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Cathedral Cold Roof Insulation


Abletree
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Hello Everyone!

I am new here. I am a novice in construction, but would like to put some sweat equity into a second home I have.

Hoping to scale down and live efficiently, I would like this small ranch on a slab to be very energy efficient.

I have done most of the gutting and the interior walls and roof has been stripped down to the studs and rafters. The main living area is very open with cathedral ceiling and a hip-roof.

It was suggested to me to make the roof a cold roof by installing 2x2's along the rafters and then 2" of high R rigid foam panels sealed between the rafters. A ridge vent and eave vents would provide an air channel.

Any tips on this? I live in a cold region. Can anyone suggest a method to use beyond the 2x2's and the rigid foam? There is still some room for more insulation. How would you recommend finishing it off?

Thanks in advance, David

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Originally posted by Chad Fabry

I'd just fill the rafter bays with icynene (if they're 2x4 rafters, scab on some sleepers to increase depth to at least 6 inches). Install foil faced rigid to create a thermal break so the drywall doesn't show the rafter outlines from ghosting,drywall and you're one. This method requires no venting.

Hi Chad,

I have never seen this method used. Why would ventilation not be required? What happens to any moisture that gets trapped within the roof structure?

I would suggest packing down the framing to provide the space for at least six inches of insulation with foam chases to provide air circulation between the vented soffits and ridge vent.

More importantly, someone that knows what their doing needs to check the structure before this is done. Is there a ridge beam or are collar ties needed. Can the rafters support the added loads?

I have seen many homes where the structure has sagged or even failed because of this type of work.

I get nervous when I hear that a "Novice" in construction is gutting a home.

Regards,

Steve H.

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Hi,

Chad's approach is a pretty good one, but if you use Icynene, which is an open-cell foam, you'll need to be very careful to include a vapor barrier on the interior before you drywall and install some type of air exchanger.

There was an article in the July issue of Energy Design Update that discussed problems a Vermont couple had in a 3-year old home insulated with icynene. It was a tiny house - no wider than 14 feet and no longer than 24 feet.They'd used Thermo-Ply sheathing on the exterior and T & G paneling on the interior walls and cathedral ceilings. No air exchanger and no vapor barrier on the inside of the walls, as recommended by Icynene for homes built in climates with 7,500 or more heating degree days. The only ventilation was the windows and a small bathroom ceiling fan that is manually controlled.

They began seeing mold along the baseboards so they went running for a lawyer. The lawyer hired a contractor and had the walls and ceilings opened up. The contractor found the icynene soaking wet. So much so that you could squeeze water out of it. Being an open-cell foam, moist air was migrating through it by vapor diffusion. When that moisture-laden air reached the Thermo-Ply, which has a vapor permeance of 0.53 to 0.63 perms - essentially a vapor barrier on the wrong side of the walls and ceiling, it condensed on the Thermo-Ply and collected there. Not only were the wall studs rotting but the plywood decking on the roof was delaminating.

In order to preserve the interior, they ended up spending $25,000 to strip off all of the siding and the roof to reinsule it from the outside by shooting the whole thing with closed-cell foam. They've sued the builder and Icynene but the thing isn't resolved yet. This was a self-inflicted wound if I've ever heard of one.

Using Chad's method you could very carefully seal the perimeter and joints between panels of the foil-faced foam with foil tape, but I'd definitely install an air exchanger or, at a minimum, through-wall vents and a fan controlled by a humidistat set to come on automatically at anything above 45%. Alternatively, you could use a closed-cell foam and still make sure you're careful about air sealing the inner face of the foam.

I agree that Chad's method shouldn't need to be vented, but, despite the science, some roofing manufacturers won't honor their warranty when foam is used and no ventilation is present, so you need to know what your roofing product manufacturer's position is on that. If you don't know, you'll want to consider some type of ventilation.

If you're not replacing the roof, you could use foam air chutes from eave to ridge, overlap them in drainage plane from ridge to eave, seal them at the joints, install a full-length ridge vent and then use Chad's method beneath - being careful to create the vapor barrier on the interior face of the wall by sealing the foil-faced foam. If you've stripped off the cover and are planning on a new roof anyway, you could consider Chad's method without the chutes and use a vented nailbase foam for the deck. That'll give you some additional R-value while preserving the roof warranty.

ONE TEAM - ONE FIGHT!!!

Mike

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I'm not disagreeing w/ any of the above, but.....

When has anyone seen a vapor barrier that worked well, let alone worked? I think it's why some folks prefer the term "vapor retarder".

Icynene is still "new" IMO; until we have about a decade of performance spec's derived from real jobs (as opposed to mfg's. spec'd jobs), I'm still nervous about the "no ventilation" methods.

What about dense pack cellulose? Personally, I'd never do it, but there seems to be a number of folks that think it's wonderful(?).

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Thanks Steve, Chad & Everyone!!!

I agreed with you Chad. I definitely will be running design past an architect to confirm structral load capacities, as well as other building code requirements.

You mentioned, "I would suggest packing down the framing to provide the space for at least six inches of insulation with foam chases to provide air circulation between the vented soffits and ridge vent." I am not sure what you mean by, "packing down". I am considering adding depth to some of the rafters that may need more space for adequate amounts of insulation.

Would it make sense to use the 2x2's running up the rafters, with a high quality 2" rigid foam panel, well-sealed, and then fiberglass insulation to fill the rest of the space, and then the the foil-faced rigid that Chad recommends to overlay across the ceiling? Would I put the vapor barrier/retarder underneath the foil-faced rigid, or as the last layer underneath drywall?

And finally, The Icyene option looks neat, but wouldn't that defeat my hopes of putting in my own sweat equity/labor? That looks like a dealer/distributor/installer-only option from what I have read.

Thanks for helping the mere novice with your sage wisdom! - David

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Originally posted by Abletree

Thanks Steve, Chad & Everyone!!!

I agreed with you Chad. I definitely will be running design past an architect to confirm structral load capacities, as well as other building code requirements.

You mentioned, "I would suggest packing down the framing to provide the space for at least six inches of insulation with foam chases to provide air circulation between the vented soffits and ridge vent." I am not sure what you mean by, "packing down". I am considering adding depth to some of the rafters that may need more space for adequate amounts of insulation.

Would it make sense to use the 2x2's running up the rafters, with a high quality 2" rigid foam panel, well-sealed, and then fiberglass insulation to fill the rest of the space, and then the the foil-faced rigid that Chad recommends to overlay across the ceiling? Would I put the vapor barrier/retarder underneath the foil-faced rigid, or as the last layer underneath drywall?

And finally, The Icyene option looks neat, but wouldn't that defeat my hopes of putting in my own sweat equity/labor? That looks like a dealer/distributor/installer-only option from what I have read.

Thanks for helping the mere novice with your sage wisdom! - David

In regards to your 2x 2 question. I don't know the depth of the existing rafters. If there are 2x8 rafters, the answer to your question is yes. If the rafters are smaller, I would be concerned about their ability to support the dead weights of the building materials in addition to snow loads.

I know that insulation and ventilating the roof structure works. On paper, the other methods can work but I remember EIFS, FRT plywood, etc...

I am with Kurt, let other people use their homes as a test lab and I would sleep better.

One other thing is to consider drilling a few SMALL holes in the hip roof framing to allow air movement between the rafter voids. This helps with the problem of the rafters that dead end.

Good Luck

Steve H.

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Hi Steve,

Thanks for the encouragement! Yes, the rafters are at least 2x8's - some are larger.

Good tip on the holes, too. I was planning on drilling them this weekend - even bought the drill bits for the job. Before I proceeded, I read up on the codes in our area and there are specific regulations about the size and placement of any holes in the rafters - even specific to the type of wood used in the rafters!

Using the holes is not a problem, but I am glad I read that code first.

Thanks again for your input.

David

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  • 6 months later...

I just saw your post on your cathedral cold roof and hope it isn't too late to jump in. I had the same problem on an oblong shaped turret, cathedral stick framed with 2x12 rafters. All the end rafters dead ended on the end of the ridge on either end. Once the jack rafters were installed how do I get air in these dead spaces. Did alot of research but this is what I came up with. The roof will be a metal with a continuous ridge filter vent by AirVent.com giving 18 squares inches net free per foot, at the soffits I used the AirVent continuous soffit vent, giving 9" nfa per foot. I will seal the rafters with 1" rigid foam as you were going to do, but a company makes a neat little clip that eliminates nailing up 2x2s along the rafter to hold the foam down to create a airway. It's called the Corwin Smart Clip. The foam and clips are installed completely from the "bird Blocks" to the ridge and all gaps and spaces foamed closed. Now you have a completely sealed cold roof. To get air to penentrate to all the closed rafters and jack rafters without drilling holes, they also make a nifty product called the Corwin Rafter Vent which can be let into the rafter or installed on top of the rafter and a 2x2 filler added to the remainer of the rafter. I installed them on top and filled in. Check out their site at:

http://www.keymarketing.us/products.shtml

The picture attached shows the rafter vents in action. It sure solved a problem that is always coming up.

Download Attachment: icon_photo.gif RafterVents.jpg

24.19 KB

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  • 8 months later...
Originally posted by Chad Fabry

I'd just fill the rafter bays with icynene (if they're 2x4 rafters, scab on some sleepers to increase depth to at least 6 inches). Install foil faced rigid to create a thermal break so the drywall doesn't show the rafter outlines from ghosting,drywall and you're one. This method requires no venting.

Abletree

For roof better to use soft roll material

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Here's an interesting sealed roof system from Dupont/Tyvek:

http://www2.dupont.com/Tyvek_Constructi ... K14127.pdf

And here's an interesting site on Unvented systems:

http://www.buildingscienceseminars.com/ ... lation.pdf

And here's a site that shows the results of doing a sealed or unvented system incorrectly...scroll down to Attics, the unvented crawlspace is interesting too.

http://www.technology.appstate.edu/cons ... Attics.pdf

Lew

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Mike,

Where can one obtain a copy of the July Energy Design Update report? I tried google with no luck. some friends of mine are building a new home with icynene through out, including the roof deck, however i believe that there builder is overlooking some of the issues regarding such a tight home.

Thanks in advance

Bryan

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  • 4 months later...

A few good websites can help you save energy. There are also many books available with quite a few tips. Here are a few little free tips.

Tip 1: For each degree, you set your thermostat above 78 degrees F in warm weather expect a 5-percent energy cost savings.

Tip 2: Install an air conditioning condenser that has a high SEER rating. Find one that is 13 or higher.

Tip 3: Use leftover incense to test for drafts. A draft smoke line, rather than a straight one, could indicate a problem.

Tip 4: Do not use the "rinse hold" function of a dishwasher. That can waste 3 to 7 gallons of hot water.

Tip 5: Recycle. Reduce base consumption where practical.

Tip 6: Buy a hybrid and use other "green technology.â€

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  • 1 year later...
Originally posted by n/a

Tip 8: Replace old single pane windows with double pane windows with high efficiency ratings.

That's well and good for greenliness in newish houses. However, if one were to yank, say, perfectly good 100-year-old 9-over-one windows and have the usual wham-bam installer cobble in new double pane windows, the homeowner would lose thousands of dollars in resale value. Nobody wants a 100-year-old house with new-but-ugly replacement windows.

And while I'm on the subject: No savvy old-house owner with tasty original light fixtures will want to install any of those wormy-looking poor-fitting CFLs. (Mercury on a stick!)

Of course, in this case, I am not wrong,

WJ

PS: Beware of foam.

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