How do you recommend improving knee wall insulation???

Replies to This Discussion

Permalink Reply by Ed Voytovich on December 5, 2010 at 5:48am

Matthew . . . barring extraordinary circumstances I always recommend moving the thermal boundary to the underside of the roof deck. There are two reasons for this: (1) it works, (2) it cannot be screwed up easily at the time of installation ***or*** later on down the road by homeowners, kids, or electricians who need to get a wire under the knee wall and "forget" to seal back up.

Using cellulose or fiberglass blown through Tyvek (just as a for instance) is a distant second choice because the top, bottom, and corners are *very* hard to get right. I was involved in a failure of this approach (seemed like a good idea at the time) that I hope never to see the likes of again.

Please, spare me any ventilation: a foamed roof is now a wall that happens to be installed at an angle, and we don't ventilate walls (intentionally, anyway).

Fill the soffit with foam as part of the process, and finish up the job by dense packing the knee wall slope from above. Remember that the bottom of the slope is now securely plugged with foam.

You will have air sealed the attic flat as a matter of course. Once that's done you may insulate that area with whatever you like. Keep in mind that only masochists who enjoy poking themselves in the eye try to get a Grade I installation of fiberglass batts up there in the Devil's Triangle.

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Permalink Reply by Ron Jones on December 5, 2010 at 1:24pm

After removing the existing knee wall insulation (if there is any existing), air sealing, then properly installing the batts, I wrap it all with FSK 75. Comes in a roll, easy to maneuver, easy to cut and fit, great in crawlspaces and attics, and with it's foil backing can become the sixth side of the knee-wall equation. Give it a shot.

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Permalink Reply by Matthew P on December 5, 2010 at 2:33pm

Thanks. I will.

Ron Jones said:

After removing the existing knee wall insulation (if there is any existing), air sealing, then properly installing the batts, I wrap it all with FSK 75. Comes in a roll, easy to maneuver, easy to cut and fit, great in crawlspaces and attics, and with it's foil backing can become the sixth side of the knee-wall equation. Give it a shot.

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Permalink Reply by Matthew P on December 5, 2010 at 2:38pm

IS this FSK 750? I do not see a 75? Thanks.

Ron Jones said:

After removing the existing knee wall insulation (if there is any existing), air sealing, then properly installing the batts, I wrap it all with FSK 75. Comes in a roll, easy to maneuver, easy to cut and fit, great in crawlspaces and attics, and with it's foil backing can become the sixth side of the knee-wall equation. Give it a shot.

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Permalink Reply by Ron Jones on December 5, 2010 at 6:11pm

I think it's 75. I buy it at a local HVAC supplier. Here's a link to the product made by Certainteed: http://www.certainteed.com/products/insulation/hvac-mechanical/317380

I'm sure there are other manufacturers as well. I've tried to attach a photo of one application of the product. Hope it appears.

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Permalink Reply by Ron Jones on December 5, 2010 at 6:14pm

Here's a shot from before the fix.

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Permalink Reply by Jon LaMonte on December 5, 2010 at 6:15pm

Matthew - I live in the south so some of the things we do are a little different because of climate. I fix knee wall issues by pulling back the existing batts (no sense in wasting them), then I air seal, replace the batts properly, attaching them at the top so they don't sag), and then go with Thermax sheets. A little trick for getting foam board through a small hatch is to score the non-radianqt barrier side and fold them in half. You can use a thin bead of adhesive when you unfold them to make them whole again. Just make sure air seal the ends.

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Permalink Reply by Jon LaMonte on December 5, 2010 at 6:28pm

Almost forgot. One of the insulation giants here in Georgia has a guy that is on the state energy code board (which is making some major steps forward). They are experimenting with the radiant barrier bubble wrap because it is both an air and vapor barrier and it's easy to maneuver in tight places.

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Permalink Reply by Jamie Kaye on December 6, 2010 at 6:47am

Ed, are you saying spray dense pack against the roof sheathing? I have been under the impression that is not acceptable without baffles. How about once you have sealed the soffits, do you do the same for the ridge vents/roof venting in place? I like the imagery with the 'masochist' comment. :) so true!

Knee wall issues are a plenty. The six sided box is the goal! I agree with Ed that not all six sided boxes (interior/exterior walls) are sealed. If they were, I don't think we would be having these discussions. Spray foam seems to be a superior product in many ways, but its application/installation is not for the novice and needs to be thorough and properly executed.

Ron, I like the photos. The problem that I have seen with FSK tape though, it doesn't always stay stuck and is hard to get a super tight seal. Maybe smearing a small layer of mastic over the top will insure it is sealed and stays that way. Looks good though.

For the radiant barrier experiment, Jon, I thought many of them, or at least the good ones, are perforated to allow vapor diffusion. I have never seen an incident like this, but I have heard they condensate on the interior side much like the Poly behind the drywall does down here in the south, so the perforations allow for the vapor drive.

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Permalink Reply by Jon LaMonte on December 6, 2010 at 6:52am

Jamie Kaye said:

Ed, are you saying spray dense pack against the roof sheathing? I have been under the impression that is not acceptable without baffles. How about once you have sealed the soffits, do you do the same for the ridge vents/roof venting in place? I like the imagery with the 'masochist' comment. :) so true!

Knee wall issues are a plenty. The six sided box is the goal! I agree with Ed that not all six sided boxes (interior/exterior walls) are sealed. If they were, I don't think we would be having these discussions. Spray foam seems to be a superior product in many ways, but its application/installation is not for the novice and needs to be thorough and properly executed.

Ron, I like the photos. The problem that I have seen with FSK tape though, it doesn't always stay stuck and is hard to get a super tight seal. Maybe smearing a small layer of mastic over the top will insure it is sealed and stays that way. Looks good though.

For the radiant barrier experiment, Jon, I thought many of them, or at least the good ones, are perforated to allow vapor diffusion. I have never seen an incident like this, but I have heard they condensate on the interior side much like the Poly behind the drywall does down here in the south, so the perforations allow for the vapor drive.

I haven't seen one of their applications yet, like I said its an experiment, mainly to deal with the issue of getting materials into the work area. I personally have not worked with the radiant barrier bubble wraps so I am not sure about their permeability. Have you ever work with this material?

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Permalink Reply by Jamie Kaye on December 6, 2010 at 8:33am

I have used radiant barrier style duct wrap for insulating ductwork, aka bubble wrap, but have not put in or inspected radiant barriers yet. I have seen them installed as part of the roof decking (the integrated kind where the radiant barrier is fused to the OSB), but not as a retrofit. I have heard from some that they have had great results, but each of these comments have come from people that are selling them and the results are the experiences from their houses....kinda makes me wonder.

I know there is that trouble though with some of them not being permeable and have been told to use the 'breathable' kind if I do install/recommend them. Around here though, the price seems to be about $1.20 sq.ft. for radiant barriers. So for a comparison against open cell foam where it is about $1.50/sq.ft., it seems like there are so many more advantages to chose the latter (ducts in the envelope, grade 1 insulation, air seal, more semi-conditioned space, etc...)

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Permalink Reply by Ed Voytovich on December 6, 2010 at 9:48am

Not dense pack against the underside of the roof. Foam. Imagine a wall this angle: |. Do you ventilate it? Now imagine this angle: /. Do you ventilate that? How about 45°? 62.5°? Where do we draw the line?

The answer is very simple: The envelope is the envelope regardless of the angle.

The only reason to put vents in there is to satisfy a building inspector whose understanding is not quite up to date.

The old argument was that omitting ventilation would invalidate the shingle warranty. At least two shingle manufacturers have now removed that stipulation from their warranties. I submit moreover that the number of people who actually got a significant pay-back from a manufacturer for something other than an actual manufacturing defect is a whole integer way less than we imagine.





Jon LaMonte said:

Jamie Kaye said:

Ed, are you saying spray dense pack against the roof sheathing? I have been under the impression that is not acceptable without baffles. How about once you have sealed the soffits, do you do the same for the ridge vents/roof venting in place? I like the imagery with the 'masochist' comment. :) so true!

Knee wall issues are a plenty. The six sided box is the goal! I agree with Ed that not all six sided boxes (interior/exterior walls) are sealed. If they were, I don't think we would be having these discussions. Spray foam seems to be a superior product in many ways, but its application/installation is not for the novice and needs to be thorough and properly executed.

Ron, I like the photos. The problem that I have seen with FSK tape though, it doesn't always stay stuck and is hard to get a super tight seal. Maybe smearing a small layer of mastic over the top will insure it is sealed and stays that way. Looks good though.

For the radiant barrier experiment, Jon, I thought many of them, or at least the good ones, are perforated to allow vapor diffusion. I have never seen an incident like this, but I have heard they condensate on the interior side much like the Poly behind the drywall does down here in the south, so the perforations allow for the vapor drive.

I haven't seen one of their applications yet, like I said its an experiment, mainly to deal with the issue of getting materials into the work area. I personally have not worked with the radiant barrier bubble wraps so I am not sure about their permeability. Have you ever work with this material?

• ▶ Reply

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