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Old 11-27-2007, 05:18 PM  
pgriz
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The code for minimum air channel is 2 inches. So by putting 2 inch vertical battens, you satisfy that requirement. However, don't forget that the purpose of ventilation is 1) to dissipate residual heat leaking past the insulation, and 2) to dry out any condensation that may be forming. If the vapour barrier is doing its job on the warm side of the insulation, then there should be little moisture or condensation to worrry about. Also don't forget the insulation BELOW the roof deck, as in your situation, you will need it.



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Old 11-28-2007, 02:57 PM  
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How does a SIP work? Foam core with OSB on both sides. Do they shingle directly on top? What serves as the vapor barrier?



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Old 11-28-2007, 07:11 PM  
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Busyflyin, we're not talking the same thing. First of all, a SIP panel does not give you anywhere near the R38-R40 of insulation you need in your roof system. So if you install a SIP panel over vertical battens, you've created a nailing surface, but you have neither a vapour barrier, nor enough insulation, and the inside surface of your roof deck (the part you see from the inside) will be at or below freezing in winter and any moisture you have in the inside air will condense there. Not a good scenario.

Let's look at this step by step. Your current situation is as illustrated in Image1.jpg.
E is your current deck, G are the 2x6 rafters and F is the space between them.

I am proposing that you fill in the space between the rafters with blue styrofoam, install a vapour barrier under this (across the rafters), and then install wood planks or boards across the rafters to give your the "wood" look to your ceiling. This is shown in the following image.

Note that the inside space between the rafters (F) is now filled completely. Then you install the vapour barrier below this (H), and nail your wood planks to the underside (I). So far, you've got the vapour barrier that prevents moisture from the cottage interior air from reaching the cold parts of the roof. However, note that the building code in some jurisdictions requires that if the space between rafters is filled with foam insulation, that a fire-proof material (like gyproc) is used as the covering layer. So you need to check the local codes.

Next, on the outside, you add the two inches of blue styrofoam insulation, place 2x2 inch purlins (oriented vertically) over these, place the OSB on top of the purlins. This is shown in the following image.

The two inches of external insulation (D) is laid on top of the stripped deck, then the 2x2 purlins (C) are laid on top, and then the plywood or OSB (A) is laid on top of these. You need to hold the whole assembly together with either very long nails or screws (J) that penetrate into the original rafters.

This assembly then gives you the vapour barrier (H) on the warm side of the insulation (F & D), a ventilated air channel (B), and a deck surface (A) on which you will need to install the underlayment and shingles. The combined R-value of the insulation should be about 36-38, which should be sufficient for your location and usage. The two inches of air space is the minimum required, but you therefore meet the minimum. In winter, the freezing point of the roof assembly will be located inside the insulation, and therefore you should not have problems with condensation (AS LONG AS YOU HAVE THE VAPOUR BARRIER ON THE WARM SIDE OF THE INSULATION, AND THE BARRIER IS INTACT).

In effect, you will be constructing a sealed, ventilated cathedral ceiling.

Anything less than this will give you problems, in my opinion.

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Old 11-29-2007, 06:44 AM  
inspectorD
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Default Nice job...

Basically you have showed them a cold roof scenario. These are also in my opinion the way to go. The issue with the Structural insulated panels I did not understand ...you are saying you need to ventilate these panels?
I have run into delamination and ants with these panels failing so I do not recommend their installation, but they do not need ventilation in my opinion ,or the testing I have read through.
www.buildingscience.com has many solutions to these questions also. That was why I wanted to know the particulars or what you are trying to accomplish as an end result.
Another solution is to install a 2x6 below the existing rafter and install fiberglass with paper faced vapor barrier. With proper- vent Styrofoam channels under the roof deck. This is the way conventional cathedral systems are installed now.
And yet another is to install a closed cell spray foam insulation directly to the underside of the deck, with no ventilation. These are expensive products, and still new to residential construction. They have been used in commercial jobs for decades, but still not actually proven themselves in residential ..to me anyway.

I like Pgrizs solution. Continuous Insulation, no breaks.

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Old 11-29-2007, 08:48 AM  
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Pgriz; what do you believe will happen if only address the underside as you described and I do not add the materials to the top of the roof? I'm concerned about the cost of the project. Thanks for the great graphics.

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Old 11-29-2007, 03:29 PM  
pgriz
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Busyflyin, you will then have an under-insulated and non-ventilated cathedral ceiling roof. Snow on top of the roof will melt due to the heat loss from the roof, run to the eaves and freeze, forming ice dams. The asphalt shingles will be persistently wet and in my experience wet shingles deteriorate faster. Without the ventilation, if you have any moisture in the roof system (whether due to condensation from incomplete vapour barrier or leakage from above), the moisture is trapped and will cause rot. A cathedral ceiling will work IF it is sealed, and if the amount of insulation is sufficient.

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Old 11-29-2007, 05:41 PM  
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Got it. Thanks for all of the help with this. I don't know what I will do, but I know what I should do.. Have a great holiday!

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Old 04-20-2008, 11:58 PM  
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Hi, After reading this topic with much interest I decided to just tack my question on the end.
I have an almost identical situation with an old cabin I just purchased in California in the Sierra's around 6300 feet.
After reading this post I have decided that it would be too expensive for me to make the drastic changes needed to insulate my roof. I don't intend to make it a 4 season cabin but just wanted to make it a bit warmer in the early spring and late fall. The only heat is a wood burning stove, I was also hoping that the insulation in the roof would keep it cooler in the summertime.

I would like to insulate the floor which is partly resting on a large granite boulder and partly on stilts. So I was wondering what is the best way to do this. I will attempt to add a photo of the underside of the floor so you can see what it looks like now.
Thanks

floor.jpg  
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Old 10-13-2010, 02:18 PM  
HillbillyRob
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Default Asphalt shingles do not last long these days

Quote:
Originally Posted by busyflyin View Post
pgriz; I'm not sure I want a steel roof... we just build an adjacent garage and used asphalt shingles... so I want to match the look. Also, aslphalt is more economical. So if I understand you, on the top side... 2" blue foam followed by verticle battens to allow for ventilation, then osb, then standard roofing w/ridge vent. This would allow for sufficient ventilation? The walls are no issue for me. We intend to gut the building and wire, insulate, and such, just as a new home. Thanks for all of your advise. It's well taken and appreciated.
Asphalt shingles are only economical in the short run. Our modular has shingles that were supposed to be guarenteed for 25 yrs..the have lasted less than 20 and this is a premium Palm Harbor and we are not the first owners.
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Old 02-13-2011, 10:48 AM  
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To Pgriz a huge THANK YOU. I was planning a much cheaper solution and believe it would have cost me much more in the long run. Thanks to you I now know how to do it right the lst time.



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