Proper Way To Insulate Vaulted/cathedral Ceiling? - Insulation - DIY Chatroom Home Improvement Forum

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Old 09-02-2016, 01:40 PM   #1
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Proper way to insulate vaulted/cathedral ceiling?

I live in a two story house and I recently starting remodeling some of the down stairs. It was time for the kitchen to be done. Because of rot and damage to exteriors walls I had to replace some exterior walls and some floor joists. After the kitchen wall was removed it lead out to our porch. We decided to extend the kitchen out about 9 feet. Adding about 13'x9' of kitchen) Everything went as smoothly as you could expect on a fixer upper until now where I have to insulate the cathedral ceiling. (My knowledge is lacking in this area.)

I want to keep the cathedral/vaulted look (although slight) raising the ceiling from its normal 7'9" to about 10'3" in the addition. The beams are only 2x6s (spaced roughly 15-16 OC) however I did add fur strips to them making them 2x8 beams (7.25"). I cannot afford to have blown in or spray foam.

I have been told several different things to do but am a bit worried about moisture issues rotting out the roof if I do something wrong. One recommended method was just to shove 8" cathedral fiberglass insulation in there and call it good.

My plan was to get some 2" poly sheets (either blue board or the polyiso), force them tight against the roof sheathing, then spray foam the gaps. And either add one or two more layers of this to get my R30 or use at least the one later of rigid foam and the rest traditional fiberglass insulation.

What worries me is no ventilation. If I go the route of stacking three 2" sheets I would have enough for a 1 inch vent gap but I am not positive how to vent a roof that connects to the side of my house. Would I be alright not having ventilation if I spray foam all the cracks and spaces? I live in NY State so I get all sorts of weather.

My main questions are can I use blue board or polyiso and put it in direct contact under my roof sheathing as long as I spray foam the gaps and edges?

If I cannot do this but I can use my three stacks of 2" poly sheets method how can I effectively vent the top of the roof where it meets the house? Any help would be appreciated.
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Old 09-02-2016, 02:31 PM   #2
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Re: Proper way to insulate vaulted/cathedral ceiling?

Welcome to the forum.

With you not wanting to do something wrong and not being able to afford doing it right it sure makes giving advice difficult.

You are off to a bad start and listening to bad advice. Here's some points you need to consider.
1. What do building codes mandate in your area?
2. What have you provides for ventilation of that cathedral ceiling?
3. Does it have a soffit?
4. How is it attached to the house?
5. What climate zone are you in?
6. Sounds like a very low pitch, that raises questions about roofing material.
7. Have you pulled all the required permits for this work, plumbing, electrical, and framing?

Stuffing it full of fiberglass insulation is not on the list of options.

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Old 09-30-2016, 05:28 AM   #3
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Re: Proper way to insulate vaulted/cathedral ceiling?

If there is no attic, there is no need for ventilation.

Closed cell spray foam seals everything off nicely.
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