Knee Wall Insulation & Venting

Hello all.
I am taking on a project to finish an attic.
The attic has a cathedral ceiling, dormers, and 2 large knee wall sections.
I will be drywalling over the knee walls and the ceiling, essentially closing off sight to the cathedral ceiling.
I plan to insulate above the ceiling drywall and behind all walls, including the knee walls.
The house has continuous soffet vents and a continuous ridge vent. I intend to use soffit baffles for ventilation.

Here are my questions...
- Does the attic/crawl space behind the knee wall need ventilation?
- How should the attic/crawl space behind the knee wall be insulated? (Rafters, floor joists, knee wall - type of insulation?)
- Should I run the soffit baffle from the (1) soffit all the way up to the ridge vent along the roof, (2) just above the insulation of the ceiling that I will drywall, (3) or should I alternate the baffles end point between (a) just above the ceiling drywall and (b) within the knee wall attic space so that both the knee wall space and above ceiling space are ventilated?

Thanks for your responses in advance.

George

 

Here are a couple of related links to get you started. Whichever approach you select, air sealing is most important. If you insulate the knee walls, then the baffles would provide ventilation to the side attic and then more baffles would provide continued air flow under the sloped section. In addition, the exposed back side of any insulation in that kneewall should be covered with an air barrier, like tyvek, not a vapor barrier which would be on the living side.

I live in a cape, so know first hand the problems with heating and insulation. While you have these areas open, consider padding out the cavities to allow extra insulation or covering the walls/ceiling with an extra layer of rigid insulation. During construction, adding more is much easier.

http://www.ornl.gov/sci/roofs+walls/...c%20floors.pdf

Energy Savers: Knee Wall Insulation and Air Sealing

Bud

 

I suspect the triangular spaces behind the knee walls need ventilation, although that should happen as part of your moving air through those areas up to the ridge. Our entire upstairs has knee walls (house built in 1946), and I suspect the better you insulate them, the more comfortable your adjacent living areas will be. Our k.w. insulation is haphazard, making the upstairs rooms a bit chilly or hot, depending on the outside temps.

 

Thanks so much for your response!
Just want to make sure I understanding correctly.

1. How should I run the baffles? Starting from the soffet all the way up to the ridge? Or should I end it right above the ceiling drywall? Or another method? Is the main ventilation issue allowing air to hit the roof? Or does air need to actually circulate through the knee wall attic space and space above ceiling?
2. To the Tyvek suggestion. I'm planning to use R-19 faced batts for the knee walls. Are you saying to insulate knee wall with batt facing faced toward the knee wall attic space, then Tyvek wrap the kneewall (living space side) I will be drywalling? And if so, which direction should the Tyvek writing face (towards living space or knee wall attic space)?

Thanks again.

 

Thanks BridgeMan.
Tell me if I'm wrong...
When running a soffit baffle along a roof rafter,
all areas that that soffit hits are considered ventilated?
Even if the end opening occurs at the ridge?
So I wouldn't need to let any of the baffles end within the knee wall space?
And additionally, any advice on where I should end the baffling?
I've seen it run both (1) all the way up to the ridge and (2) just above ceiling insulation.

 

If you insulate the kneewall, then the fresh air needs to mix and carry the heat and moisture from that side attic up and out, thus don't run your baffles right past the side attic. Once you get up to the top attic, the same thing, dump the air flow into the attic to pick up heat and humidity that has leaked, diffused, or conducted through the ceiling or other areas. The purpose of ventilation in cold climates is moisture and heat removal. During the summer months, it then removes the solar heat.

As for the r-19, Kraft to the inside as that is coated with tar and somewhat of a vapor barrier. The tyvek or as I prefer, a layer of rigid insulation goes to the attic side. If you want to increase the r-19 with an extra layer of rigid I will dig out the article explaining how thick to avoid condensation.

Remember, your attic recommendations for r-value should be the target for walls and slopes. IMO, the only reason they backed off and allowed R-21 in walls was the practicality of making wall into R-40 to 60. Walls lose as much if not more heat than a ceiling.

Bud

 

Bud...

I apologize for my slow learning curve.
Below are steps I've understood from your responses.
If you can confirm or correct them, I'd appreciate it.

1. Insulate floor joist and knee wall with batts, then wrap attic side knee wall with rigid insulation
2. Install soffit baffles. Half of the baffles will run from soffit vent and stop within side attic to bring fresh air into side attic and to allow heat and moisture to be carried up into top attic. The other half will run from soffit vent to just above the ceiling drywall and insulation so that the top attic will circulate fresh air with heat and moisture then to be carried out through ridge vent.

Thanks.

George

 

One last question (which may answer a lot for me)...
If I use multiple baffles to reach from the soffit vent, up the knee rafters, all the way to just above the ceiling into the upper attic, how should I run the baffles themselves?
Overlap them? Space them out?
I've heard that spacing them out is a good idea to prevent moisture within the baffles.
This may also solve my question of how fresh air is being provided within the knee wall attic space.

Any responses are greatly appreciated (and needed very soon!).
Thanks.

Geirge

 

The typical installation would be ALL of the baffles (or rafter cavities) open to the kneewall space just above the insulation. Then a new set of baffles start at the top of the kneewall and carry the air the rest of the way to the top attic space. I have never seen a job done with half and half as described. That doesn't mean it's bad, just that I have never run into it.

In the interest of keeping the temperatures across the roof uniform, my guess is this approach would be better than what you outlined. But that's a guess.

One thing shown in the diagrams but not mentioned as yet, is the block below the kneewall to prevent any cold air from circulating underneath the interior floor. Lacking that, cold air can blow in one side and out the other, taking your heat with it. Fiberglass insulating is noted for not stopping air flow. You should still have the block, but take a look at Roxul insulation. It is more dense and has some great specs. In an area where there will be air movement it would perform better.

Bud

 

Interesting...
So you're saying to essentially have two areas where the air is dumped?
One in the knee wall attic space. Second in the upper attic space above drywalled ceiling.
Am I right?

So the first set/level of baffles would originate from the soffit vent.
And the second set/level of baffles would originate from the top of the knee wall.
So both zones mix with the fresh air and travel up to the upper attic, out the ridge vent?

Just to be certain, if I go this route,
I do not need to insulate any of the pitched rafters, correct?
I only need to insulate knee walls and floor joists within knee wall attic space.
And of course, seal all areas that may leak air into conditioned space.

I am intending to Tyvek wrap the knee wall insulation to avoid air from hitting it.
Does the floor joist within the attic space need any similar air barrier?

If you see any errors with the entire approach outlined above, please point them out.
You've been a great help Bud!

Heading out to figure out a plan in the next 15 minutes.
If you're free to respond within that time, would be great!

George

 

The concept of dumping the air into the side attic and then letting it find its way up and out is exactly what the air does in a traditional attic, except for the squeeze above the sloped section. Yes, insulation is needed in those rafters and that is one of the weak spots for a cape, too little room. One approach is to add 2" of rigid over the inside before the drywall. Recommendations for r-value on the slope will often quote wall specifications, but the reality is that walls and slopes should all be insulated to the ceiling specifications. The practicality of that has forced them to compromise the regulations to stay within what people can typically achieve, but areas like kneewalls should be increased when it is easy. Same for the slope.

To repeat to be sure, you said "I do not need to insulate any of the pitched rafters, correct?" The area above the side attic, no, the area of the slopped roof where the baffle connects between the side attic and the top attic, yes, and that is where I'm suggesting you consider adding 2" of rigid on the inside.

Bud

 

Kind of lost me on the last paragraph...
Are you saying to add the rigid on the sloped roof,
not within the knee wall attic,
but right above it where the ceiling dry wall will be installed?
Are we using "area above" and "sloped roof" interchangeably?

Any clarification would be helpful.

Thanks Bud.