Framing / Insulating Conditioned Room over Attached Garage
Hi all,
I’m designing an attached garage in Massachusetts (climate zone 5). The space above will be heated/cooled, and will have a vaulted ceiling. The garage will be 22′ wide across the gable end, with one side of the roof sloped at 12/12 and the other side sloped at 4/12. From the outside it will look like a smaller version of my house — a 1970s Cape.
Being in cz5 the roof insulation will need to have a minimum R-value of 49. Reclaimed EPS sheets are available nearby. Is there any reason I shouldn’t just use about 12″ of EPS? I could fill the spaces between the rafters with EPS and then put 2 more layers on top of the rafters in alternating directions as shown in the attached screenshot. The upper layer would be parallel to the rafters with nailers taller than the EPS to allow air space under the sheathing for a vented roof.
If that’s an ok plan, do I need a vapor barrier, or to seal the the innermost layer to the rafters?
Thanks,
Ben
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Replies
> Being in cz5 the roof insulation will need to have a minimum R-value of 49. Reclaimed EPS sheets are available nearby. Is there any reason I shouldn’t just use about 12″ of EPS?
Yes.
> I could fill the spaces between the rafters with EPS
Tedious and not so useful.
> and then put 2 more layers on top of the rafters in alternating directions as shown in the attached screenshot.
Sweet.
> The upper layer would be parallel to the rafters with nailers taller than the EPS to allow air space under the sheathing for a vented roof.
Double sweet.
The R-49 prescriptive option assumes no exterior insulation. If you're willing to put exterior insulation outboard of the rafters, you will hit the U-factor alternative 'more quickly' than just using insulation between rafters. Use rigid foam above the rafters... use 'fluffy' stuff between the rafters. See here: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks for your reply Patrick. For some reason it didn't show up when I replied an hour later last night, and I never did get a notification, so I didn't see it until this morning.
At first I thought the nailers in the first layer of foam would eliminate the need for the sheathing on the rafters by tying the rafters together. The article you linked to mentions air-sealing that layer of sheathing, so I guess omitting it is out of the question. Rather than all the nailers I mentioned would I be better off having 2 uninterrupted layers of foam and then using long screws to attach 2x4 or 2x3 nailers on top to attach the upper layer of sheathing and allow for venting?
I looked at the "related questions" that came up after I submitted this question, and found my way to
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
and
https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation
From reading those two articles it seems like I might be better off using R-30 fiberglass between the rafters and R-20 worth of EPS above the rafters. Structurally I could get away with 2x8 rafters, but 2x10 will make insulating easier.
-Ben
If this is a new build, I would design for a simple vented roof with batts.
An easy high R value roof is I-joist rafters with two layers of HD wall batts. Since the I-joist have very thin OSB webs, there is very little thermal bridging, which means you can meet code on U-factor basis. Something like 11 7/8 I-joist with R24+R15 batt is just around an R38 assembly.
The top flange of the I-joist makes for a convenient vent baffle, you can staple cardboard or housewrap to the bottom of the flange to form the vent channel. You can also skip this if you are careful with installing the batts.
In most cases you can even save a bit of money by going up to 19.2" or 24" OC with H clips for the roof sheathing.
Your drawing shows T&G ceiling, with that finish it is important to have a solid air barrier. The simplest is usually taped drywall. You can also use 3/4" or 1" foil faced rigid insulation with the seams taped which would also give your assembly a slight R value bump. T&G can be nailed up directly through the foam with longer nails.
P.S. With I-joists make sure to order batts for metal studs as these are wider and fit snuggly between the webs.
Yes, this is a new build. The drawing was just a screen grab meant to illustrate the criss-cross arrangement of the layers of foam over the roof. I haven't settled on a ceiling yet -- either drywall or T&G. I'll keep your note in mind.
In cz5 I need R49, which is 14" of fiberglass. That would push me up to a 16" I-joist to allow room for venting. I'm not sure how the materials cost of that compares to 2x10 rafters with R20 batts plus the extra layer of sheathing plus the recycled EPS plus the 2x4 nailers.
Labor is myself and family/friends.
I have used the method AKOS describes many times in MN. This is a good way to insulate a cathedral ceiling. With the vented air space and full insulation over the exterior walls, there will be no ice dams. The length of the ridge support beam can be a limiting factor with this type roof. 16' is about as far as I would go, after that I use an engineered parallel chord truss.
Ridge support beam is 32' long, with a support post 10' from one end. Lumber supplier spec'd the ridge beam as two 1.75x20" LVLs sandwiched together. Snow load is 60psf here. The post will be hidden in an interior wall upstairs and supported by another LVL sandwich in the garage ceiling. The framing is probably worth me starting a separate discussion for, as I'd welcome feedback on that as well.
For this type of roof, it is generally worth it get high density batts (HD fiberglass or mineral wool), these are much higher R value/in which means thinner roof. The extra cost of the batts is typically less than upsizing the rafters to fit regular batts.
Almost all codes have a prescriptive and a performance based compliance option. The R49 is the prescriptive option, the U factor based compliance alternative is the R38 I was talking about. Generally there is very little energy saved between an R38 and R49 roof, for something like a ADU, not worth the extra rafter height.
With a structural ridge make sure to figure out how to air seal and get a solid air barrier transition across it. Usually the simplest is to frame a mini attic just bellow the beam and run the ceiling across. Leave some space bellow to allow for some insulation under the ridge which can help with thermal bridging.
The other option is to drop the ridge bellow the rafters. This can be nice as you can clad the beam in wood and leave it exposed as an architectural detail. In this case it is important to install some plywood and tape across the ridge beam before the rafters are set to allow for air barrier continuity.
Thanks Akos. I understand now. I didn't get the U factor part when you mentioned it before but this article explains it well: https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness
Faced insulation or unfaced plus a separate vapor barrier? Do the I-joists interfere with facing? I guess you'd have to just pull the facing back a little to fit over the lower chord of the I-joist.
There will be a couple of skylights on the 12/12 side. For venting the roof above and below those do I just need to provide a path into the adjacent rafter bays by putting some holes in the web (following manufacturer guidelines) and making sure the insulation does not block them?
I've been considering using 1x6 and/or 1x8 pine planks for the roof sheathing. It seems a lot easier to haul up on a roof than 4x8 sheets of plywood. There's a sawmill near me and when I checked a couple months ago s3s pine was cheaper than the equivalent in 4x8 sheets of CDX. Not sure whether that's still true. I'd be more likely to get some use out of leftover pine than leftover scraps of plywood.
Also contemplating diagonal plank sheathing for the walls.
Crazy or worth considering?
I prefer to use a sheet vapor retarder as it can be detailed as a secondary air barrier. This makes the assembly less likely to leak air if/when drywall seams crack.
With HD batts you do have to notch the corner to fit over the flange if you use wider flange I-joists. With the narrow flange ones, you can usually squish it in without any issues.
Around me skylights rafter bays are almost always never properly vented. Most cases you have a soffit vent for the bottom section and the ridge vent for the top section. This seems to work well enough. I've never done much more than this.
Getting extra airflow never hurts though, extra holes are good insurance, you can also get shingle vents that work well for venting bellow skylights. With any cathedral roof, the most important detail to get right is air sealing, focus on that.
If you don't mind the work, I would have no problem with plank sheathing for a roof. Most century homes around me have this and have held up extremely well even after many roof leaks.
I wouldn't do it for the walls as the only good way to air seal plank sheathing is with a peel and stick WRB. The cost of that plus the extra fussing with diagonal install is not worth it.
Thank you again Akos. I'll plan to use a separate vapor retarder and get unfaced insulation. I'll go with a few holes to vent the skylight rafter bays. I haven't decided for sure on the plank sheathing for the roof, but will take your advice and not use it for the walls.
I've been away from this for a little while due to a family camping trip.
I'm agonizing over some of the framing details, flipping back and forth between my Sketchup model and the Weyerhaeuser references. The floor joists are I-joists 16"OC, and the I-joist rafters are 24"OC. I want to have the rafters resting on the top plate of the wall, between the floor joists, but it's getting kind of tricky. I don't really want the added height (inside or out) from moving the rafters up to the floor, though that doesn't have to be a show stopper.
Putting the rafters on top of the wall will require leaving out the rim board on that side until after the rafters are installed. I'm a little worried that might cause stability issues during construction but I could add some diagonal bridging if necessary. Also the rim board will have to be in segments between the rafters, rather than one continuous length. In the attached screenshot I've got one of those segments in place, with the rest left out for clarity. A continuous rim board might be possible, but I'd have to notch it for every rafter tail, and making the rafter ends even with the joist ends will make it hard to nail on the web stiffeners and rafter tail supports.
Does this look reasonable or is there a better way?
Thanks
That looks pretty complicated to build and almost impossible to air seal as your floor joists and rafters are poking through the air barrier.
I would keep it simple and go with standard platform framing and frame the roof on top of the floors.
If you must have the extra height, one option would be to go with top flange mount joist hangers for the floor I-joists. These would let you hang the joist off your wall top plate. It does require taller studs, so it adds a bit of cost. You can than frame the rafters as per one of the Weyerhaeuser details.
Whichever way you go, make sure to figure out how to get air barrier continuity from the walls to your ceiling.
For "standard" roofs, I like to install a wide strip of peel and stick above the wall top plate but only remove half the backing. Stick this to the top plate and sheathing and let the rest with the backing on hang on the inside. Set the rafters over the peel and stick as normal, install blocking between the rafters near the wall plate and staple the peel and stick to it. You than run your ceiling air barrier (ie membrain) across your rafters down, remove the backing on the and adhere the barrier to it.
It is complicated looking, but that actually is one of the Weyerhaeuser details, aside from the joists being at the same level. See attached.
With the ground floor being an unheated garage, the air seal should only encompass the upper floor, correct? How is that usually done? It seems like the air barrier should run across the top of the floor joists, under the subfloor.
About the height... I can't make the garage ceiling any lower because where I have it now allows just enough height for the header over the door from the house. The floor in the adjoining room is about 2' higher than the garage floor will be.
I'm trying to minimize the height of the garage compared to the height of the house. Platform framing vs. putting the rafters on the wall adds about 16". (I think I'd need to have the rafter ends sitting on double 2x6s because they don't line up with the joists, but I'm not sure about that.) Another 16" isn't going to push it over the edge though, so I probably will end up going with that.
I'm not exactly following what your is driving your height limits, a sketch would help.
You can speck a thicker rim joist or make one out of a two ply of LVL as a header over a door opening. This does mean some extra hardware as you have to hang the floor joists off the LVL in this section but it would save you the height of the header bellow. See here for details:
https://www.finehomebuilding.com/project-guides/framing/maximize-insulation-in-minimum-headers
Unless you have local zoning rules driving the eaves height, I would raise them up and set the rafters over the floor joists. Much simpler. The rafters will be supported by the rim joists of the floor bellow, you don't need a double top plate, single plate will do.
The air barrier for the ADU would be the subfloor out to the rafters and up to the vapor barrier in the ceiling. The simplest is to tape the seams of the subfloor to isolate it form the garage bellow. I would still run an air tight drywall for the ceiling in the garage as a secondary air barrier. Make sure to figure out your air barrier transitions in the staircase opening leading up the the ADU.
I would still do a decent job of air sealing the rest of the garage, taped sheathing adds very little costs. A well sealed and reasonable insulated garage will always be much more comfortable, well worth the extra cost.
Yes, some background info would probably be helpful... The garage will be replacing a carport that sticks off the left side of what's currently an enclosed porch on the left side of the house. The porch is on what I guess would be called an elevated slab... it's about 2' above grade (and it does have footings that were acceptable at the time it was built). The back corner of the porch sticks out into the carport about 3' because when the carport was added they made it parallel with the property line due to setback requirements, which puts it at about a 15 degree angle to the house and porch.
I got a variance so I could go closer to the property line, but at the same angle. So my 14' wide carport will be replaced by a 22' wide garage, and the back corner of the porch will still stick into it. The 2 rooms over the garage will be accessed via stairs from the porch. I'll insulate/seal the porch and it will become a mudroom that's minimally heated in winter so the rooms over the garage will feel more like part of the house instead of going outdoors to get to an outbuilding.
The porch slab can only be load-bearing around the perimeter, so there will be a header under the garage eaves to span over it. The existing wall between the porch and carport will be replaced with a wall that will support part of the upper floor. The door to the garage and the stairs to the upper floor will pass through that wall. A beam to the left of the stairs will support a column which will support the ridge beam. The column will be in the wall that separates the two rooms.
With the rafters sitting on the floor, the peak will still be 2+ feet lower than the house, so I guess I was worrying about nothing. I just didn't want the garage to look bigger than the house.
I've attached a screenshot of my Sketchup model. The rafters are still on top of the wall but I'll raise them up in the next day or so. Easy enough to do while I'm still building with electrons. There's still a lot of other things left to be done (windows, the tops of the gable walls, support for the ridge beam, etc). I'm open to suggestions. The variance dictates the footprint, and I can't raise the porch roof to make a hallway between the second floor of the house and the second floor of the garage because that would turn my 12-year-old's bedroom into an oversized hallway, which he probably wouldn't appreciate.
Yesterday I submitted a question on the Weyerhaeuser site about both rafter placement options. I got an answer back this morning from one of their engineers. About the platform framing option he too said that a single plate would suffice, but that if the birdsmouth cut were to be shifted inward, not over the rim, adding another row of rim as blocking under the inside of the plate would be necessary to transfer the roof load to the wall below. I suspect that 2x4 squash blocks around each joist would do the job too. Either way, I think I remember reading that part of using the U-factor requires that the roof insulation doesn't taper down to nothing, leaving an insulation gap above the wall, so I'll need to keep that in mind.
I would raise the wall on the low side to match the other side. This would make the rooms there much more usable and simplify air sealing details as now it would be a two story structure just with stubby walls.
You can bolt on a gabled over hang to make it look like a single story structure. This would look much cleaner give you more space and make the garage look less bulky. A non standard roof line really only works if it is intentional and part of the design.
Curved stairs are pretty time consuming to build. Unless part of a design feature, I would go for either winder or a landing to make the turn. Your staircase also juts into the garage space, which might or might not be an issue. If you put a winder box on the bottom and a landing to turn into the garage, it would raise the stair up earlier and avoid clearance issues.
You can push the header for the garage door into the floor and have it do dual duty as your first floor joist. This would also let you get a taller garage door.
Most ridge vents are designed for a single ply ridge board, they won't span over the a two ply LVL. You can use it as is but go with two shingle vents on either side of the ridge. My preferred solution is to drop the ridge beam bellow the rafters and leave it exposed. Means ripping some tapered spacer but this way you can also skip the hangers for the rafters.
You have a lot of structural intersections at the stair and roof over the mudroom area, get a structural engineer to look at it.
Thanks for the suggestions.
The house is a Cape with the back roof bumped up the same way. My intent was to make the shape of the garage similar. I've attached a photoshopped mock-up that I made a while ago. (Cedar shingles was just one siding idea.)
I'm not sure about the idea of a two story garage next to a Cape, but I see where the false gables could help make it work visually. I've been considering using one on the high side anyway because I think it's a detail that's conspicuous in its absence (including on my own house).
The stairs have been a balancing act between space in the 10x12 mudroom, headroom in the garage, and clearance under a rafter tie in the mudroom. A landing anywhere takes away rise and pushes the staircase lower into the garage. Getting too much rise in the mudroom means bumping your head, or at least having less than the required 80" above the stairs. What I have now is essentially a 5 step winder box with 15 degree stairs, and straight runs above and below that. It's easier to draw the winder part as two 75 degree arcs, divide those into 15 degree segments, and then pull each step up to the right height. I just haven't gone back and squared off the corner because I'm not sure if I'm finished tweaking them.
The way I have the stairs now they're high enough to be well above a car that would park in that spot. They're also far enough back that they'd only be above the hood of the car. I don't have the framing all figured out for them yet, but I think they'll be a bit less than 6' above the floor at the right side of that parking spot.
I had thought about pushing the garage header up but I guess I never got to it. The LVL will be taller than the TJI floor joists. What's the best way to deal with that? Notch the ends of the header to fit over the adjacent walls? Build a ledge into the ends of the adjacent walls for the header to sit on? Size the header to stop at the insides of the adjacent walls and then fill the gaps with small lengths of rim board?
I was leaning towards dropping the ridge beam as you had suggested before, but had been spending my time on the other ends of the rafters. I hadn't thought about how it would block the ridge vent the way it's modeled now.
I think of the area where the garage meets the mudroom as essentially a very tall garage door opening which the mudroom is sort of wedged into. Neither one supports the other. The "new" gable end wall of the mudroom will support part of the floor above the garage as well as the stairs, but none of that relies on any support from the old part of the mudroom. Likewise the old part of the mudroom can (and will, for a little while) stand without support from the garage.
Actually I guess I'm not sure about the way that the mudroom roof will get cut away at an angle like that. Rafters without a structural ridge like to be symmetrically opposed to each other and I am messing with that. See other attached screenshot. (I see that I'm missing a short rafter piece between the ridge and one of the skewed rafters. Maybe another one between the wall and the other skewed rafter but that's currently an oddball 2' rafter bay anyway.)
I'm not a designer, so I can't help you too much on the looks, the attached photoshopped mockup needs a lot of work. Everything looks offset, there is too much solid wall area, the offset gable is in the wrong direction so it clashes with the house.
To get the look of this type of project right, it is well worth your while to higher an architect, it will be money well spent. I find a great way to get ideas for something like this is to go for a walk in the neighborhood, I'm sure there are houses nearby that someone has built a similar extension to.
The way I would deal with the taller LVL header above the garage door is to build the two narrow support walls on either side of the opening to a height that would put the top the LVL beam at the same height as the top of floor joists. You set the LVL on this wall but run the rim joists for the floor above all the way out to the LVL and nail it to the end same as a floor joist. You can then run your subfloor across the LVL the same as if it were a floor joist.
Your two roof intersections are outside of what random dude from the internet can help with. I would run it by a structural engineer. Walls that spread or roofs that sag over time are very annoying and can sometimes fall down, I would not mess around with it.
P.S. Maybe it is just me, but nailing up cedar shingles is extremely time consuming, many other siding choices out there that work just as well and less fussy to install.