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optimistic
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# Posted: 5 Sep 2013 02:04pm
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My roof in my 12' X 12' cabin (13.5ft ceiling) is steel panels that have felt under it then osb and finally my rafters are 2x6. I have only one ceiling light - a chandelier.
Where the ridge is, from the inside of the cabin looking up, there is a gap between the osb sheets. I wanted to fill that with spray foam, then install fiberglass Batts between the rafters, (the ones meant for for 2x6 walls that have paper on it), and then cover with 3mil sheet... Then install wood strips for finish ceiling.
Reading about it now - I see that this is a very bad idea and that I most create ventilation or something.
I already filled the gap with foam insulation and connected 2/3 of the Batts. This can be undone of course.
Any guidance as to proper insulation?
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PA_Bound
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# Posted: 5 Sep 2013 03:06pm - Edited by: PA_Bound
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Opti... I'm not an expert here, but from what I read on other posts the recommendation is to create an air-gap between the OSB and the insulation to allow air to flow from from the soffit to the ridge- removing condensation off the OSB. First, for this to work, there needs to be vents on both ends- soffit and ridge. Do you have those?
Assuming you do, the next question is how to maintain that air-gap when the insulation is installed. I installed semi-rigid plastic ridge-rafter vents that I got from Lowes. They fit against the OSB, between the rafters, and run from eaves to ridge maintaining the gap. Here is a picture taken mid-installation:
Maybe this will help, or maybe I mis-understood your scenario all-together.
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TheCabinCalls
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# Posted: 5 Sep 2013 03:37pm
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Was the gap in osb at the ridge? and was it there for you to use as the ridge vent?
Venting a roof makes the shingles last longer and keeps moisture and heat down.
If you vent a the soffit then you need to vent at the ridge and have an air gap in the rafter bay to allow for the heat and moisture to leave.
If you have the money you could just use spray foam and not vent it.
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TheCabinCalls
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# Posted: 5 Sep 2013 03:38pm - Edited by: TheCabinCalls
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Also I am not sure you want to have two vapor barriers. Make sure your paper on the batts doesn't serve as the vapor barrier. My feeling is that two vapor barriers will trap moisture.
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MtnDon
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# Posted: 5 Sep 2013 06:42pm - Edited by: MtnDon
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Two schools of thought on this; non vented and vented.
Vented: Ideally there would be a 1 inch airspace between the insulation and the upper osb. There would be continuous ridge venting and a soffit vent in each rafter bay. The total free vent area is determined by the floor area; 1 sq ft of free vent area per 150 sq ft of floor. That should be divided unequally with more vent space given to the soffits, the 'in' air. That encourages circulation.
Non-vented: Requires the use of foam against the roof sheathing. Either on the underside or on top with the roofing on the foam. The idea is to keep the underside of the roof warmer than the dew point so condensation can not happen. That can not be accomplished with batts and can be difficult when cutting rigid panels to fit in between the rafters. It can be done by cutting and then sealing along all edges with foam in a can.
How thick the foam should be is determined by climate zone.
Here is a link to an article on insulating cathedral ceilings, vented and non vented. The non vented starts about 1/6 the way down. Most of the page is reader comments do do not let the 1/6 deter you from reading.
There is a load of info including references to vapor and air barriers.
Basically you should never have 2 VB's .
~~~
The fact that this is a part time occupancy building can skew the rules a bit. Maybe. Some think so, but until someone can prove it is okay longterm, I prefer to be cautious.
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rockies
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# Posted: 5 Sep 2013 08:26pm
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I would go to www.buildingscience.com and then go into the "information" area and search for "vented" or "unvented" catherdral ceilings. They have lots of articles and drawings on how to insulate, vapour barrier and ventilate roofs, walls and buildings. Or look in their "popular topics" area for "high r-value roofs"
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Truecabin
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# Posted: 6 Sep 2013 04:53pm
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if you want good answers put up a picture of this gap a small building probably does not have the separation of inside/outside to be able to discuss this
i bet your building is too small and too simple to worry about venting or even discuss venting
the air that you want passing thru a vent is outside cold air that came from the cold outside and returning to outside this air is separated from inside air by a vapor barrier its not able to pick up any inside moisture
if its warm inside air then plug it up and stop the air
you dont want warm inside air flowing past anything cold or it will drip with water
warm inside air should only be allowed to contact warm insulated walls or warm inside insulated ceiling
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optimistic
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# Posted: 16 Sep 2013 07:41am
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Thanks for all the info. Been super busy (and still am) with work th elast few weeks....
I read that GBA article and it seems my only way of doing an unvented ceiling is with a combination of rigid foam and batts.... But because I need a minimum of 3.5" rigid foam for that - I might as well just do a 5" rigid foam insulation to save me time.. I already wasted enough with putting up the batts and now I need to take them off....
The question is... can I get away with leaving it as it is - just batts? After all -this is a weekend cabin and I am not going to spend more than a weekend every so often there.
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Truecabin
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# Posted: 16 Sep 2013 10:41am
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relax your little cabin doesnt have house problems like a dishwasher and boiling pots of spaghetti and kids taking 30min showers
yes leave it alone
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TheCabinCalls
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# Posted: 16 Sep 2013 01:37pm
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Batts are fine. Just make sure you are venting where you can.
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MI drew
Member
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# Posted: 16 Sep 2013 05:25pm
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PA-Bound.
I am doing my ceilings very soon and those are the same panels I will be using. Question. When you got the the change in the roof line did you cut or bend them to fit? Also for venting... did you have them meet at the ridge or leave a small gap?
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creeky
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# Posted: 16 Sep 2013 06:39pm
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kludge: to quickly put together a solution that gets the job done.
you could consider reopening the ridge gap for ventilation.
then leave the batts. face the ceiling with an 1" foil faced polyiso layer tacked onto the ceiling joists. once taped (i like to use window flashing tape) you have an extra r 7.5 and a vapour seal. put your board over that. i actually like the "tinfoil" look (I'm using this technique in my outhouse), but I have noticed that my female visitors are more bemused ... the fact that it's completely waterproof however is a bonus for the bathroom ... and it does bounce the light around.
to improve air flow: drill or cut holes along the top of the exterior wall sheathing to open a vent space into the "attic". the batts should settle to leave space from the OSB. now air can flow up along the underside of the OSB and out to the ridge. the metal roof ridge needs to allow air to continue to the outside / you may need to screen the new wall openings to keep bugs out.
i used a version of this idea on my solar shed and it has worked well. it provides air movement in the attic and r20+ levels of insulation. plus it's pretty simple to do.
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PA_Bound
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# Posted: 17 Sep 2013 09:21am - Edited by: PA_Bound
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MI Drew... At the bend in the roof I tried it both ways. My first thought was to cut the angled sides of the panel (which give it rigidity) and then bend them in the middle, but I found the panels awkward to work with when I did that. After two or three that way, I switched to simply cutting them completely in half and then butting the two ends together. That seemed easier to work with, and I really couldn't see any reason not to continue that way so that's how I finished them.
At the ridge, I again mostly butted them together but I was not too concerned about how tight the fit was. I figure the air will travel the path of least resistance and once I get the final paneling up that path will be through the ridge vent to the outside. A couple places I left a few inch gap, as it is there I plan to install vents in the interior ceiling to allow the cabin to "breath" when I'm not there and it's buttoned-up. We had problems with trapped humidity in my hunting cabin because of this, and we found that even a little ventilation greatly reduced the problem.
Below is a picture of my ridge. It's not the greatest job, but it works (and once it's covered with insulation who is going to see it anyways?):.
Looking up at ridge- before insulation
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creeky
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# Posted: 18 Sep 2013 09:48am
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I plan to install vents in the interior ceiling to allow the cabin to "breath"
that's a great idea. thx.
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woodspirit
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# Posted: 30 Jan 2020 03:56pm
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I bought my cabin and it has no soffit vents or ridge vents. The only heat source is a propane heater invented that looks like a woodstove with glass front. The drywall in the cathedral ceiling is bowed from moisture in the insulation so it's all got to come down. Drywall and insulation and redone the right way. Gonna be a huge mess.
I'm going to put soffit vents in, a ridge vent and put in those poly vents in from bottom to top. It's the only way to fix it. In the fall the night temps were upper 40s and I ran the propane heater with the windows and sunroof open and water was running down the windows by morning and pooling up on the windowsills. After that I'll replace the roof wit metal roofing. Then get rid of the propane and put in a woodstove.
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Eddy G
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# Posted: 31 Jan 2020 07:49am
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It draws up some controversy but you can avoid venting with sprayed in closed cell foam
You don’t need a professional with a big pumper truck
I bought 50lb tanks and carried them in with a sled.
Wasn’t a big deal.
Go to any of the leading cell foam manufactures pages like “Foam it Green†“Tiger Foam†“Dow froth pakâ€
They can explain the science and tell you the what’s and hows.
I’ve used the green foam on a vaulted ceiling with great success. 5yrs running no issues, inspected regularly for moisture etc.
I used Dow froth pacs on the ceiling/roof and walls on a 3 season porch. It’s only it’s second winter but seem fine. The difference in warmth, draft, sound is amazing
It’s more expensive at first but add the costs of all the sofit and ridge vents time and labor and future fuel savings ...not to mention the added comfort level and it’s not more expensive
Go to the web sites read the info send them questions and see what you think
Good luck post pictures before after and during if you can. It’s always great to watch others projects
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razmichael
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# Posted: 31 Jan 2020 11:51am
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I partially agree with Eddy on this. Doing a "hot" roof might be an easier solution and even cheaper considering the problems with fixing the current situation (obviously much easier if done right during construction). I had my cathedral gambrel roof sprayfoamed R36+ (and did the loft ends while at it). No vents etc needed if you have enough thickness. Certainly not cheap but not as expensive as I thought it would be and also meant the original roof construction was much simplified (as I was doing it myself).
Where I will throw out a caution. The process is not easy and there are lots of things to understand (type of foam, temperature, rate of spraying etc). Lots of horror stories of people messing up and it is not easy to sort things out after! Like many of these types of jobs, it may look easy but that tends to be when you are looking at someone who has done it many times before. Screw up some tiling and you might live with it or tear it up and try again - screw up spray foam and you are out a lot of money and in a bad situation to sort it out. If you can afford it I would consider getting an expert in.
And Eddy - I do applaud you for doing your own sprayfoam - I would never have tried it - I make enough of a mess with the small handheld cans!
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Eddy G
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# Posted: 31 Jan 2020 03:53pm
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[quote=razmichael]
And Eddy - I do applaud you for doing your own sprayfoam - I would never have tried it - I make enough of a mess with the small handheld cans
Believe me, I went round and round before making a final decision.
I did a lot of research and took it really slow.
In the end it worked out well for me.
That’s why I stress going to the websites and contacting the manufactures. Watch a lot of YouTube.
I agree it isn’t for everyone.
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DaveBell
Moderator
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# Posted: 31 Jan 2020 04:15pm
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Interesting discussion. I don't like the idea of a vented ceiling because humid outside air is allowed in the gap between the insulation barrier and roof. From MtnDon's post it seems if you use batts you should allow venting. Okay, so don't use batts. I think I would use the Closed Cell spray foam (CCSF) as Eddy G. did. Six inches of CCSF and you have R-40 ceiling.
BTW, I saw another article against spray foam in the walls because of thermal bridging through the framing. It recommended rigid foam on the exterior with regular batts on inside walls. So what about thermal bridging through the roof framing? I guess I'm going with a layer of rigid foam board on the ceiling before the ceiling finish.
My property is 1800' up on a mountain and if I insulate smartly, I should be able to reduce the amount of wood I feed the wood stove, less tree cutting, and less wood splitting.
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woodspirit
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# Posted: 2 Feb 2020 04:08pm
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Ok but either way I have to remove the ceiling an all the insulation.
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gcrank1
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# Posted: 2 Jan 2021 02:14pm - Edited by: gcrank1
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When it (whatever 'it' is) goes bad ya just gotta suck it up and do/redo it.
I never did decide what to do with my old '83-84 build cabin cath ceiling so it still has the 1/2" cdx plywood covered with asphalt shingles. Id get some frosty roofing nail tips and a few little drips off em, no big deal. When I had some shingle damage and leaking I could tell pretty much where it was at too.
Back then I had no advantage like the i-net and this forum and got lots of conflicting info. It sounds like there is still controversy....
Thermal bridging through the rafters, yeah, it'll happen. So what, its a cabin.
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Brettny
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# Posted: 3 Jan 2021 06:57am
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Quoting: woodspirit Ok but either way I have to remove the ceiling an all the insulation.
Yes it's all got to come down and you will prob have mold on the rafters. Since your going to all this trouble I would do spray foam. It will never happen again.
Also by leaving the windows open and propane heater on seeing water didnt really tell you anything. Un vented Propane adds alot of moisture in a building alone.
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ICC
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# Posted: 3 Jan 2021 09:41am
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Funny how old topics pop up again sometimes.
But yes, IMO, cathedral ceilings are mistakes
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KinAlberta
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# Posted: 3 Jan 2021 02:31pm
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I love it when old threads are updated by the early posters with the solutions they eventually chose and resulting long-term experience.
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snobdds
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# Posted: 4 Jan 2021 01:49pm - Edited by: snobdds
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I don't think cathedral ceilings should be vented, after all these years, that is what experience says.
So only two options in my book.
1. Spray foam which creates an air seal and cold roof.
2. If spray foam is not an option, foam board directly underneath the roof decking in an appropriate thickness with the edges sealed with canned spray foam for an air seal.
No batts, no soffit vents, no ridge vents, no plastic things to try and direct venting...nothing. Foam should be the only thing in contact with the roof from the underside.
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paudette
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# Posted: 5 Feb 2021 04:17pm
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I have the same type of building with a gambrel roof with very small eves and no soffits. I am planning on doing a "hot roof" with spray foam. I only have 2x4 framing on the roof but will be filling the entire cavity with closed cell spray foam with no air gap or baffles. As far as I can tell this should prevent hot air condensing on the roof sheathing. I will only be using this cabin on weekends however while there I will be monitoring the moisture and mitigating it with opening windows or using dehumidifier when needed. For heat I am using a mini split heat pump and electric resistance heaters. One criticism of this design is that if there is a roof leak it could cause more damage however I think it is worth it for me considering the alternatives. I am in Minnesota and will not meet the recommended r-value for my ceiling however it should be good enough to prevent the air reaching the dew point on the ceiling/insulation. Open to any suggestions won't be adding the insulation until spring.
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Brettny
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# Posted: 6 Feb 2021 08:20am
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Use 2x2 furring strips and pad out your 2x4 roof rafters. 2x4 isnt very thick for ceiling insulation. Your prob dealing with closer to 3in of foam. That would be about R18. That not much.
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