Knee Wall Insulation

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Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 4:05 pm

Does anyone know if I need to use baffles on the sloped ceiling before I put insulation there in the knee wall? I can't see daylight when pulling back the insulation where the floor meets the roofline. So I'm not sure if it is venting there. The house does have gable vents.

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Re: Knee Wall Insulation

Post by corneliani » Sat Nov 16, 2019 4:25 pm

I'm not fully grasping where you're insulating.. the knee wall or the rafters... but the general idea with attic ventilation is to allow airflow from the soffit vents up through the rafters and to the roof (or ridge) vent. Always allow for that path to happen otherwise your venting is pointless.

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Re: Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 4:28 pm

Insulation for the celling. No eaves or soffits on house. Only gable vents.
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Re: Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 4:29 pm

1st picture is where the venting would usually be. In most videos you can see some daylight here.
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Re: Knee Wall Insulation

Post by Grass Clippins » Sat Nov 16, 2019 4:45 pm

It's funny you should mention this....I'm just finishing up with a knee wall insulation job myself. If I were you I would definitely try to get an air gap if you have ridge vents. Even though you can't see daylight I'd bet it's pulling air from somewhere. I used the thin metal L channels that you find in the roofing section to get my 2 inch air gap, on a 2x6 rafter, and then used Roxul to insulate. My knee walls get up to 150 degrees Fahrenheit in the summer so I added to the cavity with another 2x4 to get R30 (2 layers of R15). One of two rooms is complete and the complete knee wall attic area was actually cooler this summer than the AC setting upstairs while other one was still around 150 midday.


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Re: Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 4:51 pm

Thanks @Grass Clippins. So the Roxul goes up against the L channel and the air gap is just behind the Roxul. I'm definitely confused with all of this. And then you used fir strips to secure it in?
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Re: Knee Wall Insulation

Post by Grass Clippins » Sat Nov 16, 2019 5:03 pm

@pennstater2005 You got it. I haven't used the fir strips on the second layer yet but it would be a good idea. The first layer of R15 is being held up by the 2x4's which contain the second layer of R15. Notice in the picture 2 how the 2x4's run perpendicular to the rafters in picture 1. My thought was that it would help prevent thermal bridging.
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Re: Knee Wall Insulation

Post by Grass Clippins » Sat Nov 16, 2019 5:15 pm

@pennstater2005 To get the air flowing at the bottom you may be able to use a whole saw to poke a couple holes in that black board...unless that's the fascia...is that the fascia?
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Re: Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 5:22 pm

I'm not sure what the black board is. Surprisingly the knee wall area is very dry despite no visible venting anywhere. I'll use the baffles as they pretty cheap. The house has gable vents on each side and a ridge vent. No soffit vents. That's why I've been debating on the need for baffles.
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Re: Knee Wall Insulation

Post by Grass Clippins » Sat Nov 16, 2019 6:00 pm

I got you. I'd bet that black board is the fascia. I was assuming that you had soffit vents and a vented attic, thanks for clearing that up. I'm going to go back on what I said about needing the air gap. In your case the only thing an air gap would do is use up space that you could be adding R value. You don't need Roxul either, I just used it because of it's rigidity.

If you have gable vents without soffits then you are probably losing a lot of energy from air being pulled up through your light fixtures and switch plates. I think they refer to this as negative pressure. If you feel up for a bigger project you could go up in the attic and air seal the ceiling under the insulation.
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Re: Knee Wall Insulation

Post by pennstater2005 » Sat Nov 16, 2019 6:30 pm

I wouldn't even know how to get to that area! I just don't want to end up with any moisture issues as I don't have any right now. Thanks @Grass Clippins :thumbup:
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Re: Knee Wall Insulation

Post by corneliani » Mon Nov 18, 2019 8:07 pm

@pennstater2005 - I build in Atlanta so our construction standards are a bit different but what you have seems a bit unconventional. Makes me think someone added that black board to cap off the soffit venting (in order to keep the cold out??). Venting is about the exchange of air - where cold air enters the soffit which then pushes the hot attic air up and through the ridge venting (and/or roof or gable vents). It's a bit of a chimney effect. If you don't allow for this exchange the hot attic air creates a lot of thermal pressure on both your adjacent living spaces as well as your roofing system.

In your case, if I'm understanding the pictures correctly, the soffit vent has been capped. To do this right you either open up the soffit vents as @Grass Clippins suggested (baffles are typically only needed when you install Sheetrock up against the roof rafters where the insulation can fill the rafter bay and interfere with that air circulation) ... or you completely close off the remaining gable & roof vents and completely insulate the rafters, creating a true encapsulated attic. If it's this latter option you choose then insulating the knee wall is redundant... the building envelope will now be the roof and not the ceiling / knee wall.

PS: I have a feeling the reason for these modifications are due to the cold air coming into the attic and there not being enough ceiling insulation to keep that cold air from penetrating into the living space. I say this because seeing those boards on the ceiling joists makes me think the batts are barely 8" tall there (... and energy code for your Zone 5 (I have friends in Allentown & Bethlehem) requires at least a R49. In the meantime adding baffles doesn't do anything.

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Re: Knee Wall Insulation

Post by pennstater2005 » Mon Nov 18, 2019 8:39 pm

@corneliani Thanks for the reply.

There is no soffit. I actually installed the vent baffles just in case I ever add eaves to leave an air gap. For two bucks each I figured that was cheap enough.

Cape Cods are notoriously hard to heat and cool. None of my exterior walls have insulation except the kitchen which I remodeled. House was built in the early fifties and heated originally with coal which was cheap and so insulation was skipped.

I want the attic to be conditioned air. Problem is the previous homeowners insulated poorly and so I'm removing some is that stuff as well. Insulation is expensive!!
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Re: Knee Wall Insulation

Post by GrassAndWater12 » Tue Nov 19, 2019 11:54 am

I have recently went through this exact situation and here is what I did but I know this will bring mixed opinions and reviews. My issue was a bonus room above the garage. The room was always to cold or hot due to the lack of ventilation in the knee walls to the roof, lack of properly installed insulation, and of course the garage not being a conditioned space.

I decided to get a company to spray foam (open cell) the entire knee wall encapsulating the area creating a sealed area and no need for ventilation. They sprayed the roof line and the outer wall of the room over the plywood with batt insulation. I know it was an over kill but I wanted to make sure I was done with the kids/wife complaints. Since I was going through an entire renovation I also knocked down all the garage walls and ceiling and had it spray foamed ultimately insulating the floor of the bonus room making it like a yeti cooler. The results the best feeling room in the house!! Had to actually adjust the dampers down that were installed at the air handler pushing the air to another room. Ultimately giving more air to the other rooms using less air to heat/cool the upstairs.

It has been 4 years and have been thrilled with the results. I would also like to add that I had the entire roofline sprayfoamed encapsulating the whole house and now the attic is +or- 10 degrees from the house.

Only thing I recommend is to use a reputable company and not go cheap with a diy kit or a dad and sons spray foam business as it could be a nightmare if not done correctly. I forget the site but you can search an accredited installer.

I’m always a diy if it can save me money but this one I decided to hand it over to the pros incase I didn’t achieve the desired results I could just call them back. Hope this helps and I’m not trying to start a spray foam use or don’t use argument.

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Re: Knee Wall Insulation

Post by pennstater2005 » Tue Nov 19, 2019 12:40 pm

GrassAndWater12 wrote:
Tue Nov 19, 2019 11:54 am
I have recently went through this exact situation and here is what I did but I know this will bring mixed opinions and reviews. My issue was a bonus room above the garage. The room was always to cold or hot due to the lack of ventilation in the knee walls to the roof, lack of properly installed insulation, and of course the garage not being a conditioned space.

I decided to get a company to spray foam (open cell) the entire knee wall encapsulating the area creating a sealed area and no need for ventilation. They sprayed the roof line and the outer wall of the room over the plywood with batt insulation. I know it was an over kill but I wanted to make sure I was done with the kids/wife complaints. Since I was going through an entire renovation I also knocked down all the garage walls and ceiling and had it spray foamed ultimately insulating the floor of the bonus room making it like a yeti cooler. The results the best feeling room in the house!! Had to actually adjust the dampers down that were installed at the air handler pushing the air to another room. Ultimately giving more air to the other rooms using less air to heat/cool the upstairs.

It has been 4 years and have been thrilled with the results. I would also like to add that I had the entire roofline sprayfoamed encapsulating the whole house and now the attic is +or- 10 degrees from the house.

Only thing I recommend is to use a reputable company and not go cheap with a diy kit or a dad and sons spray foam business as it could be a nightmare if not done correctly. I forget the site but you can search an accredited installer.

I’m always a diy if it can save me money but this one I decided to hand it over to the pros incase I didn’t achieve the desired results I could just call them back. Hope this helps and I’m not trying to start a spray foam use or don’t use argument.
Thank you! I plan on using foam for some areas but the entire area would be an idea. Any idea on price for your square footage?
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Re: Knee Wall Insulation

Post by GrassAndWater12 » Tue Nov 19, 2019 1:54 pm

I couldn’t find the exact per sq ft price but for a double car garage all walls and ceiling was around $700 if that gives you an idea. Total for roof garage etc was like $3,500 for a 2k sq ft two story house.

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Re: Knee Wall Insulation

Post by Grass Clippins » Tue Nov 19, 2019 2:13 pm

@GrassAndWater12 Sounds like you've spent some time over on the building science forums... :lol: I've never seen a group of people more argumentative and opinionated in my life.

From all of my research, the biggest take away I got was the importance of air sealing and creating a tight envelope. A pro Spray foam job(closed & open) definitely does the best job at getting that done but you've got to deal with the cost and finding someone whose going to do you right.

I decided to air seal my living space while leaving the attic unconditioned but I can definitely see the allure of a conditioned attic. I didn't it myself and went way overboard with R60 cellulose and a radiant barrier. Now my attic looks like the inside of a spaceship.

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Re: Knee Wall Insulation

Post by pennstater2005 » Tue Nov 19, 2019 2:18 pm

@GrassAndWater12 Thank you.

@Grass Clippins Damn!
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Re: Knee Wall Insulation

Post by GrassAndWater12 » Tue Nov 19, 2019 3:28 pm

Grass Clippins wrote:
Tue Nov 19, 2019 2:13 pm
@GrassAndWater12 Sounds like you've spent some time over on the building science forums... :lol: I've never seen a group of people more argumentative and opinionated in my life.

From all of my research, the biggest take away I got was the importance of air sealing and creating a tight envelope. A pro Spray foam job(closed & open) definitely does the best job at getting that done but you've got to deal with the cost and finding someone whose going to do you right.

I decided to air seal my living space while leaving the attic unconditioned but I can definitely see the allure of a conditioned attic. I didn't it myself and went way overboard with R60 cellulose and a radiant barrier. Now my attic looks like the inside of a spaceship.

[url=https://postimages.org/]Image[/@GrassAndWater12

You are absolutely right on the other forums. That’s why I put it out there I didn’t want to start the spray foam debate lol. Your attic looks great, I would’ve liked to do it that way myself but after spending hours up there on other projects I was glad to hand it over to someone else. And totally agree just like any job it’s so hard to find a good company in any household project, and that’s “the key to success”. Here to help with any other questions or advice.

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Re: Knee Wall Insulation

Post by corneliani » Wed Nov 20, 2019 8:59 pm

totally agree on the air sealing @Grass Clippins. It’s a pet peeve of mine how ppl spend all this money on decorative finishes but completely ignore the $500-1000 investment to do a thorough air seal, or the additional couple grand to bring the attic within the building envelope.
My first house after I got married was an 1800 sqft ranch on slab fairly new tract home. Our bills averaged $250-350/mo due to it being leaky as hell. The new home I built is more than twice the size and our bills hardly ever go above $150/mo, not to mention the comfort is soo much higher. I had calculated the air sealing and encapsulated attic had a ROI of approx 3-4 years after which it’s a savings of $1500/yr +/-. Win win, imo.

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