybe?) So I am only going to spray to the kneewall. Inside the eave space, there are the vents on each end and I am going to vapor barrierr the kneewall too. The plan anyhow is to vapor barrier the kneewall, fiberglass insulation and then maybe foam board on the eave side. So basically what I am trying to figure out is if I want to use this space for storage and lay plywood down, the insulation can not be compressed. There is living space below the eave storage and that needs to be insulated. With 2X8 joists I am limited too certain amount of R value. What I was thinking was to rip pieces, say 2" to put on top to basically give me a 2X10, that way I can put more insulation in and when the plywood gets installed it won't get compressed. Does this sound like a good plan? So I need to get faced insulation correct? Since below it is living space even though I can not staple it to anything. What are your thoughts?
On a side note, I am going with the foam for a few reasons. The roof has such a shallow pitch (shed dormer) and no soffit at all so it is a very hard roof to vent. people have tried in the past and failed. Causing moisture issues. A few people have suggested spray foam for this situation. When I looked into options, to insulate with fiberglass and install a roof venting system it is cheaper to go with the foam and it hasnt been done yet so I figured I would give it a whirl. Any thoughts or ideas are always apprieciated!!!
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Old 04-17-2010, 03:17 PM   #6
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Assuming you are in heating climate:

If you are going your route, you want to minimize moisture transfer from the conditioned areas into the non-conditioned eave storage, and you want to encourage air flow through the eave space so that moisture which does enter can escape to the out side. You also need to ensure that moisture in the eave space is not allowed to travel into the non-vented, foam insulated portion of the roof assembly.

Make sure any plumbing or electrical penetrations are sealed tight.

Isolate the eave portion of the ceiling ties from the portion that is under the living space, and isolate the eave portion of the roof rafters from the portion filled with foam above the living space. Use tight cut foam board and spray foam to do it.

Your firring idea will work fine. If you have good access, nail 1x3s on edge perpendicular to the joists. That ought to be quicker than cutting and installing the firring in line.

Leave large gaps when you nail down the plywood. Don't create a barrier by mistake there.

Use loose fill instead of batts in the floor joists. Use a vapor block primer on the ceiling under the eaves.

You can use faced batts on the eave knee wall, but many people use a poly sheet and unfaced batts.

Don't put a vapor barrier on the non-vented roof areas.

You may need a powered vent at the gable ends.

One last thought for a fully non-vented approach. The needed amount of insulation applied directly to the underside of the roof varies depending on where you live (it's all about average dew points). Spray foam is usually purchased by volume. If you are covering X sq ft to a depth of 6", 2X sq ft at 3" might cost the same. If 3" is adequate, you could fill the rest of the cavities with batt insulation. Just a thought.

Good luck.
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Old 04-17-2010, 04:11 PM   #7
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Rory Read has passed on some very good advice.

OP,look at the whole system to see what works together well.

What is the roof covering? deck etc.. conditions ...( existing and total changes you plan.. )

please don't design a system that will cook your roof covering... you will be sorry you did. "(a good plan today..is better than a headacke tomorrow)". ( sorry Geogre P......) sometimes it pays to think ahead.
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Old 04-17-2010, 04:49 PM   #8
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some things no one has mentioned. Air ceiling the attic floor must be done prior to insulating. Especially since you plan to use the worst insulation on the market. We call it filter-glass since it filters the dirt and mold and moisture more than it insulates unless you perfectly air seal everything. Blown cellulose will work much better and with the small joist spaces you can use dense-packed cellulose for both the floor and knee walls. Paper faced insulation does not last all that long on an exposed knee wall. This is another poor choice. The are above and below the knee wall need to be airsealed with a 2X8 block using spray foam to seal around the edges. If this is to be a finished conditioned space you need the local code minimum R-Value in the ceiling. this can best be done by installing the venting above the existing roof and adding 2" foam to the existing roof for insulation and air sealing. Research all the details on the points I presented and you can see the value of what is said.
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Old 04-17-2010, 05:03 PM   #9
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In Canada or the U.S.? For the poly on the walls: http://www.ecohomemagazine.com/energ...gy-claims.aspx

Be sure to seal the attic floor as R.R. said, otherwise extra moisture will be venting out the powered gable vent: http://www.ronhungarter.com/black_mold.html

Use a housewrap on the knee walls attic side. OR unfaced batts with the foam board.


"Since below it is living space even though I can not staple it to anything. What are your thoughts?" ----- It's fine: http://www.certainteed.com/additiona..._Retarders.pdf

If you are in a cold or very cold zone, I would use cellulose or other over glass batts. I would use rigid at the exterior wall: http://www.homeenergy.org/archive/he...96/961110.html

Be safe, Gary
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Old 04-19-2010, 09:48 AM   #10
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First off, let me just say thank you to everyone that has contributed info to my situation and post. A lot of good information has been given and made me open up to other ideas and possiblities. Tons of great advice. Ok, go easy on me as I am a very experienced and capable DIY'er, but like in everything I obiviously do not have the experience and knowledge to know all the in's and out's as someone who does it for a career. I do however, take a lot of pride in my work and truely want to do it right that is why I am here asking tons of questions. Which leads me to more questions.
Like I mentioned before, all the framing (rafters, joists, etc are all 2X6's. The Shed dormer on the house has no soffit and a very shallow pitch. Moisture always has been an issue because originally they jammed it with fiberglass batts and did not vent it which equals problems. Upon re-doing the space, I want to correct the problem and also make it as effiecent as possible in the mean time. That is what brought me to look into the foam. I can go with two different types of foam. The open cell will give me about an r-21 and the closed cell an R31. I planned to go with the open cell because the open cell is significantly cheaper but I am contemplating spending the extra money. As with anything, have to watch the budget and we are not planning on staying in the house forever either. For the walls, I was going to do unfaced batts with the poly vapor barrier. Now, in the eave space, I am pretty much open. It is about 7' depth and runs the length of the house but the roof slopes down to that point. There are rooms below it. We really want to use it for storage. That is are only attic space and with little ones coming eventually in the future, I know how valuable storage space is. I have no problem using the loose blown in insualtion if everyone says it is that much better and effiecient. As long as we can throw plywood down, use the space and not have the insulation be interferring and messy. Is it more effiecent? What is the R value? In this space what I was planning on doing was cutting foamboard to fit in between the rater bays below the kneewall and then spray foaming the gaps. That will seal the space below the kneewall. I can isulate the eave space to that point, anything on the other side of that won't be insualted because those are the two upstairs rooms. I will then i address the knee wall and the company is going to foam the rafters down to the kneewall. So back to the eave, The floor joists are also 2X6's. I was going to put strips of wood to get it close to a 2X10 to allow for an r-30(in I was using batts) in the eave space. Is the dense blown in more efficient? Could I still be able to use that space with the loose insulation? On a side note, I read an article on a guy who used the sray foam cans to seal all the exterior boards, before he installed the batts, is that a good idea or is it not recommended or overkill? I have boards for sheathing not plywood. oh, and for those that asked I am in a heating zone, in Mass. Anything more input or advice is always apprieciated.
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Old 04-19-2010, 08:33 PM   #11
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GBR,
great article, so is it beneficial to put foam board over the exterior wall and then put the insulation over that?
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Old 04-19-2010, 09:54 PM   #12
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This: "Insulating Kneewalls It is important to install a minimum insulation of R-19 and to provide fastening other than just stapling when adding fiberglass batt insulation to a kneewall. The problem is that the kneewall is open on the cold (out) side because the kraft paper backing that comes with insulation must be installed on the warm (in) side of the insulation. After a few years, the glue holding the paper on loses its adhesiveness and as a result the insulation can, and often does, fall down. The insulation will stay in place much longer if you use plastic mesh, wires or twine stapled in place. Pennsylvania energy consultant Linda Wigington recommends a method that is better still. Her approach is to cover the entire cold (out) side of the insulation with house wrap, another layer of batt insulation, or rigid insulation. This holds kneewall insulation in place and protects it from wind intrusion, which can seriously degrade the R-value. House wrap is particularly important for kneewalls because these vertical walls are open to the attic on the outside and exposed to wind blowing in through soffit vents." From that: http://www.homeenergy.org/archive/he...95/950309.html


Double your trouble: http://www.simplesavings.coop/simple...ee%20walls.pdf


Use boards with the air gap to let the moisture rise up and out the vents rather than deposit on the bottom of the OSB or plywood acting as a secondary vapor retarder there.


Be safe, Gary
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Old 04-19-2010, 11:22 PM   #13
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another way is to use tyvek on the inside of the knee wall and a poly vapor barrier on the warm side. Slice a small hole and fill with cellulose, then seal. In any case as you can see fiber glass is a poor choice.
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Old 04-20-2010, 08:31 AM   #14
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The common factor I see in everyones response is that fiberglass is not the best idea. So lets say that I go with blown in in the kneewall. How does everything else sound you a plan? Would it be wise to spend more and upgrade the spray foam from roughly R-21 to R-31? Is the R21 to low of an R value?
If I go with the blown in in the attic space would I still be able to use the space (for storage). I would leave a large space between plywood sheets. How does evrything sound?
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Old 04-20-2010, 11:43 AM   #15
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you can use the blown and compress it with the plywood for storage. Will not lose the insulation value when doing this like you would with the fiber glass. For the roof you need more insulation....

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