Insulation question regarding unvented vaulted ceiling
I have an older house with a vaulted ceiling that I want to look similar to this: http://images-02.har.com/e1/mediadisplay/02/hr3249802-6.jpg
The original ceiling had skylights that were taken out long before we moved in, and it just had roof sheathing put right over top of the hole. This is illustrated in the following picture (the wood you see in the skylight hole is the roof sheathing): http://imgur.com/a/yiDs6
I need to cover this up hole up, and another one like it, with beadboard to complete the ceiling. I don’t know how to handle these skylight holes though. There would be NO ventilation provided to these areas of the ceiling, besides the fact that they are just open holes. According to GreenBuildingAdvisor.com, if I’m understanding it correctly, you either need a ventilation system (baffle, soffit vent, etc.) for vaulted ceilings, or you need to put air-IMpermeable insulation right up on the roof sheathing. This is suppose to ensure that warm air from the kitchen doesn’t rise up through the insulation to the roof sheathing in the winter and condense, possibly causing the plywood/OSB to rot/mold over time.
Does anyone know what counts as air-IMpermeable insulation (not the specs, but what it is and where you can get it)? Several resources I’ve found, including GreenBuildingAdvisor.com, say that air-impermeable insulation can be a rigid foam, but don’t specify certain brands or types. Are they speaking of a particular kind or thickness? Is XPS and EPS air-impermeable? Any help tracking some down that isn’t the sprayed-in foam would be greatly appreciated! Other thoughts I had was just throwing up some stain, primer, paint, polyurethane, vapor barrier, etc. on the bottom of the roof sheathing to make sure it wouldn’t soak up any moisture from underneath. Is this a good idea?
Additional question: From the little that I can see, it appears that behind some bead board right now is about 3.5 inches of material. Some appears to be rigid insulation, white styrafoam-like, while the other is “stuff” that I’m unsure of. Here is a picture of the “stuff” from above from when the roof was getting done awhile back: http://imgur.com/a/xHOE4. Anyone know what this is and if it’s air-impermeable? Maybe this whole ceiling is done incorrectly.
Thanks!
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Replies
Michael,
To give you good advice, we need to know your climate zone or geographical location.
To learn about all of your options for insulating the skylight holes, read this article: How to Build an Insulated Cathedral Ceiling.
It looks like your roof assembly may have some type of fiberboard insulation, but it's very hard to determine what you've got, or how thick it is, from your photo.
If I were called in as a consultant on this job, I'd probably want to carefully remove sections of your ceiling to examine the existing layers that make up your roof assembly. My guess is that your insulation R-value (on the sections of roof that never had a skylight) is seriously deficient.
It could be argued that it makes little sense to do a better job of insulating these skylight patches than necessary, considering that your entire roof may be poorly insulated.
If you can afford it, you probably want to develop a plan to insulate the entire roof.
If you want to create an unvented roof assembly (working from the interior side), you need to use spray foam insulation. Spray foam insulation is air-impermeable.
Rigid foam insulation is also air-impermeable, but the use of rigid foam insulation on the underside of roof sheathing (in unvented assemblies) is not recommended, because these installation have resulted in moisture problems and rot. (For more information on these problems, see Cut-and-Cobble Insulation.)
Thanks for the response, Martin. The climate is southern Missouri. This is from wikipedia:
Missouri generally has a humid continental climate (Köppen climate classification Dfa), with cool to cold winters and long, hot summers. In the southern part of the state, particularly in the Bootheel, the climate borders on a humid subtropical climate (Köppen Cfa). Due to its location in the interior United States, Missouri often experiences extremes in temperatures. Lacking either large mountains or oceans nearby to moderate its temperature, its climate is alternately influenced by air from the cold Arctic and the hot and humid Gulf of Mexico.
Martin -
I did a little research, and it does appear to be fiberboard: about 3.5 inches of it. This puts it at an R-value of a little above 8 from what I've seen.
According to the region map in the "Cut-and-Cobble Insulation" link, I would be in region 4 where it's recommended to have R15 for your ceiling. This means my ceiling as a whole is lacking when it comes to insulation.
It is too much money at the moment to rip the ceiling down and build from that direction, and the same goes from ripping off the roof, so we definitely need to leave the insulation that exists now alone. This is a single room in our house on the first level, about 15ft. X 15 ft., and has a vaulted ceiling that goes from 8 feet to about 14 feet high. I know nothing about roofing or insulation, and while I've come to realize the ceiling as it exists right now isn't very good, hopefully it won't hurt us too badly in terms of temperature differential with the rest of the house.
All that then remains to figure out is the skylight holes. I guess I have a few options here. I have two main concerns: 1) causing rot/mold/damage to the ceiling/roof and 2) being energy efficient
1) Just leave the skylights open. My concern is of course heat/cold air loss from the house.
2) I could cram about 3" of XPS in there, followed up by some insulation tape of some sort? One of the articles mentioned that the swelling of the sheathing in the summer could move the rigid foam around a little and cause gaps where air could easily get in behind the foam causing issues. Would so caulk to seal up any cracks in the foam work here, or is there a such thing as putting a primer or sealant on the underside of the roof sheathing itself?
3) I could put an air permeable insulation up there
The roof has been on there for 40 years, and we just had it redone where only two spots needed to have the sheathing replaced. As long as I can get another 40 years out of it with minimal issues, while being as energy efficient as possible, I will be happy.
Thanks!
Michael,
In Climate Zone 4, the building code calls for a minimum of R-49 insulation in a ceiling, not R-15. So R-8 is woefully inadequate.
The R-15 recommendation is not for total ceiling insulation; it refers to the minimum R-value of the foam insulation layer in an assembly that includes two types of insulation (for example, R-15 foam plus R-34 fiberglass = total of R-49 insulation).
My recommendation for your skylight holes remains the same: If it's unvented, you need closed-cell spray foam insulation to avoid moisture problems. Go to a lumberyard and buy a 2-component spray foam kit for about $600, or hire a spray foam insulation contractor.