Soffit baffles where chimney is in the way
Home is a single-story, 1200sq/ft, 3bdr, 1ba ranch, on a slab (no basement), built in 1955. Home has an attached, two-car garage with a common (fire-rated) wall between the home & garage. The chimney is on/in/at the common wall such that we have a fireplace in the living room and the oil burner/furnace in the garage also vents into the chimney.
I am in the process of insulating and installing the pink foam O.C. soffit baffles in the garage but the chimney takes up one entire section of rafter space. It actually takes up 1.5 rafters but the 1/2 rafter space is in the common wall so not that big of a concern. I’m quite handy and nothing really scares me. I’m good with dimensional woodworking/lumber but not a cabinet maker by any means. I’ve got tools, table saw, RAS, router table, drill press, etc., I have two welders that I know how to use, can use fiberglass & composites, etc. I’m doing all this work myself and have no problem taking the time to build a ducting system such that the air space below the chimney can flow sortof over & around the chimney and connect into the high side of the rafter so it is one continuous channel for air to flow from the soffit up to the ridge vent.
Just wondering what the best way to approach this would be. To make matters worse, it is a shallow pitched roof and the area right around and below the chimney has been terrible with ice damming in the past. Of all places, I really want insulated baffles on this particular rafter space.
One last tidbit of info… Home/neighborhood was built as Navy housing back in the 50’s so proper 19.2OC spacing… Makes things even more fun with sourcing materials… Thanks for any suggestions or advice.
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Replies
Jon. Where are you located?
Consider shingle vents on each side of the chimney.
http://www.dciproducts.com/html/smartvent.htm
(see similar issue with a skylight)
Jon,
How is the rafter that ends at the chimney supported? Depending on the depth and span of the rafters, and how much of the load from the shortened one they carry, you can probably cut a 1 1/2" x 12" notch on the top of the other one to allow air to move that way.
Jon,
You'll get varying (and conflicting) advice on how to solve this problem.
I doubt that any suggestions will get you much airflow in the rafter bays in question, especially if this is a low-slope roof. My advice would be to treat this as an unvented roof assembly.
For more information on your options, see How to Build an Insulated Cathedral Ceiling.
With a masonry chimney in the way you won't be able to do soffit to ridge venting on the affected rafter bays. Your best bet is probably to buy a DIY foam kit and go unvented on those bays, installing 1-2" of closed cell foam on the underside of the roof deck, filling the rest of the rafter bay with fiber.
Be sure to leave the code-required 2" (minimum) air gap clearance to combustibles between the foam & the masonry chimney. You are not allowed to fill that gap with fiber insulation, even if it is not combustible. Installing blocking between the rafters with the 2" gap to the masonry is recommended. You can then use sheet metal air-barrier on the underside of the gap to prevent room air convection into that space, sealed with fire rated caulk.
The ratio of foam-R to total R necessary for dew point control for the unvented rafter bays varies by climate, so knowing your location would be useful.
Not sure how to respond to answers individually so:
1. Steve, we are in northern CT, near the RI border.
2. Jon, thanks. Shingle vents are an interesting solution. I didn't know they existed. My only concern with something like that would be the shallow pitch of the roof, with regards to water and ice. We are located close to the ocean. Prevailing winds are S/SW and the section of the roof in question is south facing. I had to get very creative with the chimney flashing to eliminate persistent leaks with driving rains and South/SW winds.
3. Malcolm, thanks for the idea. There are links to some photos below. As you'll see in the photos, one rafter is flush with the top of the cinder blocks and actually rests on top of the cinder blocks such that the rafter is continuous along the roof. The other side of the chimney goes into the common wall between home and garage --so it is supported by a wall/studs. I do like your suggestion and thank you for it. I'm going to have to take a look at code and see if I can sister a second section of 2x6 (or see if that is even needed) such that I can cut vent holes (hole saw) or a notch (MultiMaster tool) at the top of the rafter (between rafter and roof) so that air can pass over into the next rafter section and then back again at the top of the chimney.
4. Martin, Thanks for the reply. I figured I'd get varying advice. Treating it as unvented makes me nervous. It's a pretty low-slope roof and its 2x6'es. I'd be more worried about the summer heat than the winter, though both are a concern. This particular space is over a currently non-heated or insulated 2-car garage however I am in the process of insulating it (walls are done with R-13 & 1" of fire rated gypsum board (2x 1/2" on top of each other)) so that I can run a small AC/dehumidifier in the summer and between the furnace and a small space heater in the winter it'll stay well over 50 degrees in the winter. My concern would be that the 2x6'es don't provide enough space to properly insulate. Can I combine the two methods --vent all the other rafters and use a high R-Value foam insulation on this section/rafter space? Other concern I have (see photos below) is that this space is known for ice damming and leaks at the chimney flashing. I don't like the idea of sealing it up such that a water leak would be hard to detect. My goal was to put up the pink soffit baffles and then put faced R-13 (better than nothing... remember its a garage). I was planning on leaving a 1-2" space between the top baffle and the next one down such that water would not travel all the way down the baffle. I wanted to make it so that a leak would present itself fairly quickly on the faced pink insulation. If not it could be months (or years) before we learned of a leak.
Here is a link to the photos. Water leaks have been fixed. Leaks were from chimney flashing. It was noted in home inspection and chimney company came out and redid the flashing prior to our purchase of the home but the leaks still persisted during heavy rains and the right wind conditions. A creative combination of caulk and a little lead baffling (so water can still drain but not get in) fixed it. Wood isn't rotten but I'll coat it with an epoxy hardener before the insulation goes up.
Photos: http://imgur.com/a/1WSvV
Jon,
It can be hard to solve an ice dam problem when you have a warm masonry chimney (especially a warm masonry chimney that isn't located near the ridge).
Certainly, leaking chimney flashing doesn't help. Hopefully, you've solved the leaking chimney flashing -- and done it properly (rather than just smearing roofing cement everywhere or using silicone caulk).
A poorly insulated roof assembly also doesn't help -- and R-13 fiberglass batts certainly fall under the category of "poorly insulated."
If you plan to install a space heater in your garage, you need to insulate this roof properly. If the rafters aren't deep enough to install enough insulation, then clearly you need to deepen the framing members or install continuous layers of rigid foam on the interior side of the rafters so that you can provide enough R-value to address your ice dam problem.
The NE corner of CT is US climate zone 5A, which needs at least 40% of the total R as foam for dew point control on a roof. With a 2x4 rafter you can do 1" of closed cell foam and compress an R11 batt in the remaining 2.5", but that's woefully low performance and isn't going to do ANYTHING for ice damming.
It's fine to make some rafter bays vented, when others are not, but putting more R as rigid foam either under the rafters or (preferably) above the roof deck will do a lot for ice damming potential. With an inch of vent space you don't have a lot of room for insulation, and you may want to just skip insulating between the rafters and go straight to rigid foam, since for a low-slope roof you really want at least 2" of vent space (even though code only demands 1"
In your location there are multiple sources of used roofing polyiso within an hour's drive, eg:
http://hartford.craigslist.org/mat/5810717113.html
http://nationwidefoam.com/
http://www.greeninsulationgroup.com/
Reclaimed roofing polyiso is often cheaper per R than fiberglass batts(!)
To hit IRC 2015 code min with just continuous roofing iso takes 6", but even 4" would be a HUGE improvement. With 4" polyiso above the roof deck and R13 fluff between the rafters you would be just shy of code minimum performance and would have plenty of dew-point margin.
Thank you both Dana and Martin.
Yes the chimney flashing was fixed properly. I happen to have restored a few cars that require 'tinning' of certain body panel joints (60/40 lead/tin). I did use a bit of caulk in places but only because I wanted to be conservative with melting (brazing/soldering) the lead permanently. I check the flashing periodically and in a few years if it's holding up fine I'll pull the caulk and replace it with more lead.
Back to the roof. The whole home is 2x6 rafters and the roof is relatively new. I'm not about to tear the roof off or start replacing the 2x6 rafters with 2x8 or 2x10 lumber. That would be crazy. That said, (not sure if this would be legal (code) or not) I wouldn't be opposed to using some 5-6" long deck screws and fastening 2x4's to the 2x6'es to lengthen the rafters out to 2x10 dimensional specs. Seems crazy but less effort than replacing the rafters and I can do it a little at a time. Also open to other ideas.
I realize that R-13 is considered poor by today's standards --however the house was built in the '50's with 2x4 walls and 2x6 rafters. R-13 is a whole lot better than the R-0 that was there when we bought the place. The home already has (5-year old) vinyl siding (I hate vinyl siding with a passion). I just did blown-in cellulose insulation and putting foam on the exterior between the shingles and vinyl is cost prohibitive to the home in general. The amount of foam we would need would extend the lengths required of the vinyl siding. Manufacturer doesn't make that style/color anymore so we can't even buy a little and patch it all together. Would mean re-siding the whole home.
Similarly with the roof, I'm not going to start replacing rafters. Might as well put the house into a dumpster for $20k and start over on new construction (not an option). Is joining 2x4's onto the 2x6 rafters something I should ever consider or would that be crazy madness? End result would get me a 2x10 space to baffle and insulate.
With regards to the chimney, Can I sister a (2x6 or proper 2x10 section) next to the current rafter and just cut a small 1"x8" strip along the top of the rafter (directly under the roof deck) for air flow to pass into the adjacent rafter space? I'd do the same on the top side of the chimney as well. I don't have a problem (for that one rafter) buying a 2x10 that spans the whole roofline.
Thanks.
Jon,
In the article I linked to earlier (How to Build an Insulated Cathedral Ceiling), there are lots of ideas for ways to add all of the needed R-value in a roof assembly when you have 2x6 rafters.
You can add 2x4s, 16 inches on center, below the existing rafters (installed at 90 degrees to the rafters).
You can add 2x4s in the same plane as the existing rafters, but below the rafters (with a space between the rafters and the 2x4s), and join the 2x4s to the rafters with plywood gussets.
You can add a continuous layer of rigid foam on the interior side of the rafters.
Sistering onto or deepening rafters is a lousy strategy, because the timbers are thermal bridges undercutting thermal performance. The only time you'd want to do it that way is if you needed more structural capacity for the roof (to say, support a solar PV array?) When you add another 2x6 of depth and fill it with R23 rock wool it only adds about R19-R20 to the "whole assembly" performance, and eats up 5.5" of interior space. If instead you install 3.5" polyiso directly to the underside of the rafters, strapped in place by 1x4 furring through-screwed to the rafters with 6" pancake head timber screws on which to mount ceiling gypsum it would deliver equal or better performance, and only eats up 4.25" of interior space. The timber screws need to penetrate the rafters by 1.5" or more, so don't cheap out with 5 inchers.
If you use reclaimed roofing foam it would be substantially cheaper than R23 rock wool too (by better than half!) Most of the reclaimers in your area sell nearly perfect 3.5-4" reclaimed roofing polyiso for under $25/sheet. Even at $25/sheet for 3.5" that's 78 cents per square foot for ~R20, or about 4 cents per R-foot, which is cheaper than batts, and about the cost of open blown cellulose.
With 2x6 rafters you can install R13-R15 fluff and still have 2" of vent space (about right for low-pitched roofs), and with 3.5" interior foam you would come up only slightly shy of current code-min performance. The R20 thermal break over the rafters would significantly reduce ice damming potential too.
The open fireplace is probably a big heat loss due to 24/365 flue convection. Even with R13 cathedralized ceilings a reasonably air tight 1200' rancher will have a heat load of about 18-20,000 BTU/hr @ +5F (or whatever your local 99% outside design temp is- it's +5F in Worcester MA a few miles north of you.) The output of the oil burner is probably at least 4-5x that much. It wouldn't be insane to scrap the oil burner and get rid of the fireplace & chimney, replacing it with a heat pump solution, if you had the budget for it. If it's an open floor plan even a single 1.5 ton wall-blob style mini-split like the Mitsubishi -FH18 would probably handle it. ( A handful of years ago when #2 oil was averaging $4/gallon those things paid for themselves in fuel savings in under 3 years, even with 20 cent electricity.) With 450-600 square feet of rooftop solar the output would probably even cover 100% of your heating/cooling power use.