Is it important to get ceiling joists 16" OC?

As I’m sure some have seen from other posts, I’m figuring out plans for refinishing an unfinished attic.

The current roof rafters are about 20" on center, and the plan was to sister new rafters on those, which would then also be attached to the new ceiling joists. Unfortunately, that means that the ceiling joists would be 20" on center, rather than 16".

The ceiling would be standard drywall with plaster, is it enough to simply support the spans with strapping nailed perpendicular to the joists? Trying to get the new rafters 16" on center seems like a pretty challenging endeavour, unless there is some easy fix I haven’t thought of.

To follow about getting a high level of insulation: Are you going to build a vertical kneewall under this slope or fully insulate/ finish the slope as is? There is a fairly cheap way to get extra space; would need to know more about your plans.

And…No, it it isn’t necessary to have rafters 16" OC. For example, most truss systems in houses are 24" OC.

Using 5/8" dry wall rather than 1/2" will also help prevent sag.

Can you provide a better picture (close up) of where the rathers meet the top plate? I have a concern here.

If it is just a matter of having the right dimensions to hang the drywall, I have seen the rafters stripped with 1x4’s in the opposite direction, spaced at 12" on center. Gives a larger area to support the drywall.

In addition to Pauls concerns and to prevent rafter spread at the top plates some form of gusset or tie in should be constructed to keep the current and new rafters from spreading due to new joists and drywall weight.

Since more pictures and information would be adviseable and usefull, I figured that this might help since it seems to be in the alley of what the poster is asking.
Thanks to Thumb and Hammer for the information.

The Attic Renovation

Finishing the second floor of a 1 1/2 storey house.
This is one of the side attics in the guest room. I removed all of the old insulation and cleaned up all the debris. The fire damage is obvious in these pictures. I painted over the fire damage with Zinsser Bin spray primer which supposedly helps to encapsulate any odor. Whether it does or not may not be important, since the damaged wood will be sealed behind vapour barrier.
I installed vapour barrier in the side attic, wrapping it up and over each floor joist and taping all the seams with red technical tape. It was awkward and time consuming, but it is essential to prevent the warm moist air from the house from infiltrating the attic space.
Rather than install another suspended ceiling, I decided to install some collar ties so that drywall ceiling could be hung. The collar ties will also add strength to the roof structure.
I temporarily attached a couple of 2x4s to the rafters and made sure they were level to each other and along their lengths. I then cut 2x6s and installed them by resting them on the 2x4s and screwing them to the rafters. The 2x6s allow for R-20 insulation. An additional R-5 will be provided by extruded polystyrene boards which will be sandwiched between the collars and the drywall, providing a thermal break. I was advised against adding another layer of insulation above the collar ties as the resulting air space would be inadequate.

At this point in the renovation, I became overwhelmed with the numerous problems in the house. I sought the advice of a contractor on a number of issues and hired him to repair the structure of the back dormer as it had been compromised by the improper installation of windows. That repair is covered elsewhere on this website.
I asked “John” how much I could expect to pay if I had him finish the second floor for me, and was pleasantly surprised by the estimate. He could certainly finish the job a lot faster than I could if I continued plucking away at it on weekends.

Basic Attic Structure
This diagram is copied from Jon Eakes’ website ( and shows the basic structure of an attic.
The vertical studs are what is commonly referred to as the knee wall. The knee wall meets with the slope of the roof which creates a cathedral type sloped ceiling. The space on the other side of the knee wall is the side attic.

Side Attic Insulation

The side attic should be treated as any full size attic since it is directly above living space. The joists are 2x8s, which allows for two layers of 2x4 insulation in between. I chose to insulate with Roxul which has a higher R value than the pink stuff. 2x4 insulation has an R-value of 13.5. With 2 layers between the joists and another layer running perpendicular over top, the R-value of the insulation in the side attic is in the range of R-40.

Cathedral Ceiling Insulation
It is absolutely crucial that at least an inch or more of airspace is maintained between the insulation and roof deck to allow for adequate ventilation.
The roof members are 2x6s, which only allows for 2x4 insulation at most. John added 2x2 strapping to all of the roof members. Not only does this create a larger cavity for insulation but it also adds strength to the roof structure.
Raft-R-Mates were installed to prevent the insulation from coming in direct contact with the roof deck.
Now we could insulate the sloped ceiling to R-20. With one inch polystyrene between the drywall and the roof members, the r-value increases to r-25 with no thermal bridges. Essentially this system offers excellent insulation value because wood conducts heat and cold. The polystyrene prevents the outside temperatures from being transmitted through the wood directly to the drywall.
Knee Wall Insulation
House wrap (Tyvek) was stapled up on the attic side of the knee wall, treating it as an outside wall. This not only provides a backing to prevent the batts of insulation from falling into the attic, but also acts as a barrier to help stop the cold air from penetrating the insulation, increasing its effectiveness.
Other options for backing for insulation: nail 1x2 strapping, or other wood strips to the attic side. Some contractors opt for chicken wire or other forms of netting to hold the batts in place.
Using extruded polystyrene, while it may seem like a good idea adding more r-value to the knee wall structure, is not recommended in my area, where vapor barrier must be on the warm side of the insulation. The polystyrene would not allow the wall to sufficiently breath.

I hope this helps a little.

Marcel :slight_smile: :smiley:

also see,M1

page 69

Very interesting. That’s very informative. I hadn’t thought about using solid foam insulation on the living space side of the sloped portion of the roof. I had thought about just firring out the roof rafters a bit more in that section; e.g. using a 2 X 4 for that portion of the roof, to allow more space for fiberglass insulation. I don’t know which method would be best. Does screwing drywall through foam insulation to the stud work well? Seems like it’d be a little “squishy”.

Also interesting was the description of applying vapor barrier to the floor in the side attic. I had planned to try and have some access to the side attic for storing rarely used items (figured it’d be a decent place to throw Christmas ornaments). So, I don’t think I can do all that fiberglass insulation on the floor. I’m also going to be having some heating/cooling ductwork in the side attic space, so there will inevitably be some moisture in there. I think it’s more important I get a vapor barrier between the side attic space and the roof, than between the side attic space and the kneewall or floor below.


Going to lead to you into some alternative but more progressive ways of doing the slopes and side/outer attic:


See the last section and second drawing on “kneewalls”. By fully airsealing and insulating the outer slopes and triangular gable end walls of the outer attic, you create an airtight warm space for storage. There are benefits to this method: (1) less insulation is used; (2) you only have to airseal 2 locations instead of 3 if a kneewall already exists…sealing between the floor joists under the kneewall is a b_tch. If no kneewall is there and one goes in after insulation & air/vapour barrier, even better…there will be no break/airsealing at the kneewall/slope to airseall; (3) you now do not have to vent this attic space as it is now contained warm space; (4) you do not have to insulate and weatherstrip doors to the storage space. Sounds quite good doesn’t it…started doing these areas like this in 1983!!!

As for the slopes, see the section on cathedral ceilings here:

If you do a good job on the airsealing and your house is run at moderate humidities of 35-40% in the winter, the venting is really not that critical.

To get cheaper extra depth in the slopes: rip 8’ pieces of ½" or 5/8" chip/osb board to the appropriate depth for the R value you want and screw/nail them to the sides of the rafters. Then screw 2x2" pieces alongside thejoist extension inner edges lengthwise. After insulating and air/vapour barriering the cavities with batts, add 1x3" strapping horizontally at 16" center for the drywall; screw the strapping to the 2x2" material. If there are no kneewalls there now, this can be done for all the slopes before the kneewalls go in…just have to pre-plan for wiring services in the kneewall later.

Excellent! I very much like the idea of having the side attic be conditioned space. Space is at a premium in my house, and a little extra would be greatly appreciated.