Thread: Attic Kneewalls insulation and door insulation

Have a question. We are having our finished attic inspected after the fact (long story with the contractor). The inspector asked for a couple of fixes and needed to better understand some things.

1. The kneewalls have access doors from the warm to the cold area. The inspector said that they needed insulation and said it couldn't be the hard foam type? I was at the local hardware store and they have hard foam panels R-3.0. These seem like the proper way to insulate the door but wonder if that doesn't meet local code ( North Carolina ). Putting a higher R value material would just fall apart after a couple of times opening and closing those small doors.

2. He said that the walls needed to be insulated with an air barrier. I thought that the insulation that is faced (towards the sheetrock) would act as the barrier. He wants to put house wrap on the other side of the wall ( the side exposed to the storage side are in the attic ). Isn't this a problem? Having one side have the barrier that comes with faced insulation and house wrap on the other side? Couldn't that cause condensation to be trapped in the winter?

Any thoughts on this is appreciated.

Thanks!

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The insulation on the knee walls which back up to the attic on the other side of the knee walls needs to at least meet the minimum requirements for the insulation in the ceiling for your area, and that includes the attic access door.

If your area only requires R-19 on ceilings, then the knee walls and access door need to be R-19; however, if your area requires R-30 on the ceilings, then R-30 is needed on the knee walls and the access door.

I think you should ask the inspector to clarify. His concern about the foam may have to do with fire hazard, not insulation quality. Toxic fumes are released when foam is burned.
House wrap such as Tyvek is porous and allows air to pass, so it should be ok. Again, this may be due to an air quality concern if you are using the attic space for storage, frequent entry, etc. In your climate, vapor barrier rules are different from what we do here, so local info might differ.
We don't know what the inspector is thinking or what guidelines he is using, so those are a few ideas, nothing more.

Right or wrong it is fairly common practice to insulate access panels with rigid foam. Panels that fit into a ceiling access can easily be insulated using the same batt materials used to insulate the rest of the attic but panels that are installed in walls need some means to support the insulation and behave more like a door. The rigid foam would be on the “outside” of the door so while fumes would occur in the event of fire they would not vent freely into the home. If for some reason your area insists that foam be covered simply cover it with a layer of plywood and framing forming a sandwich and also allow weather stripping.

Tyvec is a moisture barrier and is not a very good air barrier. A rigid product (i.e. thermo ply with the seams taped) is a better air barrier.

Tyvek can be an air barrier if it is detailed correctly. That would be tapeing the seams and edges. There is no reason you can not have more than 1 air barrier. So even if the sheating is the primary air barrier it is worth it to detail the tyvek to be an air barrier. Some people do use Tyvek as the primary air barrier but I would not recommend it.

What you dont want is a 2nd moisture/vapor barrier. A building cavity needs to be able to dry either to the interior or exterior.

I think the OP was saying that the knee walls have kraft faced fiberglass or a plastic vapor barrier with fiberglass. The back of the kneewall is still a wall and code R value for the wall would apply. The problem is that the effective R value in this installation is barely measureable. Putting drywall over the fiberglass will improve its performance but it sill will not be good.

The saying is that fiberglass needs to be in a 6 sided air tight enclosure to perfrom well. So when you can see fibeglass it will not perform anywhere near the R value stated on the package. That makes fiberglass a poor performer in an attic floor and when it is exposed on the back of a kneewall.

Any insulation works by trapping air. Fiberglass is the most air porous insualtion. When air flows over the insulation , referred to as wind washing, it pulls the air through the insulation are reduces its effectiveness. Fiberglass also is the least effective at stopping radiant heat flow. In an attic during the summer the primary heat from is the radiant heat from the roof. The hot air is not much of the load as it is bouyant and floats out the roof vent.

Originally Posted by Robert Hronek

The back of the kneewall is still a wall and code R value for the wall would apply.

Robert,

Maybe Florida is the only state which does this, but:
- N1102.ABC.1.2 Walls considered ceiling area. Wall areas that separate conditioned living space from unconditioned attic space (such as attic knee walls, walls on cathedral ceilings, skylight chimney shafts, gambrel roofs, etc.) shall be considered ceiling area and have a minimum insulation value of R-19.

I should look it up here. I know the houses I am in I see a bat for a 2x4.

Just a second or third though. The R-value ratings are for walls & ceilings cavities, I know of no door or window or skylight that have a R-15 or 19 rating. So if I was to inspection said property I would recommenr 1 or 2 layers of 1.5" blue Dow board and call it as good as one can get.
The only other way around your propblem would be to insulate the exposed rafters and joist in the atti area and then no insulation is needed in the transition wall between attic and living area.

Food for thought

Brad Peterson- Tri-City Inspection Agency LLC, Nunn, CO.