How to better insulate my roof
I have a log home with a high ceiling. It has 2by 12 roof rafters. Which means I have 10inches of insulation. I live in northern NY. I want to know if I put 2inch foam panels and a metal roof will it increase my r rating? I seem to lose a lot of heat through the ceiling and it’s very hot in the summer.
I was told by friends to put spray foam but that would mean to completely remove the roof or the ceiling inside! I was also told to put a cold roof on with foam boards. I would like to put a metal roof on. The house seems to use a lot of energy in the winter to heat. How can I make it more efficient and not cost a fortune? It’s about 3000sq.ft.
Any suggestion would be helpful.
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Replies
Rodney,
Q. "I want to know if I put 2 inch foam panels and a metal roof, will it increase my R rating?"
A. Yes, it will -- but 2 inches of rigid foam isn't enough to keep you out of trouble. You need enough exterior rigid foam to prevent moisture accumulation in the roof sheathing. In your climate zone (Zone 6), the rigid foam layer installed above roof sheathing needs a minimum R-value of R-25 (about 6.5 or 7.5 inches of EPS, 5 inches of XPS, or 4.5 inches of polyiso). For a full explanation of the principles behind this rule, and more information on the topic, see these two articles:
How to Build an Insulated Cathedral Ceiling
How to Install Rigid Foam On Top of Roof Sheathing
Remember, if you decide to go ahead and install rigid foam above your roof sheathing, you will have to seal off the soffit vents and ridge vents. This needs to be done conscientiously, to stop all air leakage.
Q. "I was told by friends to put spray foam but that would mean to completely remove the roof or the ceiling inside."
A. Spray foam insulation is one option. If you are planning to install a new roof, it will obviously be easier to do this work from above (by removing the roof sheathing) than below (by removing the drywall ceiling).
Q. "How can I make it more efficient and not cost a fortune?"
A. You don't want to try to do this job on the cheap. Halfway measures can easily cause moisture problems and ruin your roof. Do it correctly or don't do it at all.
Log homes are notoriously air-leaky, which may be a large fraction of the total heat loss.
If your logs aren't a foot thick your conducted wall losses are going to be quite a bit higher than the roof losses through the ~R30-R38 fiberglass between the rafters.
If you blow cellulose into the rafter bays by drilling from above, compressing whatever insulation is in there to a higher density you'll be at about R40-R43 center cavity. If you are in the US climate zone 6 part of northern NY you would then need an equal amount of R above the roof deck for dew point control which is probably pretty difficult & or expensive, but in the zone 5 sections the exterior R only needs to be 40% of the total, which may be do-able as a retrofit. Which climate zone are you in?
I am in zone 6. I am up near northern Lake Champlain. Would using 2 inch foam board on the roof then putting a metal roof over it with a space Above the foam and below the metal? The metal would allow the snow to slide off easier and not melt causing ice dams to form. I would remove the original vents and the vent would be between the metal and the foam. I think that's a cold roof. Would that help?
If I understand correctly, I would need to have another 6inches of foam board above the roof I have now?
In zone 6 half the total R has to be above the roof deck for dew-point control of the wintertime moisture drives. With only 2" of foam the roof deck will be MUCH colder than the dew point of the conditioned space air, and would take on too much moisture over the winter, leading to mold & rot risk.
Rodney,
Once again, I urge you to read the articles I linked to in my first response. These articles explain why your suggestion won't work.
You are, as most people do, focused on insulation when the issue is as much air sealing as anything. You can have tons of fiberglass insulation, but if you have huge air leaks you will be cold. As Dana pointed out, log cabins are notoriously leaky; both from the joints between the logs and from the board ceilings which allow lots of air through, and are often covered by poorly installed fiberglass. First, figure out where all that heat is going, then close up your leaks. Obviously it may not be easy, but you are wasting time, energy and money adding insulation without dealing with the elephant in the room.
Ever tried to air seal a log house?
It's not easy- the dimensions change with temperature & humidity. Leakage varies with the seasons, and drifts over time. The best thing to do with most log houses is to install housewrap detailed as an air barrier over the logs, and add 2-3 of inches of rigid foam on the exterior, with some standard siding over it.
Air sealing the chinking and the transitions to the roof with the usual goop is something of a fools errand, and bound to fail over time. Flexible sheet goods for an air barrier and sufficient exterior wall-R is a good place to start, at the cost some of the visual charm as viewed from the driveway. For 10" logs R10 foam would be enough to keep the exterior side of the logs from accumulating too much moisture in climate zone 7. For thinner logs it's less. Rigid rock wool would be better if you can't go high enough R to keep the cold side of the logs warm enough, since that would allow good drying toward the exterior even in winter. With really fat logs of sufficient thermal mass and total-R you might just skip the exterior-R and use rainscreened siding on furring over the housewrap.
An R30-ish vented cathedralized ceiling isn't a terrible energy hog (unless it's leaking air, which is fixable) and I'd be gobsmacked if more roof-R were the best place to spend the energy-efficiency budget on this house.
Most log homes are oversold with fantasy type thermal performance claims based on the thermal mass characteristics. But while the thermal mass of the wood makes a difference, they all pretty much suck until/unless you air seal them. Even the DOE seems to have bought into some of those claims from log-kit vendors:
http://energy.gov/energysaver/articles/energy-efficiency-log-homes
"The R-value for wood ranges between 1.41 per inch (2.54 cm) for most softwoods..."
Really? Name those species!
Unless you know the species & density, assuming anything over 1.2 per inch is wishful thinking. Doug fir is about R1/inch, as are some species of hemlock. Even most species of cedar & spruce don't hit as high R1.4/inch.
But even if it were R100 per inch, a well insulated wind-tunnel is neither comfortable nor energy efficient, and air sealing is job #1.