Knee Wall Insulation? - Contractor Talk - Professional Construction and Remodeling Forum

MEL · 02-09-2009, 03:43 PM

What is the best way to insulate a knee wall? I have a customer with a bungalow that is about 50 years old. A bungalow is a type of house with a bedroom up stairs and part of the ceiling is angled like the roof.

The angled part of the ceiling is insulated and has an air space for ventilation.

The knee wall is already insulated with 3.5” fiberglass with a foil face. The foil is facing the cold side of the wall.

The customer wants me to add insulation to this wall. What would be the best way to it?

I am going to be adding 17” blown in fiberglass to the attic behind the knee wall. I’m also going to be adding soffit vents, baffles and ridgevent.

I would appreciate a quick response; the customer is waiting for me to call back with a price.

MEL · 02-09-2009, 04:38 PM

The existing insulation on the knee wall has the vapor barrier facing out. In Michigan the vapor barrier should be on the warm side, facing in. Should I rip it out?

Would I create a bigger problem if I insulate over it with another vapor barrier?

What about adding Foamboard insulation?

SLSTech · 02-09-2009, 05:43 PM

Mel, I don't know if it is the best - but regular batt insulation in the knee wall cavities would be my answer at the moment. (Sorry, but I mainly push / spec closed cell spray foam with no ventilation for the roofs, walls sprayed foam & use batt insulation on interior walls to help with noise) You could put rigid insulation on the back of the wall but I would think that would be a pain.

Tinstaafl · 02-09-2009, 05:57 PM

I too (without seeing it) would go with batts on the knee wall. Definitely get rid of that inside out vapor barrier. Installing the batts from the "wrong" side of the wall makes the new vapor barrier a bit tough to get any kind of seal. Maybe some tightly fitted foam board installed and foamed tight prior to the batts would be the best compromise, then use unfaced batts.

Kgmz · 02-09-2009, 08:13 PM

The best way is as SLSTech mentioned with the roof inulated all the way to the outside wall.

The problems with this type of construction with the knee walls, lower ceiling in below attic out to outside wall and roof above upstairs space. Is that you have cold air coming in at the soffit and supposedly going out ridge, but sometimes it will infiltrate through the knee wall insulation if it is not tight and sealed and then with the cold air hitting the back of the sheetrock you will have condensation there. And with batts faced or unfaced you do not get a tight seal with 2x4 studs and then if you put in a thicker batt then you have the gap where the studs are not out as far as the insulation. Also don't forget to seal off the space under the floor from the attic, you don't want cold air blowing under there.

I have built several of these type of bungalows, room in attic. What we used to do. Solid blocking under wall in joist space to reduce air inflitration under floor and blocking out from top of knee wall to hold ceiling insulatiion and to give a place to attach foam board. Knee wall gets 3.5" batts and cover wall in attic space with 1.5 or 2 " foam board running it up in to rafter space fitted around rafters leaving a 1" gap for ventilation and all the way down to sheetrock of ceiling below fitting it in between the ceiling/floor joists. And then some canned foam to fill in any gaps and tape seams. And then blown in for the ceiling below attic space. This will reduce any chance of air infiltration on that wall.

What we do now, spray foam either closed roof or under ventilation space built under roof. And then this conditioned attic space is sheetrocked and used as storage if stick built or just left as is if trusses. And way better overall if there is any ductwork in there.

MEL · 02-09-2009, 08:59 PM

Quote:

Originally Posted by Kgmz

The best way is as SLSTech mentioned with the roof inulated all the way to the outside wall.

The problems with this type of construction with the knee walls, lower ceiling in below attic out to outside wall and roof above upstairs space. Is that you have cold air coming in at the soffit and supposedly going out ridge, but sometimes it will infiltrate through the knee wall insulation if it is not tight and sealed and then with the cold air hitting the back of the sheetrock you will have condensation there. And with batts faced or unfaced you do not get a tight seal with 2x4 studs and then if you put in a thicker batt then you have the gap where the studs are not out as far as the insulation. Also don't forget to seal off the space under the floor from the attic, you don't want cold air blowing under there.

I have built several of these type of bungalows, room in attic. What we used to do. Solid blocking under wall in joist space to reduce air inflitration under floor and blocking out from top of knee wall to hold ceiling insulatiion and to give a place to attach foam board. Knee wall gets 3.5" batts and cover wall in attic space with 1.5 or 2 " foam board running it up in to rafter space fitted around rafters leaving a 1" gap for ventilation and all the way down to sheetrock of ceiling below fitting it in between the ceiling/floor joists. And then some canned foam to fill in any gaps and tape seams. And then blown in for the ceiling below attic space. This will reduce any chance of air infiltration on that wall.

What we do now, spray foam either closed roof or under ventilation space built under roof. And then this conditioned attic space is sheetrocked and used as storage if stick built or just left as is if trusses. And way better overall if there is any ductwork in there.

Can I just leave the foil faced insulation alone if there are no sign of mildew?

What would happen if I put foamboard over it?

Could I put tyvek over the batts to cut down air infiltration?