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Blowing dense-packed cellulose into Larsen truss walls and open rafter bays

This is what is keeping me up at nights:

Larsen trusses have no stud cavity walls and in my building the trusses are also open to the rafter bays (i.e no top plates).

I assume it’s the resistance to the blow by the walls and plates of the cavity which enable the density. I will be blowing cellulose into spaces with no boundaries so how to do I get the density and how do I know I have reached it?

Is it all about feel (i.e. resistence to the hose) and sound (i.e. back pressure while blowing)?

Asked by Oak Orchard
Posted Mon, 01/21/2013 - 12:27
Edited Mon, 01/21/2013 - 13:28

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5 Answers

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1.
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Oak Orchard,
You might find this link interesting.
It's not quite a true Larsen truss but there are some photos and Robert answers some questions about dense packing in his open truss design.

I believe the Larsen trusses of Thorsten's Chlupp's Sunrise Home were simply loose filled, but with the top of the truss cavity open to the attic and extra loose fill blown over the truss cavities.
I gather the idea is that as the cellulose in the truss cavities settle, the extra attic insulation should settle down into the wall to prevent a void at the top of the cavity.

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Answered by Lucas Durand - 7A
Posted Mon, 01/21/2013 - 13:09
Edited Mon, 01/21/2013 - 13:10.

2.
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Oak Orchard,
As my article on Larsen trusses pointed out, it can be difficult to install cellulose insulation in Larsen trusses unless the truss bays are broken up into small compartments.

In that article, I wrote:

"Another possible disadvantage [of Larsen trusses]: if you decide to fill your Larsen trusses with blown-in insulation, it may be hard to find an installer with enough experience to do the job well. “In 2008, I was able to examine the entire house with a borrowed infrared camera,” Belknap wrote. “Doing so pointed up many places where the insulation had settled (or never got) and there were now gaps in the insulation coverage. This required reapplying insulation (blown in from the inside).” When I talked to Belknap about the issue, he said, “The problems I had with the insulation were attributable to the fact that the cellulose would drift from one bay to the next, and some of the bays would never get filled.”

"Experienced installers of dense-packed insulation have developed techniques to fill deep walls, including Larsen-truss walls and double-stud walls. However, depending on the skills of the insulation installer may be risky; some architects and builders prefer to install air-permeable netting on each Larsen truss to separate the stud bays (see photo below), a technique that facilitates insulation installation."

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/21/2013 - 14:20

3.
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I would check w/ Thorsten on whether or not his walls are loose filled; I believe they are dense packed. I spoke w/ a couple of his installers a few years ago, and the one gent said he walks around in the walls to pack them if they feel it is necessary; I don't think he was kidding. Also, Robert Riversong says he dense packs his Riversong trusses, and uses two or three hoses in the wall at once, leap frogging from one to the other. When one area gets "full", he jumps to the next hose, then the next, then comes back to the first when the first "layer" is in. He said that saves a lot of time pulling hoses up and down.

Answered by John Klingel
Posted Tue, 01/22/2013 - 03:10

4.
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John Klingel,
Your comment prompted me to look back at the ARCTIC detail Thorsten sent me.
The detail does indeed specify dense packed cellulose in the truss cavities.

Interestingly however, the detail also specifies that extra insulation be blown in over the truss cavities "in case of settlement of wall cavity insulation"...
Not sure why he would be worried about settlement in a dense packed wall...
Although, from what I gather, Sunrise home was pretty experimental so maybe he was just covering his bases.

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Answered by Lucas Durand - 7A
Posted Tue, 01/22/2013 - 10:28

5.
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I read Martins article but in my case electrical wiring and my supply/return pex for my wall hung Stelrad radiators are run through the walls. This makes it impossible to attach insulweb to form boxed-in cavities, truss-by-truss. I also read the various articles and posts by Mr. Riversong.

The reported R value on cellulose seems inconsistent; but it appears that loose fill and dense packed cell are not too far apart: loss being 3.5 – dense being 4.0. Am I then to understand that the dense packing is not done for extra R value but for prevention of loss of coverage due to settling?

Given your replies, you are increasing my motivation to do it myself if I can get the right machine. At least I can take the time to be extra careful to ensure the coverage is complete. My concern is that I do not dislodge or damage any mechanicals or electricals during the process.

I can work the tube in every cavity and snake it around until I get the resistance/back pressure. I understand it will take some learning time.

It sounds like I have to do two or more passes per bay; first flowing in the cell to back fill the cavities up to the insert hole and then moving on to the next truss bay until I have a substantial quantity of cell in the lower section of the perimeter walls and cross-truss bay spillage stops.. Then return and do this again to fill above the insert and working up to the top. During the second pass I may be able to get the resistance and therefore density I need by working the tube in all directions from each insert hole. I can even use a second insert hole at the top of the Larsen truss bays if necessary (i.e. since I will get a lot of settling and cross flow that will keep the pressure from building)

Based on what you all say, at least for most rafter bays I can create a bottom end barrier to the bay by adding insulweb at the top of the truss cavities where a plate would be in conventional framing.

Answered by Oak Orchard
Posted Tue, 01/22/2013 - 11:10

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