Best Practice for Retrofit Insulation on Modern Knee Walls in Attics

I'm wondering what folks are doing to add air-sealing and insulation to knee walls in modern attics. Methods I think of with questions/concerns are:

1. Foam it

    a- very expensive

2. Batts in cavities with location-approved foam board as continuous insulation

    a - getting foam board in attic

    b - cost of foam board

3. Batts in cavities with FSK faced R-19 insulation as continuous insulation

    a - how to attach insulation for longevity w/o crushing it and having reasonable air sealing

4. Do something like BIBS with blown cellulose or fiberglass in the pockets.

    a - what to use as membrane (FSK, regular BIBS membrane (not good air seal), etc.?)

    b - thin where membrane is attached 

Are there articles, posts you would recommend reading?

Thanks for your input,

Martin

Tags: best, bibbs, board, foam, fsk, knee, practices, walls

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It gets down to the size of the crawl - I can't recall where I saw it but one contractor cuts a whole through on the gable end which allows for passage of foam boards, etc... Don't forget you have to put an air barrier up for most products 

It is important to also remember the key junctures where the top of the kneewall meets the roof, and where the bottom of the kneewall meets the floor/ceiling.  Getting an air barrier installed at those two details isn't always done well.  

There are 3 problems you are trying to solve.  First, that the face of the insulation is not making good contact with the back of the interior drywall, second is that the batts are in danger of falling down over time, and third is the wind washing on the back of the batts reducing their R-value.  Our solution for the last 35 years has been to be sure that the existing batts (if any) fill the cavity completely in depth and side-to-side, then install and seal appropriate foam board over them, from 1/2" to 1-1/2", depending on budget.  We cut the 4X8 foam board down the middle to make 2X8 pieces and these will almost always fit through hatches that exist or that you make.  Don't forget the extremely important step of closing off the open cavities into the house just below the kneewall.

See NREL SWS Tool, SWS 4.1004.2

Our favorite way was roll up fiberglass and spray with two part foam but most contractors do whats in the SWS.

Thanks for your replies! I appreciate the reference to the SWS. Haven't seen that before.

This can be more problematic than it seems.  In Northern climates, thermal loss and active air circulation cause ice dams.  I might recommend an approach that in effect isolates the area inside the thermal envelope, meaning the floor, knee wall, end walls and rafters (using Proper Vent (foam chute) to dissipate wall moisture) and insulated with fiberglas.  Plastic sheeting could be used as a continuous air barrier.   A weatherstriped inspection hatch on each side is just that:  Get out, stay out and find somewhere else for the Christmas lights.

One option I used to use was a net and fill with cellulose, however certain types of netting we have found are not designed to be left exposed and deteriorate over time. In that case housewrap is beneficial to install over the netting to prevent exposure.

I have seen Bubble wrap radiant barrier types of materials used, and while this may provide an air barrier, it also provides a vapor barrier depending on the material choice, which in a vapor barrier on the attic side does not feel right to me (other climates that may work, I prefer to keep most walls vapor open)

I prefer Open Cell spray foam in kneewalls, but also use various thicknesses of rigid board in smaller areas.

For small kneewall areas such as side attics especially when they are currently unvented; I prefer spray foam on the roofline and end walls over treating the kneewall.

For larger homes with tall kneewalls I prefer spray foam. I find that 2" rigid board and 5" open cell spray foam in kneewalls have a similar cost once you reach about 350-400 sqft. Smaller than that and the cost of sending a foam rig out is too high.

Getting full sheets of foam board into an attic can be a challenge. If you have a small access and cutting a larger hole is not an option, use 1" material and cut it into strips that fit through the access. Then install 2 layers overlapping and tape the seams. Don't worry about landing on a stud, 2 layers will provide enough support for durability.

I recently foamed 8" of open cell foam on a kneewall surrounding two bedrooms that each were supplied by the main HVAC and each had a supplemental window unit. After blowing the attic above, there just wasn't enough difference, so we applied the foam and we were able to remove the window units altogether.

It's all about treating the surface area and eliminating the thermal bridging.

rolls of reflectixinc.com 5' X100' are easy to get in small holes.    HVAC tecks use this for duct wrap.  If you can get in the hell hole - Knee wall attic -  crawl space - access hole - mylar sheets can be used.   If you bring in all the HVAC into the building, good start!  

I have used rolls of fiber glass used for steel building to cover the studs with large headed nails or lath screws to hold up the rolls  

I like to use a reflective thermal barrier with a minimum of 1/2" foamboard core.

The specialized thermal barrier in 1/2" performs like R-7 due to its mylar caoating, although because its a thermal barrier is not really rated in R-Value.

I leave the knee-wall fiberglass batt in the cavity, encapsulate the knee-wall with the thermal barrier, tape all seams with the same mylar foil tape and spray foam all edges with closed cell spray foam insulation to ensure a tight seal.

I have plenty of infrared images on knee-walls treated this way where the treated knee-wall is 20-30 degress cooler (or warmer) depending on season. 

The material I use comes in 72' x 4' rolls which fits through most attic accesses and covers 288 SF.

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