Insulating 2×8 Cathedral Ceiling
Good morning, I want to introduce myself. I am brian and I am from Long Island New York, and I have a question related to insulation. Ill try to give all the particulars I can. My house was a Levitt Style Ranch built in the 1950s, which can be compared structurally to most Cape Code Homes in the northeast. The upstairs was originally an unfinished attic, which people then converted to livable space with 2 bedrooms and a hall closet. I am re-doing only one of the rooms upstairs, so my daughter can move out of the nursery up there.
The roof was constructed of 2×6 rafters and 2×4 collar ties face nailed to every rafter. I furred down the 2×6 rafters to 2×8 nominal size. I can’t go any larger than that because of the size constraints of the room and the vaulted ceilings. The Town Building Department only requires I put in R19 in my upstairs vaulted (cathedral) ceiling since it is a remodel…. The flat portion is a different story and not really concerned about.
I know I can slap R19 up there and be to code and call it a day. But I would like to make it a little more efficient and comfortable if I can. Due to the cost and that I am only doing one of the rooms instead of the whole upstairs I am staying away from spray foam for now. I have the required 1 inch airspace using baffles.
Below are the 3 options I am thinking of with the last one being the one I was leaning towards.
1) Use R19 per the building department
2) Compress R30 batts (9 ½) and get approximately R25 (per Owens Corning). My concern here is if I compress this into the 2×8 cavity, will I crush those Styrofoam baffles and kill my airspace? I know I don’t get the full R30 value, but it would be a little better than R19. Compression Cathedral batts are not available in my area unless I special order for a supply place (which I have had problems with selling to me since I’m not a contractor)
3) Use Un-faced R13 Batts (3 ½ inch) and then cover these batts with Faced R15 (3 ½ Inch). This would bring me up to a total of 7 inches of insulation. The compression would be less than the R30 crammed in and probably get close to 25-27 (in theory). I’m only compressing a little bit less than an inch for the most part. But I’m more concerned of there is an issue of the “gap” or space in between the two batts. I know when you layer batts on top of each in a flat attic space you alternate the way you lay the batts to cut down on heat loss and air gaps. So I know you can layer the batts in a flat space but was unsure of issues in a vaulted ceiling. Thank you for all your insight, Brian
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what I did!
Brian,
It so random that I found this post while signing up again for FH. Well, you and I have/ had common problems.
I too renovated the second story of a 1920 home, balloon frame, 2x6 roof rafters, ridge board only and 2 x 4 collar ties. The home is a story and half so the roof is actually the ceiling for most of the ceiling(Cathedral).
Here is what I did. Furred down the ceiling with 2X2's in line with the rafters. I used R-21, 6" insulation. It was a special order from home depot. Yoy never want to compress fiber glass becuase it drastically looses R-Value. I allmost went with foam to get more R-Value but I chose not to because of cost. Also, I wanted to do the insulating myself to save some money. I didn't have to bring it up to code at R-30 because it was not new construction so i could go with less R value.
Here are three things that made all the difference in my design. I would highly recommend you consider doing the same.
1 - Radiant Barrier
2 - Roof Baffles made out of the radiant barrier that kept a 1 inch air gap between my roof sheathing and the radiant barrier baffles.
3 - Taped all my kraft faced insulation joints, rips, tears and puctures with Tyvek Tape.
I have attached some pictures below. This was not a cheap do it yourself but it had made a huge difference in my comfort on the second floor for both heating and cooling. My eneergy bills have gone down drastically.
I used Pink OC Foam ripped in to strips to set my depth off the roof sheathing and then I staples up pieces of radiant barrier that I had cut to fit. I pushed the barrier up to the foam strips on the rafters then stapled the foli to the rafters. The center strip of foam kept the center air gap constant so that the insulation didnt push the barrier up against the sheathing by accident. I then taped over the foli that I had staped to the joist to help seal it. I used foil tape but If I did it again I would use some thing like a protector wrap or a butyl tape that will with stand all the temperature variation. This was a belt and suspenders approach but I wanted to seal those baffles as best as I could.
Once the foam strips were in place, I sent the baffles and taped them up and then I put my R-21 OC fiberglass in. I was careful to place the FG in correctly and not smash it or crush it. I trimmed it where needed for friction fit. I then lapped the tabs over each other and stapled them which hid the ceiling rafters. This will prevent you from gluing the drywall rafters but I believe its worth it.. Once all the insuation was installed and stapled. I went to town taping everything I could tape with Tyvek Tape.
My drywall said it was the best insulation install he has ever seen.
Its very important to note that you must be moving air from your soffit through to your attic vents. You want to move as much air as you can. I even installed electric fans to pull air through my soffit vents. Gravity is Ok but on really hot days its nice to have some additional ventilation to move air through baffles into and then out of the attic to keep it cool so I dont have to use as much AC.
Also, the radiant barrier is not a vapor barrier and neither is the kraft paper but by taping the as much as you can you will create a pretty efffective air barrier so that air does not come in to the insulation space but any moisture can dry to the inside or the outside.
Let me know what you think. I am very pleased with the performance of this for both heating and cooling. It made this area the most comfortable space in the house and easiest to heat. It also much easier to cool the space than ever. The radiant barrier with the air space to move hot air during the summer gives me a 20 degree drop over attic temperature. I am no longer battling a 120-130 blanket covering my second floor. I now have to cool the room from 90-100 becuase the air gap and the insulation drops the temp about another 10 degress. So getting a 20 degree drop from my furnace is much easier and my second floor stays cool when I need it to be cool.
Check out the pics.
I would be cautious..
...about using fans to increase the attic ventilation. Without debating the merits of attic ventilation, here's the concern. According to Building Science Corp., soffit and ridge venting should balance. If anything, you want more soffit venting than ridge venting. Excessive ventilation of the top of the attic may suck conditioned air from the house. And most building scientists question the ability of roof venting to decrease attic temperature in any event. There's a very long history here, dating back to the 1940s, at least. Check it out at greenbuildingadvisor.com. Search under the names William Rose or Joe Lstiburek.
Insulation.
You have limited space for insulation, therefore it make sense to maximise the space your have.
Batts are useless as insulation as both warm air and water vapor pass easily through them.
Spray on foam has its problems, see online the horror stories (and of course the cost).
Your best way forward is to fit sheets of polystyrene/blueboard/polyurethane foam between the rafters.
These closed cell forms of insulation are air tight and water vapor proof to an aceptable level, they work by having millions of small air bubbles enclosed in thin plastic bubbles. Air is one of the best forms of insulation, it is very bad at carrying heat.
The polystyrene sheets need to be cut to a tight push fit and pressed up tight against the roof. If you can. completely fill the spaces, but keep in mind that an air space of less than 19 mm wide, is too small for a convection current to start and the air will not rotate and transfer heat across the space. Fill any gaps and cracks with canned foam. Fix a water vapor proof plastic sheet across the rafters below the polystyrene, take care to seal any joins with tape. Then fit another layer of inch thick polystyrene below the plastic sheets and finish with drywall.
This last layer of polystyrene will stop the heat from your room, from moving through the wood of the rafters, wood is not a good insulation and it is in direct contact with the cold air outside - wood forms a very large part of any roof and in itself will cost you a lot of money in lost heat over the years.