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Small Cabin Forum / Cabin Construction / Cathedral insulation mistake?
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optimistic
Member
# Posted: 5 Sep 2013 14:04
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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?

PA_Bound
Member
# Posted: 5 Sep 2013 15:06 - 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.
ridge-rafter vents
ridge-rafter vents


TheCabinCalls
Member
# Posted: 5 Sep 2013 15:37
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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.

TheCabinCalls
Member
# Posted: 5 Sep 2013 15:38 - 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.

MtnDon
Member
# Posted: 5 Sep 2013 18:42 - 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.

rockies
Member
# Posted: 5 Sep 2013 20:26
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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"

Truecabin
Member
# Posted: 6 Sep 2013 16:53
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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

optimistic
Member
# Posted: 16 Sep 2013 07:41
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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.

Truecabin
Member
# Posted: 16 Sep 2013 10:41
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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

TheCabinCalls
Member
# Posted: 16 Sep 2013 13:37
Reply 


Batts are fine. Just make sure you are venting where you can.

MI drew
Member
# Posted: 16 Sep 2013 17:25
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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?

creeky
Member
# Posted: 16 Sep 2013 18:39
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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.

PA_Bound
Member
# Posted: 17 Sep 2013 09:21 - Edited by: PA_Bound
Reply 


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
Looking up at ridge- before insulation


creeky
Member
# Posted: 18 Sep 2013 09:48
Reply 


I plan to install vents in the interior ceiling to allow the cabin to "breath"

that's a great idea. thx.

woodspirit
Member
# Posted: 30 Jan 2020 15:56
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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.

Eddy G
Member
# Posted: 31 Jan 2020 07:49
Reply 


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 pak”
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

razmichael
Member
# Posted: 31 Jan 2020 11:51
Reply 


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!

Eddy G
Member
# Posted: 31 Jan 2020 15:53
Reply 


[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.

DaveBell
Member
# Posted: 31 Jan 2020 16:15
Reply 


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.

woodspirit
Member
# Posted: 2 Feb 2020 16:08
Reply 


Ok but either way I have to remove the ceiling an all the insulation.

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