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Old 09-27-2012, 02:38 PM   #16
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How would you Insulate THIS?


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Originally Posted by GBR in WA View Post
--- you need at least R-25 foam, not R-14 for your location with a closed roof, since you are not going that direction- mute point anyway. Just don't want other readers in Zone 6 to get the wrong info.

With the foam on the inside- you are keeping the f.g. cavity insulation warmer (bringing the first condensing surface to the back of the inside f.b.) during the summer and run the risk of wet f.g. from night-time radiation at other times. I would use 1-1/2" wide 1" f.b. strips on the rafters and the 2" f.b. in the cavities (against 1-1/2" f.b. spacers) instead of the plastic baffles. Any moisture in the cavity can still dry through the f.b. (which would eliminate the radiation/condensation on the f.g.).

http://www.buildingscience.com/docum...bout-diffusion

4th para. below fig.3: http://www.buildingscience.com/docum...on?full_view=1

To use an interior v.b., you might get away with less foamboard, you need to figure the calc's., otherwise you would have a double vapor barrier (vapor sandwich) with a vapor barrier on EACH SIDE of the cavity insulation. Depends on the vapor drive/temperature/pressure differences.
2" would barely let moisture through; http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

Gary

Gary: So with a 1.5" spacer, plus 2" foam board, my 2x12 cavity is now 7-3/4". Max I can get for F.G is R-21 now unless I fur out the rafters or use expensive 2" foam board for the final.

Keep in mind this ceiling will vent into the small attic and ridge vent. It's not a TRUE cathedral.

Also, I am using T+G pine so I will certainly need a better barrier than just the 1.1 perm 1" foam.

What a pain this is.

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Old 09-27-2012, 02:58 PM   #17
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How would you Insulate THIS?


Insulate at the exterior side of the roof with vented nail-based roofing insulation panels (http://www.buildingscience.com/docum...1_roof_web.jpg). Then fill the rafter bays with batt. insulation. Spray foam is NOT the way to go...

If you get the rigid panel thick enough it will negate the need for a vapor retarder at the interior. that R-value for climate zone 5 is R-10. Might be higher in your neck of the woods.
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Old 09-27-2012, 03:05 PM   #18
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Insulate at the exterior side of the roof with vented nail-based roofing insulation panels (http://www.buildingscience.com/docum...1_roof_web.jpg). Then fill the rafter bays with batt. insulation. Spray foam is NOT the way to go...

If you get the rigid panel thick enough it will negate the need for a vapor retarder at the interior. that R-value for climate zone 5 is R-10. Might be higher in your neck of the woods.


So, rip off my brand new roof and decking? I can't use nail base - the roof is already built and the cost of that stuff was astronomical. I considered it. Not even close to affordable. Not sure where R-10 comes in. It's R-49 where I am (there are some exceptions).
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Old 09-27-2012, 03:17 PM   #19
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2009 IRC Table R601.3.1 allows for Zone 6 to utilize class III vapor retarder at interior (poly finish) provided there is an insulated sheathing layer of greater than R11.25 provided.
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Old 09-27-2012, 03:20 PM   #20
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I was bored...

I appreciate your drawing, but you are clearly missing the fact that the shingles are already on the roof and nailbase is not an option.
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Old 09-27-2012, 03:20 PM   #21
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So, rip off my brand new roof and decking? I can't use nail base - the roof is already built and the cost of that stuff was astronomical. I considered it. Not even close to affordable. Not sure where R-10 comes in. It's R-49 where I am (there are some exceptions).
No, No, No...nevermind it all...
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Old 09-27-2012, 03:20 PM   #22
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I appreciate your drawing, but you are clearly missing the fact that the shingles are already on the roof and nailbase is not an option.
Didn't see that little tidbit till after it was all posted...good luck!
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Old 09-27-2012, 03:25 PM   #23
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That's it!? Tell me how to do it!
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Old 09-27-2012, 03:29 PM   #24
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That's it!? Tell me how to do it!
That's how you do it...sorry you build the thing before you considered how to insulate it...The discussion so far has been hashing out the only options you have left.
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Old 09-27-2012, 03:51 PM   #25
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I did consider it. You need to read the thread a bit more closely - you are missing some key points. Not everyone can afford that stuff. Thanks anyways, and thanks to all whom offered suggestions. I'll figure it out.
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Old 09-27-2012, 09:49 PM   #26
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Sorry we couldn't help you more.

You didn't see my mention of 1-1/2" x 1" f.b. strips on the rafter face to increase the cavity depth... You would have 1-1/2" f.b. at the 1-1/2" air space = 3" minus 11-1/4" = 8-1/4" plus the 1" strips = 9-1/4" R-35 (R-38C compressed) plus R-7.5 XPS = R-42.5
OR change the XPS to PIC = R-44.75

You probably wouldn't get summer drive with poly inside; fourth paragraph below figure #3; http://www.buildingscience.com/docum...on?full_view=1


Cost vs. value, just go with 98% (10"): pp.2; http://www.enersavesystems.com/pdf/E...Insulation.pdf


I framed 3 houses similar to yours. All had a 6x6 post at the top of the stairs for the "great room" ridge beam support opposite the windows. The window wall held up the other end of the massive glulam structural beam. A single ridge board won't work. Upstairs; with the rafter ties pushed up past 1/3 (plate to bottom of ridge board) the 2x12's are carrying the roof load on cantilevered sticks, like walking on stilts when I was way younger. Make sure the rafter ties are not collar ties because one works, the other doesn't, 1/2 way down page: http://myconco.com/ComEngProb.html Make sure you have a structural ridge over the great room, just to keep you and family safe.


Gary
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Old 09-28-2012, 06:23 AM   #27
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The rafters have sampson strong tie hurricane ties fastening the rafters to the top plate all the way around the house. Not the answer to sag, but maybe I'll add a pine beam or two for that.
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Old 09-28-2012, 07:17 AM   #28
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Aquasport,

Everyone is trying to help here an GBR and AGW are great resources. I am guilty of not reading the threads from time to time as well.

I think, given the the shingles are on, the best route is the way we discusses previously with putting rigid foam in the rafter bays (in lieu of the vents chutes) insulating with high density batts, and putting foam to the interior is the way to go. You maintain a vented assembly and still get the higher R-Value out of the roof. You also thermally break the wood structure from the inside of the home with the rigid foam prior to drywall. Whether or not you need the vapor retarder level with the rigid foam all depends on the perm rating of the foam that you choose.

Foam to the outside of the surface, while not an option, would have been best but 20-20 is always perfect.

GBR knows the codes and links like a RainMan so I will defer to climate specific recommendations (although I think R-14 to the outside with a 12 rafter bay that is insulated is quite enough), I will defer to him.
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Old 09-28-2012, 10:38 PM   #29
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The hold-downs for the rafters at the wall are only for up-lift, not keeping the walls from bowing out, pp.36.

You really need a ridge BEAM rather than a ridge board; pp. 18, 22: http://www.awc.org/helpoutreach/ecou...seV09-2007.pdf

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Old 09-29-2012, 04:01 PM   #30
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aquasport,

I see you have been given excellent information and advice from GBR and Windows and do not know what more I can add. I understand it can be frustrating at times when advice given may not match what you wanted to hear.

It is also difficult in reading many many many posts and absorbing all the information. You may have already answered some of the questions I will pose, and if so I apologize. Also I do not know the level of knowledge of yourself or your builders and any comments/questions are not intended as a slap in the face or putting down anyone. It is difficult to observe all conditions in a couple of photos.

I understand your comment about no inspections, however there is a building code and construction needs to be in compliance with the code or issues may arise should you ever decide to sell you home. Not only that but codes typically come about because of issues in the past that have caused either damage to structures, or injuries to occupants. If the safety of your family, or your investment in your home does not concern you there is no need for you to read farther. If you do have concern then by all means please continue to read.

After viewing post #1 and your photo I have concerns regarding the construction of your roof. Here is my understanding of the construction of your roof, please correct me if I am incorrect:

1. Rafters are 2x12, they sit on a knee wall and run up to a ridge board forming a cathedral style ceiling/roof assembly.
2. I do not see any collar ties in the upper third of the space from the top of the knee walls to the ridge board. Where ridge straps installed over the wood structural panel sheathing of the roof at each pair of rafters? This strap would have run from the top of one rafter over the ridge and attached to the top of the opposing rafter.
3. I cannot verify if the ceiling joists, which would be acting as rafter ties are within the bottom 1/3 of the space from the top of the knee walls to the ridge board. Can you verify this? What is the measurement from the top of the ridge to the knee wall, and the measurement from the top of the knee wall to the ceiling joist?

I refer you to:

R802.3.1 Ceiling joist and rafter connections. Ceiling joists and rafters shall be nailed to each other in accordance with Table R802.5.1(9), and the rafter shall be nailed to the top wall plate in accordance with Table R602.3(1). Ceiling joists shall be continuous or securely joined in accordance with Table R802.5.1(9) where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building when such joists are parallel to the rafters.

Where ceiling joists are not connected to the rafters at the top wall plate, joists connected higher in the
attic shall be installed as rafter ties, or rafter ties shall be installed to provide a continuous tie. Where ceiling joists are not parallel to rafters, rafter ties shall be installed. Rafter ties shall be a minimum of 2-inch by 4-inch (51 mm by 102 mm) (nominal), installed in accordance with the connection requirements in Table R802.5.1(9), or connections of equivalent capacities shall be provided. Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice.

Collar ties or ridge straps to resist wind uplift shall be connected in the upper third of the
attic space in accordance with Table R602.3(1).

Collar ties shall be a minimum of 1-inch by 4-inch (25 mm by 102 mm) (nominal), spaced not more than 4 feet (1219 mm) on center.


Rafter ties are installed to prevent rafter thrust from pushing out the tops of wall. In a traditional flat ceiling the ceiling joists act as rafter ties. If the ceiling joists are installed too high they are not able to eliminate the rafter thrust. This is why I'd like you to verify the measurements, to insure you do not have issues with your walls pushing out. When this occurs the ridge tends to settle and sags. This will cause cracking and damage to your ceilings and walls. Would it collapse? probably not, but it will cause damage not only to your ceilings, but potentially to the sheathing and roof covering which may allow water intrusion. And we all know what happens with that.

Therefore your rafter ties need to be in the lower 1/3 of the kneewall to ridge space, or a properly sized structural ridge beam needs to be installed with columns to support the load.

So we know your knee walls are load bearing, and this load transfers down to the parallel chord floor trusses and out to the exterior walls. The truss acts as a beam. Is this correct, or were there structural beams installed to support the load of the roof/knee walls? Can you confirmed that if the trusses transfer the loads they were designed to do so?

Now to your insulation

I understand you are with Climate Zone 6 which Table 402.1.1 of the 2009 International Energy Conservation Code

http://publicecodes.cyberregs.com/ic...9_4_sec002.htm

states you need a ceiling R-value of 49, which you stated you are aware of.

Are you however aware that section 402.2.2 Ceilings without attic spaces allows R-30 within those spaces as long as it does not exceed 500 square feet or 20% of the total insulated ceiling area, whichever is less?

This would be those areas from the top of the knee wall to where your rafter ties are installed. If you can confirm these areas you may find there is no requirement for R-49.

I would recommend reviewing your structure for the issues that have been raised before enclosing to prevent any future issues.

I would further ask you to review the comments and questions that have been posed to you by others and consider their advice, after all they have been doing this for a long time and know of what they speak.

Good luck!

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Last edited by GBrackins; 09-29-2012 at 04:10 PM. Reason: typos
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