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Community and Q&A

Insulating Room In Attic Trusses

martyrg | Posted in General Questions on

I’m constructing a shop building for myself – 44′ x 30′ with a second floor/attic space.  The main floor will be heated workshop space with a ceiling hung unit heater.  I plan on using the attic space for a woodworking shop, unheated for now.
I am using Room In Attic trusses for the roof construction.  The top cords are 7 1/4″ deep.  The bottom cords are 11 1/4″ deep.  Looking for advice on the best insulation strategy.  I want to avoid spray foam due to cost and quality issues.  I plan on using rock wool for its fire resistance and higher R-value compared to fiberglass.  Should I?:
A).  Insulate the ceiling space between the main floor and the attic space with 11 1/2″ of rock wool?  Also insulate the 7 1/4″ top cords in the attic space with 5 1/2″ rock wool (leaving an 1 1/2″ ventilation channel above the rock wool to the bottom of the roof sheathing – I’d use Accuvents for this).

B).  Fur out the 7 1/4″ top cords to get enough room for the 1 1/2″ ventilation channel and 12″ of rock wool?  My plan would be to use metal studs, attached on edge, to the bottom edge of the truss cords.  The “C” shape of the metal studs allows for easy attachment to the wooden truss cord.  The rock wool will fill the space in the metal stud, helping to eliminate thermal bridging.

C). Your idea :).

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  1. onslow | | #1


    Before getting to any insulation ideas, do you know the load limit for the room. Woodworking can be a vast range of endeavors not involving heavy machines. Do you go old school with all hand tools? You said the main floor is also workshop, but apparently other crafts. Is electricity and lighting loads all accounted for?

    Heating the bottom and letting the warm rise may be more than enough. Insulating the entire ceiling of the bottom room may be counterproductive. If no vehicles are under the attic room, you might be permitted to put in a few floor grates like the turn of the century homes with gravity furnaces. You haven't mentioned windows for the upstairs room, so are there code things like egress to plan for?

    I am curious to see the truss plans, as the way the room sits inside your roof profile may suggest particular insulation approaches. I also have an interest because I am working on plans for a new place and am currently planning on framing differently. The one experience I have had with in room trusses has taught me the floors have a lot of bounce in them. An 11+ bottom chord sounds appealing.

  2. martyrg | | #2

    Hi Roger, Thanks for your reply. To answer your questions: The Room in Attic trusses are rated for 40 pounds live load. The woodworking is hobby related - pretty basic - table saw, small joiner, drill press, bandsaw, work bench, etc. I'm feeding everything with a 100 amp feeder from the house, which will be more than enough power. Lighting will be LED fixtures. I do have one egress window on the gable end of the attic space. The main floor is an insulated slab on grade, 2x6 walls and 6" thick insulated barn doors (3 ea.). Main intent of this space is restoring cars, mostly air cooled VW's :).
    The trusses span 30 ft plus 16" of eaves on each side. The attic space measures 20 ft wide and 8ft 1" tall. The knee wall is about 42" tall. The pitch is 10/12. Floor sheathing is 3/4" Advantec glued and nailed. The floor was pretty bouncy until I installed the bracing, which really stiffened the floor. The truss engineer says it will get even better if I drywall the first floor ceiling (screws and glue). The trusses came from Midwest Manufacturing and ordered through Menards. All and all, so far so good. Just need to figure out the best way to insulate this building that does not involve spray foam.

  3. onslow | | #3


    Holiday delays. I am guessing you are probably in CZ5 in the midwest somewhere. Menards does come pretty far west, but not as far as I am. You have a very nicely done major league shop there. Good thing the only beetles you will be harboring are the classic ones.

    The trusses are a surprise to me, as they appear to be made of very nice wood. The bottom chord dimension makes me think it might be LVL or similar. The picture doesn't enlarge well. If it is 2x12, then I am very jealous of the quality. My experience with trusses is limited, but not favorable. I expected to see much more complicated elements in your attic truss. The pattern of yours looks very much like floor joists, roof rafters and collar ties. Just a whole lot wider than I am use to seeing. Even 2x12's don't clear span 30 feet. My attic room was barely 10'6" wide, but feels like the bottom chord is 2x3.

    The insulating task you have at hand is potentially rather like a cape cod with the knee wall issues. Insulating the ceiling over to the knee wall before turning up to the roof line will create a large cold triangular prism shape the length of both sides of the shop roof. In a perfectly insulated world with perfect air sealing, this might not mean much to your roof. However, any warm leakage getting to this space would risk creating an ice damming potential. And, insulation that leaves a prism shape will just create a hot box in the summer and a freezer box in the winter. If you are really, really good at sealing things you could try, but amount of square feet of insulation is actually a bit less be following the hypotenuse. The trusses are so clean and simple looking that it should be fairly easy to cap the top plate areas and establish one long chute going up.

    Depending on your final R goals and how you want to make the vent chutes I would suggest thinking about stringing 2x3 stock up the sides of the trusses. Splice the runs such that on piece makes the jump from knee wall to collar tie. You might find a simple flat tie plate to take some bounce out of the span. If you need a sketch, I can try to scan something in.

    Code for home roofs requires R49 now in most regions, but maybe shops are not subject to that. 12" of Rockwool batts will get you R36-39+ which sounds super for a shop, but local authorities rule. If you do go for the full 12" of batts, you need to allow about 14" from underside of the roof deck, which is almost double your 7 1/4 stock. If you can sacrifice a bit of room width, go for a full 2" vent chute to encourage a less stagnate path. The knee wall height will shorten toward 36", but you will gain a much cooler attic space come July and August.

    If you go with just insulating walls and the underside of the roof, your main thermal management issue will be keeping heat back down stairs. I assume there is a stair way up, so unless you have sealed it off from the bottom shop space with walls and a door, then your heat will migrate up fairly quickly. If it is sealed off, then the floor of the upstairs shop should slow things down enough that you might find insulating the bottom shop's ceiling is not necessary. The balance of loss from below will be driven by the slab. The slab will suck up a fair amount of warmth. Plan on a few ceiling fans to stir things downward at first and then, if you can wait, after a season of living with the uninsulated ceiling, decide if time spent downstairs warrants holding heat out of the upstairs.

    A fudge for heat management could be a reversible ducted fan or, super simple, a couple of floor grates that can be covered. A ducted fan might be nice during the spring when the roof warmth will tend to drive your attic up into uncomfortable range. Blowing down the heat to the shop might help temper the heating demands. For summer, windows at each end gable to cross ventilate would be another option to cool both spaces down if you would rather not have critters visiting through open garage doors.

    Your custom and insulated doors look great. It also sounds like the first floor insulation is under control. Only question I have is what form of ceiling hung heater you are going with. If it is gas, then a whole set of new considerations might come into play.

  4. martyrg | | #4

    Hi Roger,
    You are correct, I live in Southeast Michigan - CZ5.
    The truss cords are made with high quality lumber - 2x8 for the top cords and 2x12 for the bottom cords (no LVL's involved). The lumber is clear and knot free, straight grained, and very hard. I'm using a good quality Bostich framing gun and it doesn't have enough power to fully drive a 10p nail.
    Ventilation - wise I plan on using Accuvents in each rafter bay from soffit to the attic, where I have a ridge vent installed. Subtracting for th Accuvent thickness, that will give me between 5 & 6 inches of space for Rockwool. By furring down another 6 inches with the metal studs I'll have room for a full 12 inches of Rockwool = R42. If the local athority wants more R value I can add a layer of sheet foam(EPS, XPS, Polyiso, etc.) before I drywall.
    The stairway is closed off with walls and a door at the top, so keeping the two spaces segragated won't be a problem. I had thought of either floor grates on the second level or small in duct fans to move heat between levels. My plan for now is to leave the first floor cieling open for a year or two. This will allow me to make changes in layout of lighting and mechanicals until I get thing just where I want them. Window-wise I do have double hungs on each end of the building.
    My heat source is a ceiling hung, sealed combustion, Reznor unit heater -83% efficient on natural gas. What are the considerations with regards to gas heat?

  5. onslow | | #5


    My only concern would be if the unit wasn't sealed combustion. I have seen some shops with older units that appeared to be without venting, though I'm not sure anyone would have noticed given the level of fumes from the trucks. Some people also try to get away with the propane fired heaters which produce lots of moisture to go with the fumes. The insulation would tend to load up with moisture over time. Plus the fumes of course tend to build up in you. Sounds like you are doing everything right.

    Based on some venting issues I have fixed for others, be sure to pitch the exhaust run to outdoors down at least a few degrees. There will be some condensation build up at the end where the pipe will get cold, particularly between cycles. It is best to help it drain fast to reduce the risk of ice building up. Ladders in February as seldom fun.

    I sure hope I get as nice wood delivered to site this coming spring. I lose hope when I see some of the stuff being pushed in recent months. Your shop looks great, the beetles should be very happy.

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