Cathedral Ceiling Insulation Options

Started by North Sask, January 23, 2014, 12:23:21 AM

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North Sask

I didn't get a lot of traction with this post in my project thread so I thought I would dump it in the general forum...

I have reviewed some of the old threads on cathedral ceiling insulation and I have reviewed the Green Building Advisor article on insulated cathedral ceilings. From that I have come up with three alternative roof insulating scenarios (vented and unvented) for my 16 x 24 cabin with a 9:12 roof pitch. It seems like Option 1 and 2 would not meet code as shown below (insulation not thick enough???) but they are a step in the right direction. Any tips on determining what the insulation thickness would need to be for the setup shown in 1 and 2 (to meet code)? I think it would be appropriate for my situation to assume -30 C (-22F) outside and 21 C (70 F) inside as the worst case. I don't really know what to assume for humidity levels. Are there any resources/references for that or can you just use the humidity provided by the weather forecaster? Keep in mind there will be very limited winter use, no showers, no laundry, wood stove heat, and no interior plumbing whatsoever. I don't need to comply with any codes but I want an insulation system that will allow me to keep some heat in the cabin for the occasional winter trip. I also don't want to create a mold/rot disaster.

I am leaning heavily toward Option 1 (or some variation - I could also add rigid foam insulation to the interior of Option 1 or 2). Part of the reason is because the upfront cost would be low because I could/would insulate some time down the road. Option 3 would have a large upfront cost. Option 2 is a slight variation of Option 1 with a home made rafter vent rather than a factory made vent.

Option 1


Option 2


Option 3
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

Alan Gage

From all my reading and no personal experience with cathedral ceilings I'd go with none of the above. Skip the plastic vapor barrier (which is almost impossible to get right from all accounts) and spray foam the cavities. That gives you max R value and it also acts as the vapor barrier. I believe that's what building science recommends.

Alan


JRR

I prefer option #3.  I use two layers of 2" on roofs (South East).  I use a drain field/air gap/purlins under the roofing.  My sheathing goes against the rafters.  I like the idea of having all (most) of the structural wood inside the insulation.  In this way the wood stays as warm and dry as "inside".

MountainDon

I saw this before and then got sidetracked.  :o

IMO, it is difficult to build an energy efficient cathedral ceiling / roof especially in a cold to very cold climate. The rafters are almost always never deep enough to provide the appropriate amount of insulation. Not to mention that a room or building with a cathedral ceiling is harder to get warmed up from cold than a shorter ceiling. But that is another matter.

I believe the best cathedral roofs would be made with (1) spray in closed cell foam on the underside of the sheathing.  (2) Exterior foam sheets laid over the roof sheathing. Spray foam is probably out of the question because of the distant location. So that leaves rigid sheet foam. Probably 6 inches of XPS as a minimum.  More os likely needed to meet whatever energy codes up there might suggest. Being a part time residence doesn't usually exempt one from codes; but so far north as you are, well who is looking?  The insulation R-value is between you and your wallet. The wallet being measured for both the expenditure for the insulation and for the future heating bills.   

Enough foam R-value up there will keep the inside warm enough to prevent condensation and ends the worry about venting and vapor barriers. . It gets more complicated and expensive when you then have to add another layer of sheathing to attach the metal or shingles to. And all the long screws to attach that and the foam to the rafters add up and add to the workload. You need careful measuring & marking; straight driving of screws.  Anyhow one of those would be my ideal cathedral construction.

Just a comment; my experience has been that building and then adding / modifying, has also cost me more and been more difficult in the end than doing it all from the get-go.

As the green builder article pointed out the biggest good thing you can do is eliminate air movement between the interior and the exterior. More good probably comes from great air barrier installation in a roof / ceiling than extra insulation.

Buildingscience.com has good info on insulation and air and vapor barriers too. They focus on the best methods for different climates.

I think installing and especially building air vent channels for a vented cathedral roof / ceiling is too much effort when an excellent non vented roof / ceiling can be built and be better insulated.

As a further aside, in the vicinity of our cabin there is a small development. Some people there are part timers but most are full timers. There are many homes there that I am certain have cathedral ceilings. Those are the ones with big icicles hanging off the eves. Insufficient insulation to prevent the roof snow from melting and re-freezing at the overhangs.

I like the appearance, the open feeling, of most cathedral ceiling homes and cabins I have been in. However, my mind runs to all the negatives I have seen over time. But that is just my outlook.
Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

I'd sheet foam over the exterior of the roof and walls and on top of the floor sheathing inside, then sheath roof and floor again with 1/2" and rainscreen the walls. Minimal insulation with maximum sealing. Wood heat, outside combustion air, throw another log on. Have a well screened vent to the exterior high on a gable and thru the floor at the opposite end that you can leave open when absent.


John Raabe

I really have to say that I think the option one or two is a good inexpensive and well tested method of insulating a single rafter cathedral. You don't need to worry as much about the vapor barrier stopping vapor but it should be a good air barrier. The cavity under the sheathing can vent and keep ahead of any imperfections.

I'd check with experienced builders in this climate and maybe talk to neighbors with similar structures. I'll bet many were built as you show. Doing a good job on the underlayment, flashing and roofing is very important. Many times the roof design gets blamed for moisture problems caused by sloppy roofers.
None of us are as smart as all of us.

North Sask

I don't think spray foam is a great option for a boat access cabin that is about two hours past the middle of nowhere.

Quote from: MountainDon on January 23, 2014, 02:29:38 PM
Probably 6 inches of XPS as a minimum.

Pardon my ignorance. XPS is what now? Extruded Polystyrene?

I appreciate your thoughts, Mountain Don, but I have an irrational desire to have a cathedral ceiling. I acknowledge that I am pushing practicality to the side for this decision. I don't think there will be any convincing me otherwise.  :)

There seems to be a large range in price for the rigid foam panels. It would cost anywhere from $1800 to $4400 to put a 6" layer on the roof. I guess that would get me to R-30 and it would be pretty well sealed.

What thickness would be required/recommended for the walls and floor? Around R-15 or R-20?

I like the idea of wrapping the house in rigid foam but I think such a large up front cost might be prohibitive. I was planning to insulate with fibreglass in year 2 or 3.

Don_P, can you elaborate on why you would leave the vent open when absent?

It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

North Sask

Thanks for your feedback John. Ultimately I might choose Option 1 so that I can spread the cost over a few years.
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

MountainDon

QuoteXPS is what now? Extruded Polystyrene

Yes, that is it. R 5 per inch thickness. Best rigid foam next to polyisocyanurate which is 6.5 but also more money most of the time. But that also depends on where you buy it. Locally I have access to a company that sells rigid foam of all types. I can get polyiso from them a slightly less than XPS at any of the big box stores.

~~~~~~

QuoteWhat thickness would be required/recommended for the walls and floor? Around R-15 or R-20?

R20-ish would be nice for walls.  Maybe think about 2 inches or so of XPS on the exterior. Two layers with staggered joints. Taped properly. That'll seal the air barrier very well. I think I said above, that an excellent air barrier is worth a lot of insulation thickness; maybe more important.  Then later add fiberglass infill between the studs if you want, before completing the interior wall faces. Again I have to balance what I would want ideally (if I was living there full time) vs what is pretty well guaranteed to be a part time residence. So with that in mind R10 - R15 on the exterior could be okay.    Floors ?  Energy code says R19 minimum IIRC.

~~~~~~

I understand wanting something and saying to heck with the potential downside. It's called compromise. Everything I contemplate with my trailer project has a compromise. 

~~~~~~

I think Don_P's idea of the venting is to have the venting head off any interior condensation issues as the structure sits there at whatever the ambient temperature is. Heck, maybe I should have implemented something  like that. Once the cold weather sets in the interior temperature is colder than the exterior daytime temperature every time we go up to our cabin. Last time it was 28 F inside vs 40 F outside.   :o    I'm sure he'll expand on that.

~~~~~~

Have you heard of Reschek? Free download. It is a US oriented tool for calculating how the energy efficiency of the building stacks up against the energy codes. It lets you play R-values of walls, ceiling, floor against each other and the windows and doors; trade one thing for another without having to go by the cookbook building code method. We in NM are required to use it when submitting plans for a permit.


Just because something has been done and has not failed, doesn't mean it is good design.


North Sask

Thanks, MtnDon. I had not realized that the air barrier was so critical. Looks like I will have to do some more reading on the subject matter. I'll have to take a look at Rescheck. I am not at all familiar with it.

Looking forward to more updates on your trailer project. I really like that you opted to build your own.
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

Don_P

QuoteI think Don_P's idea of the venting is to have the venting head off any interior condensation issues as the structure sits there at whatever the ambient temperature is. Heck, maybe I should have implemented something  like that. Once the cold weather sets in the interior temperature is colder than the exterior daytime temperature every time we go up to our cabin. Last time it was 28 F inside vs 40 F outside.

That was my thought. I was advocating an unvented insulation approach, a tightly sealed box that will be the dampest when you close it up to leave for extended periods. To my thinking letting some fresh air move through it while you're gone might be a good idea... If I'm wrong it's simply a matter of closing the vents and leaving it sealed. I guess the active way of doing something along those lines would be a solar powered vent fan switched by a humidistat. When the sun is shining if the humidity inside is above a set point the fan kicks on.

Redoverfarm

Quote from: Don_P on January 24, 2014, 10:42:42 AM
That was my thought. I was advocating an unvented insulation approach, a tightly sealed box that will be the dampest when you close it up to leave for extended periods. To my thinking letting some fresh air move through it while you're gone might be a good idea... If I'm wrong it's simply a matter of closing the vents and leaving it sealed. I guess the active way of doing something along those lines would be a solar powered vent fan switched by a humidistat. When the sun is shining if the humidity inside is above a set point the fan kicks on.

Don would not some of the crawlspace, thermostat vents work? Of course this would only apply in the wintertime colder temps that they would close and summertime they would open.

Don_P

Cheaper and lower tech and it probably would work. When it gets warm is when the trouble with mold starts. Just like at the sawmill, best thing about winter is you don't have to worry about fungi this time of year.

Erin

#13
QuoteI can get polyiso from them a slightly less than XPS at any of the big box stores.
And it's a popular material for building recycler's stores. 
I'm about 200 miles for Denver and commercial buildings are always going up and coming down.  Consequently, material like polyiso can be found fairly cheap!  We wrapped our house in poly-iso that I picked up for about 1/2 the price of new at Menards...

BTW, our cathedral ceiling looks like this:

The roof truss, 1x8T&G, roofing felt, polyiso, 2x4 strapping tied through the polyiso into the roof truss, and topped with steel. 
The wise woman builds her own house... Proverbs 14:1


old_guy

This is a great discussion.  Even though most of us need not worry about floating materials to the building site, cathedral ceilings are of common interest.  The 3 option pictures are very informative and help (help me, at least) a lot in clarifying the applications.  I like option 3 for a variety of reasons, but am confused about what happens at the exterior walls.  I expect to have significant gable and eave overhangs, and an unsure (that gives me too much credit) how to implement them using this option.

The gable ends seem easy - let the T&G layer extend out a couple feet (or whatever is aesthetically appropriate) past the gable framing and add framing above it to support the roof sheathing.  The eaves have me puzzled.  Are the rafters (even if big ones at 48" or greater spacings) allowed to extend past the wall as tails to support the T&G?  Are smaller (2x4?) rafters sistered to the big ones, and perhaps added atop the bottom couple of feet of T&G between them, to support the plywood sheathing and the roofing?

Every option I consider brings with it a series of technical or aesthetic problems.  How are overhangs generally handled with this type of roof?

Erin

How much of an overhang are you wanting? 
You can create a bit of an overhang just from your steel roofing...
The wise woman builds her own house... Proverbs 14:1

old_guy

The eave overhang would be about 18 - 24 inches.  Too far for just the metal, I think.

Sorry for the lack of info.

- John

North Sask

Quote from: old_guy on January 27, 2014, 02:58:50 PM
The eaves have me puzzled.  Are the rafters (even if big ones at 48" or greater spacings) allowed to extend past the wall as tails to support the T&G?  Are smaller (2x4?) rafters sistered to the big ones, and perhaps added atop the bottom couple of feet of T&G between them, to support the plywood sheathing and the roofing?

Every option I consider brings with it a series of technical or aesthetic problems.  How are overhangs generally handled with this type of roof?

There are a number of ways to frame your overhang (e.g., exposed rafter tails, boxed-in overhang). I would recommend searching for pictures on Google and selecting the style that you like. Some framing books have different eave options. I am planning to go the boxed-in route. The rafter tails extend beyond the wall and support the roof sheathing. You attach a ledger to the wall sheathing and then attach a lookout from the rafter to the ledger. A picture is worth a thousand words:

     


Here is a view from my own plans. I suspect that if you were going to proceed with option 3 (unvented), your blocking would extend from your top plate on your wall to the top of the rafter (or underside of the sheathing).
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

Don_P

This is one way, lots of work, but I know it works. Typically when I've done that type of roof the rafters are heavy timbers widely spaced, generally something like 4x10's on 4' centers and the rafters often extend out to support the eave overhang. The decking is then 2x6 T&G and extend out to form the gable overhang. A 2x is then ripped to the width of the insulation depth, say 5" plus the thickness of any strapping (2x usually) over the foam. That is attached on top of the T&G flush to the outboard gable end of the overhang. I'll often make an L by attaching a 2x4 to the side of the ripped 2x and screw down thru the 2x4 into each piece of 2x6 T&G to suck them up flat to the ripped 2x edge. Do that again on the outside edge of the gable wall to avoid having to fill the overhang with foam. Do the same along the bottom edge. Just outside of the eave wall, an insulation dam dropped to just insulation thickness is run. Fill the field with foam, offsetting seams. Then there is usually 2x4 strapping running up and down the roof on 2' centers over the rafters (good screwing) and midway between rafters screwed to just the 2x6 T&G decking (check your oly screw length carefully!). The 2x4 strapping runs over the lower insulation dam and butts into the backside of the bottom ripped 2x. Then ply over everything. The lower eave overhang is vented into the insulation free channel, the 2x4 strapping provides a vent space. Run metal straps over the ridge to connect the strapping together over the ridge.

I've also done more widely spaced heavy timber trusses, timber purlins running horizontally across the roof and T&G decking running up and down the roof, then foam, strapping, ply.

When we first did this in the late 60's through the 70's I was a kid working for my Dad. There was the timber decking, We were using 3" thick T&G and very wide on center rafter spacings with 4x12 glulam rafters, it wasn't unusual to have only 2 sets of rafters and the gable walls.  2" of foil faced polyiso, and shingles nailed directly over the foam with 3" roofing nails. We were in high cotton, cutting edge, smarter than the average bear and everything. Mom used the scraps of foam to do macrame' on, hey it was the 70's and I had a cool macrame belt on my elephant bellbottoms  ;D.  That foil facing burned the shingles up from underneath... what I learned was, let someone else be the guinea pig out on the bleeding edge. By the time the problem showed up there were a number of roofs completed that way. The extra layers of foam and the vent channels with ply over top for a nail base solved the problem.

Erin

Quoteand shingles nailed directly over the foam with 3" roofing nails.
Aside from the overheating issue, was this water tight??  Just thinking of how many nails one uses for shingles... 
And why didn't you just use steel roofing?  ???  Granted, the good stuff wasn't quite as accessible as it is today, but even then, there was galvanized, corrugated stuff that was available at any lumber yard...
The wise woman builds her own house... Proverbs 14:1


Don_P

 I can't remember there ever being a leaker, (now my 9th grade english teacher's roof... that was my first leaker, talk about stressful. Never work on your teachers roof, especially if it's not a subject you're doing well in  d* Then again, maybe she was helping me, understanding that NASA was not going to be calling)  We did have to learn how to drive the nails since snug was it, you didn't want to pound dimples into the foam, but we could walk the roof without any trouble. Metal was common prior to and after that period, it was an ag/outbuilding/commercial product in consumers minds at that time. To be honest I still prefer shingles. Ours are coming up on replacement and we're torn as to which way to go... I'll probably decide the morning of  :D.

North Sask

Here is an Option 4:



It is the same as Option 1 but with the addition of 2 inches of XPS on the interior. The XPS would serve as the air barrier and would up the R value. According to some of the reference material at Building Science, this should be a perfectly acceptable vented cathedral roof/ceiling. It seems the keys to success for a vented cathedral ceiling are simple roof geometry and minimizing ceiling perforations. I am going with a simple gable roof and I should only have one perforation for the chimney (no plumbing so no vents, no electricity so no lights, etc). I believe my cabin would be in Climate Zone 7 so according to Table R806.4 (IRC 2009) I should have a minimum R value of 30. As drawn, my Option 4 should come in at R-38. This option will allow me to insulate when I decide to finish the interior of the cabin (one or two years down the road). If money was less of a concern I would probably proceed with Option 3.

For reference purposes, I think the consensus on this thread, with respect to preference for cathedral ceiling insulation, would be:

1) Option 0 - unvented with spray foam (I did not post a picture of this option so you'll have to use your imagination, or follow the Bldg Science link)
2) Option 3 - unvented with multiple layers of exterior XPS
3) Option 4 - vented with combination of XPS and air permeable (e.g., fibreglass)
4) Option 1 - vented with air permeable insulation (might have long-term performance issues)
5) Option 2 - same as above but more difficult to construct

The above order would be listed in terms of performance but it does not consider cost.
It would be greatly appreciated if you stopped by my thread and left your two cents.
Great Northern Saskatchewan Adventure...Round 2

Don_P

Option 4 is basically the roof on our house. I used 2 layers of 3/4" foam under the rafters and then screwed a layer of 1/2" osb thru the osb and foam into the rafters to provide a nail base for a T&G ceiling finish.