Insulation for Cathedral Ceiling

If the roof rafters (trusses??) are not deep enough to accommodate R-30+ insulation, what other options are there to provide higher than R-19 insulation protection? Can a combination of rigid foam insulation board and R-19 be used? Thanks.

 

CharlotteAnn,

Yes it can as long as you maintain proper ventilation from soffit to ridge. The plans should indicate such.

If you have concerns about this, contact your local building official.

Hope this helps!

 

as you can probably see from previous posts, we're experiencing some moisture "issues" . . . our roofer indicated that the R-30 insulation would have to be scrunched down to accommodate the venting baffle, thus reducing the insulation value of the R-30, and he said it would probably be better to just use R-19 (which is already in there, house is 17 years old) I'm thinking once we open up the airway, the R-19 might not be sufficient.

 

CharlotteAnn,

I agree. Are these scissor trusses or roof rafters creating a vault?

Good Luck!

 

well, I don't know much about the correct terminology, but I'm thinking they are just roof rafters running straight up to the peak of the house. the outside roof is on the top and there is insulation and then drywall underneath that is the ceiling of the rooms below. We have a cape cod with kneewalls and have noticed on the north side of the house condensation on nails, some mold/mildew in the sheathing, and some wet insulation in spots, mainly located on the far sides of the house. in some places where the insulation touches the underside of the roof there is mold/mildew/wet sheathing, and in other places where the insulation is touching the underside of the roof everything is dry as a bone. Frustrating.

 

CharlotteAnn,

I am assuming then that you have 2x10 rafters or TJI's.

Does any of this help?

http://dougaphs.smugmug.com/gallery/364233

 

I added another detail section that can be used as alternative. Maybe this will help.

http://dougaphs.smugmug.com/gallery/364233/1/62200787

 

Thanks for the links! Since all this is foreign to me, I'll have to study it. We have I guess what's called a "true" cathedral ceiling where it is minimual space between the sheathing and ceiling drywall. The R-19 insulation in the ceiling runs straight up the roof to the tiny attic space which does have a ridgevent. The kneewall area has insulation on the inside wall area. On the other side of the house the roofer opened up the ventilation by replacing the roofing plywood, using venting baffles and installing new insulation, but i noticed there is rust on the new nails, although the insulation does not seem wet. On that side that we opened up, we only have two soffits in the kneewall area. Perhaps not enough airflow still? Again, all this seems to be concentrated on the north side of the house.

 

I guess the paper part of the insulation is considered a "vapor barrier", so if we installed the rigid foam insulation board on the bottom and then the r-19 fiberglass insulation on top of the rigid foam board, we would not be compromising any vapor barrier issues, would we? is there any other type of roll or fiberglass insulation that could be used. thanks again. your input is much appreciated.

 

CharlotteAnn,

It is very important to increase the air flow - if you don't have enough, you will have problems again.

Try to install more, if possible.

Good Luck!

 

CharlotteAnn,

Tha vapor barrier (paper side) should always be right against the drywall - doing it the other way will be an issue of problems to come if the rigid is not cut properly.

 

I noticed you indicated on the link that we can place four layers of the rigid foam insulation board (am I correct so far? that would be instead of one rigid foam insulation board and then R-19 fiberglass insulation) and then the ventilation baffles, but they are the baffles supposed to run all the way up to the ridge vent? On the other side of the house, without the full cathedral ceiling, the newly installed baffles run up to the flooring of the attic (there is a small attic space that you can crawl around in on one side of the house) and on the side of the house with the full cathedral ceiling that currently has no ventilation, there is a tiny tiny opening that is just enough space for someone to lay down in .. . should the vent baffles run all the way up to the ridgevent, or stop at the attic opening? I promise, no more questions.

 

not to put down one row of rigid insulation board with the fiberglass R-19 on top of the rigid insulation board? either all rigid insulation OR fiberglass, but not both? thanks again.

 

CharlotteAnn,

There are different methods to do insulation when the total thickness outweighs the amount of space you have.

You can have R-19 against the ceiling drywall and lay R-10 (possibly 2 layers) on top of the batts

or

you can install 4 layers of R-10 and forget about using batt insulation

Regardless, you need to allow at least 1 - 1 1/2" of space for air flow.

Does this help?

 

exactly what I needed to know!! you are great! other than the vent baffle issue (straight up to the ridge vent or stopping at the small open attic space) i think i have a handle on the situation. thanks so very, very much!!

 

CharlotteAnn,

Great!!

You're very welcome!!

Sounds like you have a good roofer!

Good Luck!

 

should the ventilation baffles run all the way up to the ridgevent or stop at the small attic area below the ridgevent? that's all. i promise. no more questions. your kindness is greatly appreciated

 

CharlotteAnn,

Look at this,

http://dougaphs.smugmug.com/gallery/269705

Hope this helps!

p.s. Don't worry about asking anything on any subject, we are all here to help, members and staff alike!

 

CharlotteAnn,

The example would even apply to your vaulted space, just wanted to clarify that as you can see the location of the baffles that I noted.

 

I'm having a hard time comprehending . . . do the styrofoam vent baffles stop where the little attic flooring begin, or extend all the way up to the tip of the roof? Sorry . . .

 

CharlotteAnn,

They stop just after they get past the little attic horizontal member. Once the baffles extend into this space, that is all you need. At that point, all the air will be vented out through the ridge vent. Look at Method B (right side of the gable cross section).

The baffles are only installed to provide an "air channel". When using rigid, the baffles are not really needed as the space left over is the pathway for the air to freely flow.

Air baffles are used more commonyl when you have batt insulation and what little might be compressed, the air baffle ensures the clear pathway.

Blow insulation is another issue whereby the air baffles ensure that the air gets path the overall thickness of the insulation as shown in Method A.

Does this help?

 

I understand now. What I'm hearing is that if we used the rigid insulation board, we wouldn't need the baffle vents. Does the rigid insulation board provide a "vapor" barrier? And, since we have a ridge vent running the length of the house, should we close the one gable vent we also have, or also leave that open?

 

CharlotteAnn,

Well, lets ask this first - We are installing all of this from the roof side? or Inside? My assumption is the roof side.

1 Are we installing batt insulation and then rigid? The answer would be no vapor barrier needed.

2. Are we installing 4 layers of rigid? Then a vapor barrier (6 mil poly) would be laid over the rafters and down against the drywall, then rigid laid into place.

3. If we have existing gable vents, leave them alone. They will assist in cross air flow.

How's that?

 

CharlotteAnn,

if you use rigid insulation you do not need airflow at all. It is the advantage of the product especially with your high moisture issue. 1" of rigid is equal to r-7, so 5" would give you r-35. If you use batt insulation you need 2" of airflow over the top to work correctly. If you have rafters I am assuming since you have not stated sizing and thAt r-30 would be squished that it is more likely a smaller truss or reafter size more like a 2x8. If you squish the insulation you have no airflow and are creating more issues. If r-19 barely fits you still have no airflow. In your case i would go with rigid and use as many layers at it takes to fill the space and call it a day.

I hope this helps.

Brian Garrison

 

we'll be installing from the actually roof (not removing the drywall from the inside be rather replacing roofing from the outside). i now understand that we can use the fiberglass insulation along with the rigid foam insulation if the fiberglass goes down first, paper-side on the drywall/ceiling, and then the rigid foam board next. i'm assuming no baffle vent would need to be installed if the rigid foam is applied to the top of the fiberglass. and i understand that we can also use r-19 and then lay r-10 on top of that and use the ventilation baffle on top of that to ensure airflow (how am i doing??). when our house was built 17 years ago we only had a gable vent on the side of the house without the cathedral ceiling and nothing on the side of the house with the cathedral ceiling, and a few soffit vents. When the house was re-roofed, a ridgevent was installed. So, I'm hearing just leave the gable vent open and let it work along with the ridge vent. How did I do??

 

that another option is to completely fill the rafter cavity and not have any air flow at all? if so, what would be the proper way to vent the small kneewall area that flows to the ceiling? thanks.

 

CharlotteAnn,

my concern is that you are now trapping mositure between the rigid insulation and the sheetrock allowing the batts to hold the mosture. I would almost want to place the rigid against the sheetrock and move the batt up to where the airflow is over it. Doug is more familiar with this than I. I personally would choose one method and install it correctly. Again the most critical thinkg is 2" of unobstructed airflow to allow the air to convection properly and allow air flow per code.

Brian Garrison

 

CharlotteAnn,

I beg to differ with Brian as the air flow is required. If you are in snow country, this is essential and based upon what you describe, you need to do it this way.

The vapor barrier will provide protection from moisture as long as it is overlapped, the right way.

You have a kneewall area and this requires baffles.

My advice, to ensure that you are not making any mistakes or what we are suggesting is proper, call you local building official. Brian and I live in 2 different climate regions and since you have R-38 requirements, you need to ensure that all is up to Code and that you have no problems.

What I suggested is the best way in my opinion.

Hope this helps!

 

I will definitely contact our building inspector and now I'll be informed before I call him. We actually live in Virginia, and to be honest, I was told that R-30 was required now (but not when the house was originally built), but I hadn't looked it up in the code, so that will definitely be a question he can answer for me. I so appreciate the input everyone has provided. Thanks so much!

 

CharlotteAnn,

Unless you tear off the roof decking (sound like you are doing - once) you only have one chance to do catherdal ceilings correctly (contrary to open open attic space that can be worked on for years).

In your case, I would:
1) ensure that you have ventilation under the roof sheathing/deck. Baffles that run ALL the way from soffit to the area above insulation above your little flat ceiling "mini attic)
2) ensure that the baffles cover the entire width of the rafter run. Looking at photos in some posts, I see the (blue) styrofoam Durovent baffles that does not cover the entire width. Stay away from these!
3) do not trust the kraft faced paper on the insulation to act as a vapor barrier in a cathedral ceiling (OK for walls but NOT a catherdal ceiling).

You need of course first to ensure that you have an effective vapor barrier that prevents as much of the moisture transportion from the inside of the house into the wall/ceiling cavity. That said, there will always be some moisture that travels that way, and when it hits a point inside the cavity (normally inside the insulation) where the temperature is the same as the dew point, the water vapor will condensate. In sour cases, it will condensate inside the insulation and sort of get absorbed. If you are using styrfoam baffles, there is no airflow possible from "inside" the insulation to the air gap that exists underneath the sheathing/deck.

If you have the space, you be better off w/o any styrofoam baffles, as long as you can guarantee that there is at least about 1.5" of space between insulation and deck, and that air has free movement from soffit to ridge. I bad cases water can condensate on the inside surface of the styrofoam baffles, soaking the insulation badly.

If you are using rigid foam boards (for insulation instead of batts), a vapor barrier is still as important, as is the air gap under the deck. Rigid foam boards might not allow for any vapor to travel trougfh, but cracks, electrical boxes, where the boards butt up to ceiling joists, walls, etc, do! You need the vapor barrier here to prevent the air movement, and hence the transportation of vapor that WILL condensate on cooler surfaces.

My favorite insulation baffles is to put 1"x2" stringers along the rafters, and the install a baffle out of "peg board" (perforated masonite board). This will hold insulation back, but at the same time allow it to "breathe" into the circulating air - for as long as you live in the house and for future generations as well.

Then, not knowing where you live, but R-38 sounds to be a requirement, so you must be in a colder climate. In very cold climates, condensation can be even more of a problem (even though the outrside air is very dry), but you will not notice it until the spring when all humidity that has been trapped (in frozen form) inside the wall cavity starts to drip out in liquid form - water.

Too much insulation is possible, unless moisture control is properly implemented. If you have R-19 today, and the cavity really does not allwo for more, stay with it, but ensure ventilation and moisture control. You will not gain much more in comfort or reduced heating bills in getting to R-38 (laws of diminishing returns).

Controversial, yes, but I have as late as this week seen droplets form on the inside of these Durovent baffles - forcing a complete removal of insulation (thank god before the catherdral celing was finished).

 

Another alternative would be to add to the width of the rafters in the angled section of roof/ceiling. Line Wellcraft19, I would attach 1x2 strips alongside the rafters just under the decking and then use a perforated pegboard material to create rigid baffles. On a recent project, I edge-glued and lag screwed 2x8's to get the thickness I wanted.

 

We have such a mess in New Jersey! We are building an addition on our house. We have a cathedral ceiling that is built off 2 beams which form a T with a valley running down from each side of the intersection of the T. We installed, per our township official's direction, the styrofoam Raft-R-Mates in every section between the rafters and then placed the R30C insulation below that. We then installed sheetrock and a faux wooden plank ceiling by Armstrong. We installed faux beams over the 2 valleys. Over the last few months, we have found water dripping from the lower ends of both faux beams. When we removed the beams, wooden ceiling, and sheetrock next to the beams today, we found the insulation and styrofoam and wooden sheathing to be soaking wet. Since we did not have a problem all the months that the rafters were open, we are sure this moisture is from condensation but here's the problem....what do we do now? Why did it happen? How can we correct the problem and still conform to our township's requirements? Our roof has a ridge vent. There are no soffits where these particular rafters end. Any help would be greatly appreciated.