[Greenbuilding] rigid foam inside Larsen Truss?

Thu Jan 4 16:05:09 CST 2018

If you are up for saying why -- I love to learn! 
 Beatrice Dohrn 

      From: Gordon West <gordon.west at rtnewmexico.com>
 To: greenbuilding at lists.bioenergylists.org 
 Sent: Thursday, January 4, 2018 12:43 PM
 Subject: Re: [Greenbuilding] rigid foam inside Larsen Truss?

Forget the foam, stick with cellulose.
Gordon 

On Jan 4, 2018, at 12:00 PM, greenbuilding-request at lists.bioenergylists.org wrote:
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Today's Topics:

   1. rigid foam inside Larsen Truss? (Reuben Deumling)
   2. Re: rigid foam inside Larsen Truss? (David Wentling)
   3. Re: rigid foam inside Larsen Truss? (RWT)

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Message: 1
Date: Thu, 4 Jan 2018 08:32:58 -0800
From: Reuben Deumling <9watts at gmail.com>
To: Greenbuilding <greenbuilding at lists.bioenergylists.org>
Subject: [Greenbuilding] rigid foam inside Larsen Truss?
Message-ID:
 <CAE5fceAvHZHxN4uShFNPCysJcYD3R2mD8tVaxz_EBk0na+JNVQ at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

I have a line on 3-1/2" rigid foam panels for pretty cheap. My 1894 house
has (of course) 2x4 walls into which I've blown dense pack cellulose. For
stage 2, I was going to add a Larsen truss (another 2x4 wall with a big
cavity between inner and outer walls) and blow cellulose into the rest for
an eventual 11" thick wall. But with these panels showing up I'm wondering
if there are reasons not to place these as the middle layer of a cellulose
sandwich? Seems like a quick and in this instance cheap way to get a lot of
'r'. But I also don't want to do anything stupid when it comes to moisture.

Thoughts?
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Message: 2
Date: Thu, 4 Jan 2018 09:54:59 -0700
From: David Wentling <dpwentling at ymail.com>
To: "greenbuilding bioenergylist.org"
 <greenbuilding at lists.bioenergylists.org>
Subject: Re: [Greenbuilding] rigid foam inside Larsen Truss?
Message-ID: <0C7356AE-A21F-45E3-8008-5B0EE1CCD6AA at ymail.com>
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Historically, we have used the ratio of 1/3 R-value outside of any vapor retarder.  Depending on the vapor characteristics of your foam, the amount of insulation R-value you add, and the framing and sheathing present, will determine if there would be a potential problem in the future. 

There are simple vapor models you can feed wall characteristics into and they estimate where condensation may occur.

You are right to be concern. Perhaps modeling may show just install the foam board on the exterior and apply siding over, skipping the cost of the Larsen truss. Or not.

You do to know what you can not measure.

David Wentling

On Jan 4, 2018, at 9:32 AM, Reuben Deumling <9watts at gmail.com> wrote:

I have a line on 3-1/2" rigid foam panels for pretty cheap. My 1894 house has (of course) 2x4 walls into which I've blown dense pack cellulose. For stage 2, I was going to add a Larsen truss (another 2x4 wall with a big cavity between inner and outer walls) and blow cellulose into the rest for an eventual 11" thick wall. But with these panels showing up I'm wondering if there are reasons not to place these as the middle layer of a cellulose sandwich? Seems like a quick and in this instance cheap way to get a lot of 'r'. But I also don't want to do anything stupid when it comes to moisture. 

Thoughts?
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Message: 3
Date: Thu, 4 Jan 2018 17:34:27 +0000 (UTC)
From: RWT <archilogic at yahoo.ca>
To: Green Building <greenbuilding at lists.bioenergylists.org>
Subject: Re: [Greenbuilding] rigid foam inside Larsen Truss?
Message-ID: <1842894756.428117.1515087267398 at mail.yahoo.com>
Content-Type: text/plain; charset="utf-8"

 Reuben;
In the Olde Days before sim programs existed (ie before Win-Doze)? if one didn't want to bother graphically determining the dew point for a double stud wall assembly, the rule of thumb was to place the thing behaving as a vapour retarder/air barrier membrane within the first 1/3 (from inside -> out) of the total R-value of the wall assembly, in primarily heating climates like mine (Ottawa Ontario Canada).?
?For? 11" wide double-stud wall that used the same insulation in the three layers (ie 2x4 interior? bearing wall + 3.5" cavity + 2x4 exterior curtain wall) this meant that the VDR/AB membrane could be placed on the outside of the bearing wall (usually backed up by structural sheathing) and the outside 2/3 of the insulation placed in the walls before the walls were tilted up? ... meaning that the walls were very well insulated and air-sealed before the interior was sheathed (same procedure for ceiling) so that the interior was toasty-warm in the middle of winter before any HVAC was installed.??
?It also meant that services could be installed in the bearing wall cavity without having concerns (nightmares) about sub-trades compromising the integrity of the VDR/AB membrane.
    On Thursday, January 4, 2018, 11:34:48 AM EST, Reuben Deumling <9watts at gmail.com> wrote:  

 I have a line on 3-1/2" rigid foam panels for pretty cheap. My 1894 house has (of course) 2x4 walls into which I've blown dense pack cellulose. For stage 2, I was going to add a Larsen truss (another 2x4 wall with a big cavity between inner and outer walls) and blow cellulose into the rest for an eventual 11" thick wall. But with these panels showing up I'm wondering if there are reasons not to place these as the middle layer of a cellulose sandwich? Seems like a quick and in this instance cheap way to get a lot of 'r'. But I also don't want to do anything stupid when it comes to moisture. 

Thoughts?
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