Hello, first time poster/long time lurker;
*Note: for various reasons I am not going to use spray insulation; I will be using fiberglass batts and rigid foamboard. I am located near Windsor, Ontario, Canada where high winds and low winter temps/high summer temps.
I am about to start to finish the top floor of my 2.5 story home. Currently the ceiling of the 2nd floor is insulated with vapor barrier (this will be removed).
I am wondering about the a) best way to insulate the knee walls as well as b) the sloped ceiling (note: the roof does not have ceiling rafters but has trusses that go up to create an 8′ ceiling under the 12/12 pitch roof (which will allow me to maintain airflow).
For better or worse the HVAC trunk travels the length of the knee wall. I think the best way to insulate would be to build the knee on the outside of the HVAC trunk thereby enclosing it within the insulated envelope.
My plan is to build 2×6 knee walls and insulate with R-20 pink batts. Should I further put some type of rigid foamboard on the inside of the studs? Then, using air baffles to maintain airflow, my plan is to use pink batts up along the sloped ceiling. Since the trusses aren’t spaced very well my plan is to put the 16″s batts vertical between the slats thereby increasing the R value from R-20 at 6″ to something much greater. I would then put rigid foamboard on the inside of the slope/ceiling with the drywall on top of that.
What are your opinions on that? I am open to all suggestions. Please see the attached pictures! Thank you!
Chris
Replies
This is the view where the knee wall will be built. I slopes right down to the floor/eaves. You can see the trusses sloping up toward the ceiling. There is plenty of space for batt insulation and then, I believe, it will further add to the insulation to apply the rigid foamboard on the INSIDE of the living space, taped, with drywall on top.
If it is foil lined rigid board I can just tape it as my vapor barrier, correct?
I think you have a basically good plan.
A few thoughts.
some of the duct work needs some help.
make sure your attic ventilation (outside your insulated envelope) is split top and bottom, and of enough effective area for the roof size.
You might want to pull up the floor close to the top of the wall below and make sure the rim of your floor trusses is insulated and sealed.
You may also want to consider a fire break between that floor cavity and the cold space under the roof deck and over your new walls/ceiling.
Ideally, your vapor barrier will meet that of the wall below.
You can make the insulated knee wall outside the HVAC have a slant to allow more insulation at the top. an additional internal knee wall can be installed to hide the duct work if you like.
Thanks UncleMike,
It is good to know I am on the right track. Yes, I will fix the ductwork as I go.
I am not sure what you mean by having the attic ventilation "split top and bottom and of enough effective area for the roof size". Does this mean utilize baffles from the soffits/eaves up through the knee wall?
I will double check the rims but I believe they are insulated and sealed. I will confirm that the vapor barrier meets.
What do you mean by a fire break?
That's a great idea about the slanting insulated knee wall and yes, I will create the internal knee wall with HVAC access points.
Thanks for your feedback! It is really appreciated. I am open to all suggestions and would prefer to do this correctly as I start the project (in the next two weeks or so).
https://www.finehomebuilding.com/?s=attic+ventilation
https://www.gaf.com/en-us/for-professionals/tools/ventilation-calculator
Lots of discussion on roof venting. Basically you want to have enough net vent area in proportion to the area of your attic. you also want this split evenly between the high end and low ends (peak and eave)
A fire break is a set of features to delay spread of a fire. As you convert your attic into living space, you want to seal up any places where fire or combustion gasses could move from the floor below into your new space, or to the attic above. There are a couple options for such sealing, including plywood, osb, drywall, and most caulk and foam sealants.
The open truss system under your attic floor would allow fire to spread between the bays, so it is important to pay attention to the sealing after you remove the insulation and vapor barrier that is currently in place.
From your pictures, it is not clear to me that you have any ventilation at the bottom of the roof slope. (For that matter, the photos you shared do not show any roof venting anyplace)
I took your advice and peeled back some of the plywood along the outer wall. No, the rim joists are not sealed. (Not surprising though because the ceiling for the 2nd floor has the vapour barrier).
Note: there is a ridge vent that runs the entire length of the roof line for air flow as well as soffit vents
Pic1 shows the floor in the attic removed, the ceiling truss, and the eaves
Pic1.5 shows the rim joist
Pic2 shows the rim joist cavity with the insulation removed
Pic 3 is a view looking down the ceiling truss towards the eaves (has soffit ventilation every 4 feet or so)
Your advice on the following is appreciated:
1) I don't know how best to seal the rim joists. Rigid foam board cut to size then sealed with spray foam around the edges- then insulation?
- I want to continue the vapour barrier up from the second floor, across the rim joists, and then up the inside trusses (creating a building envelope along the outside of all living spaces - I will then build a non insulated kneewall in the envelope within the living space)
Pic 1 shows the red where the vapour barrier will go
2) how best to utilize rigid foam board in the attic living space?
-Can I put the foam board on the inside along the trusses and then put pink insulation between the trusses for more R value? (Drywall would then go up over the foam board with extra long screws)
- If so, can I tape the foam board as my vapour barrier?
- if so, does the foam board need to have the foil side (on warm side)?
There is lots of space for airflow from the eave vents up to the top roofline vents
(see pic 3) this is looking down one of the truss bays toward the eaves (light blue). The red represents the vapour barrier/foam board, the dark blue is the insulation between the trusses, and the light blue arrows show airflow along the roof line from soffit vents to the top ridge vent. note- in this picture the floor exposing the rim joist has not been removed yet.
I was thinking 1" rigid foam board. Not sure if it needs to have a foil side.
Thanks for your advice. Since I have the opportunity to do so now that the space is open I want to do it as correctly as I can.
Windsor and Ontario have energy and insulation requirements spelled out in building codes. R50 for ceilings with attics, and R31 for ceilings without attics. The requirement for walls above ground range from R22 to R27.
https://www.citywindsor.ca/residents/building/Building-Permits/Documents/EEDS_Performance%20Form%20A.6.2.pdf
https://buildersontario.com/wp-content/uploads/2015/02/Supplementary-Standard-SB-12-Energy-Efficiency-For-Housing.pdf
This means you need at least 5 1/2 inches of fiberglass along that rim joist, and in your insulated knee walls, and around twice that for the ceiling below the roof above the knee wall. More is better.
It looks like your rim joist (or the sheathing that is installed there) is insulated, which is good, and you want to make sure it remains that way.
My sealing suggestion is not intended to provide air sealing along the rim joist from the exterior, but to make sure your attic floor cavity is not connected to the roof venting path. (focused on spread of fire) The plywood along the narrow edge of the attic floor likely is a fine barrier for this purpose, as long as any penetrating holes for wires and the like are sealed. Half inch drywall is good for this purpose as well.
Rather than install rigid foam boards, You might want to model your solution after the way they installed the ceiling below. Hat channel (or wood furring) installed at right angles to the studs of your knee wall and the bottom of the roof trusses. Unfaced fiberglass outside, and a continuous vapor barrier inside, which is covered with drywall.
Sounds like you have adequate roof venting, split between eves and ridge, which is good. You wan to make sure your insulation installation does not impinge on the free flow of this venting air.
So the top of your insulated knee wall should be far enough in from the exterior wall below to let you install the required 10 plus inches of fiberglass in your sloped ceiling while leaving enough space above to vent the roof deck. You can use baffles above the insulation if it will be close, to make sure the venting air flow below the roof deck is preserved.
You can use whatever sheet vapor (vapour) barrier you like to extend your insulated knee wall barrier to the existing barrier on the ceiling below. This will mean some fussy detail work to seal around the floor trusses, but that is part of the fun. You can leave some extra to overlap the existing barrier, and along truss member surfaces, and use tape and/or spray foam or other sealant to join to the existing vapour barrier.
Will you have a permit and inspections for this work?
Great! Thank you for the feedback. Yes, permits and inspections are in order. Homeowner build/renovations.
Rim joists will be insulated and sealed with vapo(u)r barrier in a continuous manner from the second floor.
I appreciate your input on the attic ceiling insulation. I will leave out the rigid board and use furring strips, pink insulation and a continuous vapor barrier.
Also will utilize 10" plus of insulation in the ceiling. I will confirm that there is enough space for ventilation above the knee wall.
Holes in the plywood will be sealed to limit fire spread.
Thanks again!
Ok. Insulation is goin in soon. I am using Raft RMate blockers to keep the soffit vents flowing up to the ridge vent
See pictures- I have 2 sections between after trusses that are approx 48" wide. They are braced with 2x6s. There is about 1" of space between the lowest one and the rim joist. Some air can pass through. Is this all that is needed? Will the other soffit vents down the rest of the length of the roof provide enough airflow or should I cut out some of the 2x6 (see other pictures.)?