Cape Cod knee wall/attic insulation


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Old 06-24-17, 01:59 PM
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Cape Cod knee wall/attic insulation

Hi, I've spent the past few hours researching options for Cape Cod/knee wall treatment, and I have a few questions. I've attached a diagram of the house:

Picture

Facts:
1. The left side of the house had a mold problem on all the roof sheathing where there was insulation between the rafters. This mold problem extended into the upper attic also.
2. It was a damp winter in the crawlspace. This is something I will be attempting to fix with trench around the inside to a french drain and putting in a better vapor barrier.
3. The upper floor on the right side of the house is very warm in the summer and cold in the winter. The upper floor on the left side of the house was a good temperature year round, until I had to pull out all the moldy insulation.

The former owner may have tried to "move the attic inside" on the left side of the house by insulating the rafters. The problem is that there was a lot of condensation created, and thus mold. As mentioned above, I had to tear out all that insulation. As far as I can tell, there is no insulation or airflow blocking in any of the floor joists under the second floor.

I have a plan to do the following:

1) Block airflow under the knee walls at (B) and (F).
2) Insulate under the side attic at (A) and (G).
3) Currently, knee walls at (D) and (E) are thinly insulated. I'd like to better insulate these?

My questions are the following:

1) I'm not 100% sure where all the moisture came from to create my attic mold problem, though I'm sure the crawlspace may have played a part. I'm working to fix that. In the meantime, by blocking the floor joist airflow at (B) and (F), and insulating at (A) and (G), I don't want to move a potential mold problem inside the house. Thoughts on this?
2) The knee walls already have thin insulation inside the wall studs. I'd like to add more, assuming it should help?

Thanks in advance for any help!
 
  #2  
Old 06-24-17, 05:25 PM
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Yes, the crawlspace needs to be dry, but if you have a water issue it will float a plastic vapor barrier. Resolve any water issues from outside as much as possible. If water still enters then a drain system with a pump may be needed.

Are your soffits vented?

You will want insulation above all of the upstairs space.

You need to check the amount of insulation in the garage ceiling.

Locate the plumbing vent pipe exiting the roof to locate where it passes through that second floor. Often it goes through a kneewall and is not air sealed, provides a path from the crawlspace. Just a guess.

Any recessed lights?
All access points into the attics need to be air tight and insulated.

Kneewall insulation in your state suggests R-13 in the south and R-20 in the north. You can get R-15 in high density fiberglass or Roxul. But to reach R-20 if needed you would have to cover the back of those walls with a rigid insulation.
https://energycode.pnl.gov/EnergyCod...?state=Indiana

As you now know, fiberglass insulation can't be installed against the bottom of the roof.

You need to plan out the ventilation, soffits to ridge or gable vents. Ventilation needs a high and low air path.

Bud
 
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Old 06-24-17, 06:54 PM
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Are your soffits vented?
Every other soffit is vented, yes.

You will want insulation above all of the upstairs space.
My thought was to use a thinner insulation along the roof above the upstairs rooms, to leave room for airflow from the soffits up the side of the roof?

You need to check the amount of insulation in the garage ceiling.
Yes, I'm concerned about how much is there.

Locate the plumbing vent pipe exiting the roof to locate where it passes through that second floor. Often it goes through a kneewall and is not air sealed, provides a path from the crawlspace. Just a guess.
Thanks for the tip, I will look for that to seal that airflow off. Any other typical problem areas for crawlspace air getting up to the attic?

You need to plan out the ventilation, soffits to ridge or gable vents. Ventilation needs a high and low air path.
I understand the soffits to attic ventilation now. Can you clarify if the need goes beyond that?

Thanks for the help!
 
  #4  
Old 06-25-17, 03:53 AM
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"Every other soffit is vented, yes."
If every rafter channel shares a common soffit cavity then every other can work, if all soffit vents add up to the suggested NFA (Net Free Area)

"My thought was to use a thinner insulation along the roof above the upstairs rooms, to leave room for airflow from the soffits up the side of the roof?"
The thin insulation with air flow above is somewhat necessary where the ceilings slope, but the flat area should have a foot or more of insulation directly above the drywall. The drywall is your air barrier and should be in contact with the insulation.

Here is an air sealing guide: https://www.energystar.gov/ia/partne...ide_062507.pdf

The calculate amount of NFA needed is your attic floor area, in your case that is increased by the slopes, and divide that by 150. The result is the number of sq ft of NFA. Divide by 2 for half high and half down low.

Why is there a dividing wall between the attic and the ceiling over the master bedroom? You will need to inspect that area for mold and insulation and provide air flow. Will you be able to add soffit vents on the back?

Do you have a ridge or gable vents for high venting?

Bud
 
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Old 06-25-17, 10:09 AM
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Why is there a dividing wall between the attic and the ceiling over the master bedroom? You will need to inspect that area for mold and insulation and provide air flow. Will you be able to add soffit vents on the back?
I do not know why it was constructed with that dividing wall. It's certainly a unique design. I'm getting quotes to cut a hole in the master bedroom ceiling for attic access, but haven't done it yet. The priority right now is addressing the other bedroom (baby #2 on the way). On both sides the soffits are vented.

Do you have a ridge or gable vents for high venting?
There is a shared attic with a front gable vent and 4 box vents on the larger attic. There are 4 box vents on the master bedroom side of the attic.
 
 

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