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MntGoat
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# Posted: 29 Aug 2017 01:22pm - Edited by: MntGoat
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I am about to insulate my story and a half cabin. I have 2x10 rafters. My plan is to install R-19 fiberglass roll insulation between the rafters which will leave a gap between the insulation and roof deck of about 3.75" +/-. I have a ridge vent and full length soffit venting so I figured this would be great to encourage venting.
Do I need to worry about installing a foam proper-vent (baffle) from the soffit to the ridge provided I don't block the lower end of the rafter cavity where the soffit is?
It will not be heated year round, only when I am there (one or two days every other weekend)
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snobdds
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# Posted: 29 Aug 2017 04:35pm
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I have basically the same set up, even down to the rafter size and insulation choice. It provides plenty of venting and it has worked really well with no condensation issues.
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Bushwhacked
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# Posted: 29 Aug 2017 06:02pm
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The last house I worked on that was 2x12 rafters, we could fit R-30 in there. Thats quite a bit more r factor that the 19. I don't know where you are located but 19 really doesn't do a whole lot unless your in Texas or Florida. Even where I am in WA you are supposed to have r-38 in ceilings, though I think its less for cathedral or vaulted ceilings. I would think about adding a filler to the bottom of the 2x10's you have, a piece of ripped down 2x4 would work, then you could fit r-30 in there.
You do need to use the baffles the full length of the rafters to ensure the insulation never gets pushed up to touch the back side of the roof sheeting. (think maybe adding a light fixture or shoving speaker wire in behind the drywall or whatever wall finish you choose, you wouldn't want that to push the insulation to the back of the sheeting). Even depending on the pitch of your roof, the baffles will ensure the insulation doesn't "slide or slip" to pack into the rafter bays.
Just my two cents.
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snobdds
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# Posted: 29 Aug 2017 06:25pm
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For a cathedral ceilings, it's better to not fill the entire cavity of the rafter with insulation. You need an air gap between the insulation and roof for ventilation. This is critical to prevent condensation build up. There is a lot of science to this and lots of reading out there. With this air gap, it make baffles useless.
I can tell the OP has done his research on this...
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NorthRick
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# Posted: 30 Aug 2017 08:50am
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Quoting: MntGoat which will leave a gap between the insulation and roof deck of about 3.75" +/-.
A two inch gap is enough, so you could add thicker insulation and get a higher R value. R19 is low for a roof.
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MntGoat
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# Posted: 30 Aug 2017 03:29pm
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Thanks guys. There's so much info on the net about what's best here. I have friends who have used foam baffles from the eave to the peak on their cathedral ceilings, but they pack it with insulation. In my opinion you are then relying on that small space the foam baffle provides for proper ventilation which keeps the roof deck cold (in winter).
Well the bottom line is, in winter, if the cold roof deck doesn't get warm, you won't have moisture. How do you keep it cold? By washing that space with outside air AND keeping the heat on the interior of the building.
It's a balancing act to me. Again, these are all my thoughts only. I am not saying it's right.
It would seem to ME, that by leaving a large gap between the insulation and the roof deck, it provides A LOT more room for air movement.
It seems to me that airflow is the main item most buildings lack now, with all the insulation we pack into them.
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toyota_mdt_tech
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# Posted: 30 Aug 2017 05:05pm
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MTG Goat, I think you have it perfect. 
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smallcabin
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# Posted: 31 Aug 2017 03:22pm
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Mntngaot....I think you're good! I did the same with mine. Install baffles in between rafters from the soffit to the ridge vent...and then insulation. I was even thinking about adding plastic vapor barriers on top of insulation, but decided not to. Don't as me why? HAHA
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Bushwhacked
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# Posted: 31 Aug 2017 04:20pm
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Just to mix it up even more. I have known guys that swear by packing the entire cavity full of insulation. Their idea is to keep the moisture out of the rafter cavity to begin with. HA 
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snobdds
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# Posted: 31 Aug 2017 06:17pm
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And to mix it up even more, spray foam is better than leaving an air gap or packing it full.
I've always question the packing it full theory. Doing it that way moves away from a cold roof to a hot roof. A hot roof and a cold exterior is the point at which moisture falls out of vapor and condenses, which is at the roof sheeting level, thus sheeting rot. I've never seen how to completely seal off a roof cavity with normal building materials like wood or drywall. Moisture will work it's way through eventually.
I would rather leave a gap and let the natural vacuum created with a ridge vent suck out the moisture before it can condense on a warm roof. If the roof is cold, there is nowhere for the moisture to condense on.
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Bushwhacked
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# Posted: 1 Sep 2017 12:23am
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Quoting: snobdds I would rather leave a gap and let the natural vacuum created with a ridge vent suck out the moisture before it can condense on a warm roof. If the roof is cold, there is nowhere for the moisture to condense on.
I second that.
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Steve_S
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# Posted: 1 Sep 2017 07:42am
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@MtnGoat...
You have received some good advice and some not so clear. I would strongly recommend you have a look at THIS site which covers Cathedral Ceiling very well. https://inspectapedia.com/Energy/Cathedral_Ceiling_Ventilation.php
A VERY GOOD EXPLANATION with tech drawings and more is Here !
http://www.civil.uwaterloo.ca/beg/Downloads/Roofs-BuildCanada2005.pdf
There are many ways to do a Cathedral Ceiling but only a few will last as often there are things missed. Humidity & temp management is a critical thing for obvious reasons... who wants rotting joists & roof sheathing and THAT will happen if there is a goof up. Seen many over the years.
A Major issue is the insulation type used and each will require slight differences... some will require vapour barrier (fibreglass, rockwool) whereas Poly-ISO, XPS, EPS & Spray foams do NOT ! Even the "foams" have differences... but understand that ISO breaths but does not transfer moisture, XPS, EPS & Spray are closed cell and do not breath and create their own vapour barrier. Fibreglass will retain & wick moisture through it (ever see darkened mouldy f-glass ? & not to mention mice habitat), whereas Rockwool (Roxul) will not retain or wick moisture and is critter unfriendly... NOTE that Batt Insulation needs Galvanised Wire Retainers as over time there will be slippage & settling down creating small voids resulting in problems.
Passive Venting with "Batt Type" insulation is essential and must be done right allowing airflow from eaves up to venting strip at ridge beam. The foam shields cannot be crushed or interfered with (airflow wise) and they will crush if pushed up.
I have mentioned Cool Roof Design previously, I seriously suggest you have a look at the info... Here is a quick video with an excellent description & info... https://youtu.be/H1Zmo7D6Qqs But you can Google "Cool Roof Design" and you'll it is gaining in popularity and in some places becoming part of code... People are realizing how it saves on Heating & Cooling and is better for the building envelope.
Hope it Helps.
PS: As in all cases when building a home, the most important part of a roof system is proper design and installation... This is one place where "an Ounce of Prevention, is worth a Pound of Cure" has never been more true.
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NorthRick
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# Posted: 1 Sep 2017 09:58am
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Quoting: snobdds I would rather leave a gap and let the natural vacuum created with a ridge vent suck out the moisture before it can condense on a warm roof. If the roof is cold, there is nowhere for the moisture to condense on.
That's backwards. Water vapor condenses onto cold surfaces, not warm. Think a glass of ice water on a humid day.
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snobdds
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# Posted: 1 Sep 2017 10:33am - Edited by: snobdds
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Your right, but the context I was speaking to was a cold roof, where the interior is still relatively "warmer" than the outside.
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