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Trying to make an unvented roof assembly that will work?
Last Post 16 Oct 2009 08:57 AM by Dana1. 39 Replies.
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aeridyne
New Member
Posts:60
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| 29 Aug 2009 07:18 PM |
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I'm trying to wrap my head around creating an unvented roof assembly that will work properly so I don't get condensation problems etc.
Thus far after doing a lot of reading, the first assembly that I came up with was;
Start with 2x12 rafters - Sheath that - Blow in cellulose into these cavities for 11.5" of cellulose - Use 2" xps over the first layer of sheating, and go over that with another layer of sheathing and put roofing materials on top layer of sheathing.
But then I was thinking, if I used 2" of xps, it might be more difficult to build, I'm not a builder by a longshot, and I'm trying to figure out how to build everything on this house myself, so simplicity is a major factor by the way.
I thought maybe 1.5" of xps might be easier to build because I can use standard 2x4s laid flat along all the edges and for purlins in between the sheets to walk on. However I wondered if these might create too much of a thermal bridge and aid in potential condensation, making it a bad idea and thus I should try and leave these out and walk carefully on the foam?
I live in Flint Michigan, which I know is a zone 5 for climate.
I thought about using 1.5" of xps on the top of the roof deck, and then using 1" of sprayed foam on the underside of the lower layer of sheathing too, but I'm not sure how much that would cost, if it would be too much? I would have 1404 sq. ft to my main roof, and another 300 of so feet that would need the same treatment.
I've been reading every article I can find on buliding science and other areas, and I've read lots of Dana1's responses to people's questions (I really hope Dana chimes in! lol) but I'm still having difficulty figuring out if it would work, or if the spray foam would cost too much, etc.
All help is greatly appreciated! |
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aeridyne
New Member
Posts:60
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| 30 Aug 2009 10:10 PM |
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So I did a bunch of research all day long, again I've come up with some saying this, and others saying that.
I was thinking that if I only used 2" of xps between the layers of sheathing, and a good roofing underlayment such as titanium udl-30, it might be ok, however I think this might only address the potential issue of vapor drive into the roof assembly via solar radiation, and I'm still not sure if it would be enought to stop potential condensation on the warm side of the lower layer of sheathing? |
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Jimmery
New Member
Posts:25
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| 31 Aug 2009 06:19 PM |
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Why don't you want venting? A few months ago I was in the same position because I wanted to do a complete cavity fill with dense pack cellulose. I hear that no air space or venting is required, since the dense pack cellulose prevents air leakage from inside the house into the roof, and therefore moisture within the air will not be able to travel into the roof to require venting.
But since my building inspector requires a 2" airspace above the insulation, I have a solution that may also suit your purposes. I am considering doing a dense-pack cellulose in between TJIs in the roof, with OSB sheathing above this. To create the necessary air apace, I would be strapping above the OSB, and adding a second layer of OSB sheathing above this. From inside to outside, the cathedral ceiling roof is as follows: drywall, vapour barrier, TJIs and dense pack cellulose, OSB sheath, strapping for air space, OSB sheath, and shingles.
Would such a system work for you? |
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kjseuferer
New Member
Posts:17
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| 01 Sep 2009 10:54 AM |
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Aeridyne,
By the time you go to all that trouble and expense with all those materials, why not just build a standard roof and blow 2 pound closed cell foam against it? You will be done, have the best roof you can possibly get and you will not have all the hassle you are talking about. Not only does the closed cell foam provide a vapor barrier and insulation, but it acts like glue to strengthen your roof structure by up to 4 times what it was. You will also get air infiltration sealing that is unmatched by pretty much anything else. |
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The Sipper
Basic Member
Posts:232
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| 03 Sep 2009 12:38 PM |
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SIP roofs don't require venting, even fewer "hassles" than typical framing, and blown in foam, with minimal thermal breaks, if any. |
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| The Sipper |
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aeridyne
New Member
Posts:60
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| 03 Sep 2009 01:02 PM |
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I had thought about doing a double deck over the foam for a ventilation space, but after a lot of reading I think I can get away without one, do builders normally create a ventilation space over SIPs when used for roof decks?
The problem with spray foam is cost, as far as I know it's something like 1.50 a board foot, which is absolutely ludicrously out of reach for me, at just 4" deep were talking like 35,000 dollars, (1404 sq ft main roof + 264 back room + 390 garage + vertical walls) who the heck can afford that stuff? Not me.
I know of a place that I can get a board foot of xps for $0.27 - so basically a 4x8 sheet for around 7.50 a pop, and I can get it in different thicknesses.
I don't have a mint to spend on this house, but I'm not out to build something that might as well be rebuilt as soon as I'm done either. So this was the best way I could see doing it without a run of the mill cold attic, which I'm totally opposed to.
I thought about using 1" of 2# foam on the underside of the sheathing all around the house for a good seal, however, that would cost me a mint too at that price, I just can't afford it. What I'd like to do, and what I can realistically do are two entirely different things.
That's a contributing factor as to why actually manufactured SIPs are kind of lofty as well. So this way, basically I'm making my own SIPs. It's just a tad more complicated and different from using a premade SIP, and that's what I'm trying to tune and build for.
So at this point, that's why I'm thinking 3 or 4" of xps, not sure if 3 would work in 2 - 1.5" layers. If I could make a design with 4" work and actually be able to frame it together correctly.
Aside from that burning question, the only place I could see using spray foam where it might be cheap enough to get some of the benefit was on the inside of the bottom layer of sheathing, but put 1" of xps over it between the rafter bays, and use spray foam around the edges to seal it in tight, this would require only a small amount of the foam, and potentially achieve very close to the same effect. My worry though was sandwitching that lower layer of osb in like that, i wondered if it might have a moisture problem?
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cmkavala
Veteran Member
Posts:1649
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| 04 Sep 2009 07:37 AM |
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Sips will give you you thermal and structural component all in one, no need to re-invent the wheel. Stick with a proven method.
just a note on EPS board at .27 cents, density of foam dictates R-value............. many sheets are made with regrind foam and low density, giving poor thermal performance |
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 04 Sep 2009 05:52 PM |
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Posted By cmkavala on 09/04/2009 7:37 AM
Sips will give you you thermal and structural component all in one, no need to re-invent the wheel. Stick with a proven method.
just a note on EPS board at .27 cents, density of foam dictates R-value............. many sheets are made with regrind foam and low density, giving poor thermal performance[/quote]
As I said previously, sips are expensive, I don't have the option of easy. Also I can get whatever foam I want, .27 for xps, 13 cents a foot for eps, and 24c a foot for iso, but I'm probably going to use xps, because I can walk on it while installing. |
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aeridyne
New Member
Posts:60
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| 09 Sep 2009 10:10 AM |
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No comments on the real world strength of ISO and XPS?
Also, I have thus far been finding equal amounts of information regarding venting and not venting the roof deck with at least a 1 1/2" air space. Some say it's absolutely critical, or that it is a great benefit and even with a ton of insulation it will make a big difference in degrees of the attic when insulating a cathedral ceiling. Can anyone confirm any of this with real world scenarios, exactly how it was built and fact?
Much much appreciated. |
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cmkavala
Veteran Member
Posts:1649
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| 09 Sep 2009 10:59 AM |
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Posted By aeridyne on 09/09/2009 10:10 AM
big difference in degrees of the attic when insulating a cathedral ceiling. Can anyone confirm any of this with real world scenarios, exactly how it was built and fact?
Much much appreciated. there are no "degrees in the attic" when using sips , by the time you try to reinvent the wheel you will have an inferior subtitute for a sips roof, that I would wager you will not save any money trying to accomplish |
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 09 Sep 2009 02:45 PM |
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So you are telling me that there is not even a one degree difference between upper and lower levels in a home that uses SIPS. Again, I'm not here to act like I know what's up, but I find that very hard to believe.
Do me a favor, give me an idea of how much your sips cost.
And there is a big difference between trying to reinvent the wheel vs. making a cheaper wheel out of necessity. Seriously man, guys like you are starting to bug me, not everyone can build with ICFs and SIPS, not everyone can afford to buy a stainless steel car, not everyone can afford the best of anything.
So why not stop touting your one vainglorious model that you think is the pinnacle form of building material and standard, and open up a little bit and help people find some viable alternatives when they are not affordable for someone?
60 dollars for a 4x8 sheet is not what most people can buy.
I can build a 3" thick ISO double sheathed roof for $1 a foot or so and that's the best I can do. Any constructive input toward that? If not, please don't tell me I'm a fool, and if I am, back it up.
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cmkavala
Veteran Member
Posts:1649
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| 09 Sep 2009 03:47 PM |
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Posted By aeridyne on 09/09/2009 2:45 PM
So you are telling me that there is not even a one degree difference between upper and lower levels in a home that uses SIPS. Again, I'm not here to act like I know what's up, but I find that very hard to believe.
I can build a 3" thick ISO double sheathed roof for $1 a foot or so and that's the best I can do. Any constructive input toward that? If not, please don't tell me I'm a fool, and if I am, back it up.
No, but what I am saying is there is not an extreme, in a normal 8ft. tall room the temp will be cooler at the floor than the ceiling and the same is true with a vaulted SIP ceiling/roof but the difference is minute, the bottom side of my sips even in the enclosed dropped areas is still cool to the touch in August even though the roof temp just 6" away is. I just checked my own home with a infrared thermometer at floor level - 73.1 at 8ft AFF 74.9 and at the bottom side of the roof panel (16ft. AFF) 76.8 , exterior temp right now is 93.1No I don't think you are a fool, but you are deceiving yourself at $1. a foot. please post the breakdown I don't think 3" of ISO in Flint is going to make it |
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 09 Sep 2009 03:58 PM |
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Now were making progress, constructive criticism is fine with me, as is a forward thinking conversation, not so much a one up debate. I'm glad to see a change of gears for the thread.
I figured that the difference would not be much, nothing is impossible though of course. Many of the articles and such I've read for a standard home accept a ten degree difference as good for an attic that is part of the building envelope, and a three degree difference is seen as excellent. I would agree that a three degree difference would be excellent that that is about the range I would like to strive for.
the other concerns I am trying to hold as a priority are condensation control mainly and feasibility of the build itself of course.
$1 a foot is difficult to pull of, but I think I might be able to do it. I can get ISO for $0.24 a board foot. So at 3" thick, about .075 a foot. Take into account two layers of OSB, one must be strong for the base level deck, 1/2 should do the trick, the top layer can be, um, not entirely sure, 7/16ths? Say that the cost of both sheets is $10 a pop, that's $10 for 32 board feet of osb, which should actually be higher than what you can find it for on any given day if I'm not wrong. So say then that adds up to 72 cents plus 32 cents plus 32 cents (the difference in the osb can account for the delivery cost of the ISO) and you come in at 1.36 a foot. throw in hardware etc and you should still be well under $2 a foot. That's as good as I've been able to break it down in my head, which seems like a pretty good bang for the buck to me.
What can you tell me about sips? granted I'm probably not going to be able to afford them anyway, and I'd be utterly lost as would anyone I'd have to help me on how to install them, I guess basically I'm just curious.
**edit** oh, and you may be right about 3" not being enough, however what I'm basing that on is assuming R6 per inch, which would give me a R18 at 3". One of the articles I read on building science showed that a 1/3 to 2/3 ratio should be maintained, and also something about needing R20 above the deck to protect against condensation in zone 5. The idea is to keep the roof deck (don't remember the exact numbers) but something like 45 degrees or higher at 45 percent humidity or less.
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cmkavala
Veteran Member
Posts:1649
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| 09 Sep 2009 04:25 PM |
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aeridyne;
since SIPs are the structural and thermal component of a roof system, you have no way to support your sandwich panel? so my guess is you need to add the 2x12s back into the calculations and are you going to glue the osb and foam together? and labor (yeah I know its free) how will you handle spans over 8 ft.?
We normally would not ship to your area due to freight charges, but you have companies like R-control right in you own backyard that will probably be comparable in price to your final "TOTAL" roof structure calculations |
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 10 Sep 2009 11:14 AM |
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No they are not structural, so yes, I do plan on constructing a stick framed roof beneath it. I actually prefer it that way though. I don't want to open a can of worms but I trust solid 2x12 a lot more than I would two layers of osb and foam, I just have thing about trusting glue to hold up for 100 years plus, hence I really don't care for LVL either, ive heard stories I didn't like.
what do you mean how will I handle spans over 8 ft? They are 2x12s for pete sake, I can run them 24 ft if I want. I do a standard catherdral ceiling construction with collar ties, it's a 30' total span, and i create my roof deck over that. So yeah, the 2x12 add to the cost as well, but still it's only about another buck a foot really or less including the ceiling joists and rafters.
I'd have to "import" a builder who knew his head from a hole in the ground when it came to sips to even have them installed anyway, I know beyond a shadow of a doubt, they would come out to be much more expensive. I really just can't believe that you anyone would sell a sip that is going to get R60 for cheaper than I can build a stick frame double deck w/ rigid on the deck and cellulose in the rafter bays, I really don't know the cost, but it just seemed like a no brainer to me that SIPs were going to be a heck of a lot more. |
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cmkavala
Veteran Member
Posts:1649
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| 10 Sep 2009 08:08 PM |
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your DIY sips is limited to 8ft. my point is........... in order to replace a simple SIP,
you are needing 2x12's, dble osb, insulation, glue,etc.
The sips are less expensive and are already pre-assembled in modular sections, you would need to check with your local mfg. but I am sure its well under $4./sq.ft. for panels
does not seem like it worth the effort to me? if you are going to attempt to fabricate your own sip assembly, it seems to me you can certainly figure out how to install them.
They are by far the simplest building method out there
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 11 Sep 2009 07:53 AM |
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Posted By cmkavala on 09/10/2009 8:08 PM
your DIY sips is limited to 8ft. my point is........... in order to replace a simple SIP,
you are needing 2x12's, dble osb, insulation, glue,etc.
The sips are less expensive and are already pre-assembled in modular sections, you would need to check with your local mfg. but I am sure its well under $4./sq.ft. for panels
does not seem like it worth the effort to me? if you are going to attempt to fabricate your own sip assembly, it seems to me you can certainly figure out how to install them.
They are by far the simplest building method out there
Well, another factor to wanting the stick frame was the strength, I know of one guy that has been on this site and the mikeholmes forum as well and he said he already did a bunch of research, and was worried about the strength, he said they can sometimes crack, etc. If I was able to somehow get help with it, I wanted to do a green roof, and I didn't think a SIP would hold it. Granted yes it is a lot more work, however, I don't have time on my side either to research SIPs even if I did have a good feeling about them, and I think I can come in pretty close to $2 a square foot using my method. I get what you are saying about 8ft now, because everything is a 4x8 sheet. Another thing I thought of that could potentially be a strength of this roof is that if I needed to ever replace the top layer roof deck because it rotted or things like that, it would be fairly modular, while if I had used a SIP, what happens if that top layer starts to rot out? I would imaging you would have very serious problems on your hands at that point. Another point was this, I was thinking about floor trusses because it would simplify my floor structure, so I was doing a bit of research on those, I had pretty much decided floor trusses would make my life easier, my duct runs are going to be a ludicrous pain, but then the last thing I read was from a firefighter on the holmes forum. He first of course said that he has been a lifetime firefighter, and that he did not like the engineered wood products when it comes to trying to rescue people, he said that once the wood gets charred about 1/4 on floor trusses, the gussets start to fail. I thought about that, 1/4 really isn't that much, and I've witnessed my own house engulfed in flames once before when I was a teenager, that was one of the scariest moments of my life. I plowed our van literally through the the traffic that had built up to get to our house and make sure everyone was ok. That was something I had not even thought of, since osb is not thick, a little bit of burning would damage it a lot, I'll be building my home out of sticks at least if not brick...
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cmkavala
Veteran Member
Posts:1649
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| 11 Sep 2009 08:24 AM |
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aeridyne;
SIPs are 3 times stonger that frame
if you have a fire concern ,then there are other SIP options with metal clad and fibercement skins to make your home fire resistant |
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Chris Kavala
info@southernsips.com
1-877-321-SIPS |
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aeridyne
New Member
Posts:60
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| 11 Sep 2009 10:31 AM |
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Posted By cmkavala on 09/11/2009 8:24 AM
aeridyne;
SIPs are 3 times stonger that frame
if you have a fire concern ,then there are other SIP options with metal clad and fibercement skins to make your home fire resistant Again I'm not trying to argue with you by any means, but that seems like a very generalized and hard to believe statement. So you are saying, a 2x12 rafter roof, with two layers of sheathing, one at 1/2" & another at 7/16", is only 30% the strength of ANY sip?
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Jelly
Advanced Member
Posts:554
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| 13 Sep 2009 10:09 AM |
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aeridyne, the strength issue is not really open to debate. SIPs are definitely stronger than frame. The synergistic effect of the skins and rigid foam core have the same structural properties as an I-beam, the foam being analogous to the web and the skin analogous to the flange.
Another analogy to draw with SIPS is an exoskeleton of an insect or fuselage of an airplane, versus the skeleton of a framed building.
Price, however, is where we can still have a debate... |
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