Bill's Larsen Truss Retrofit

 
Bill did a very nice Larsen Truss retrofit to the 2nd story of his house.  This allowed him to greatly increase the insulation level for the 2nd floor. 
 
The new 16 inch thick Larsen Truss walls with no thermal bridging and filled with cellulose probably provide about R60 + the insulation in the existing wall!
 
While the Larsen Truss has been used more in new construction than on retrofits, it was actually invented for retrofits, and as this project shows, this can work very well.
 
More on other Larsen Trusses projects here ...
 
Thanks very much to Bill for providing this material.
 
 

Bill's Description of his Larsen Truss Project

Before

 


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The front, back, and side views of the house before the Larsen Truss was added to the 2nd floor.

 

 

During Construction

The entire insulation cavity, along with the triangular "skirt" whatever it is really called, were filled with dense packed cellulose, 4 lb/ cu. ft.  The insulation cavities are 16" thick.  Why 16"?  Because the framing and sheathing materials and labor would be about the same as they would have been for another 8".  The cost of the additional cellulose material wasn't that much, so why NOT 16"?

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This shows the structural arrangement for the
area where the Larsen Truss was added.

 

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The center picture shows the wall after the old roofing was removed. The area without the roofing felt shows the existing structural plywood sheathing.  What is important about this picture is that the structural sheathing was left intact, which I believe is necessary.  Because of that, the new truss system is not structural.  It only needs to carry the weight of the roofing plus the insulation.

I would have preferred plywood sheathing because of better moisture permeability, but the framing contractor already had the OSB delivered to the job and going up, so we went with what he had.  It is important to have some strength to this sheathing layer to provide integrity to the trusses during a strong wind.

 

 

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 In a few spots that it was not practical to put in the Larsen trusses, such as between the small windows on the back, we added 3-1/2" of extruded foam.
 

You can see that the windows have been replaced, as well The old leaky casements were replaced with Fibertec triple glazed with 2 low-e panes.  I think they are R-5 or R-6 windows, about double the insulating value I could have had with the more common window manufacturers.

 

The Finished Project

This is the completed job with the Larsen Truss and the new windows.

 

The upstairs seems much more comfortable now.  We feel absolutely no drafts around the windows, and the walls do not exhibit any cold wall radiation effect. I brought a thermal imager home from work, and was still able to see some cool spots in the corners and where the wall meets the ceiling.  There is not a lot of space for insulation in the attic near the outside walls.  We plan to have those areas spray foamed right up to the ventilation chutes, the other leaky areas in the attic sealed with foam, and a heavy layer of cellulose blown in later this winter.  That should help some more.
 

Looking at the heating bill that came yesterday, it was about half of what it was for the same month last year. I'll have to check the heating degree days, but I'll bet this year has been significantly colder than last year.  I think insulating that triangular section sealed off quite a bit of air leakage into the joist space, and that is why the savings are more than I would have expected.  The other thing that contributed to the reduced heating costs were converting the existing wood fireplace to an 84% efficient gas fireplace with its own fresh air intake stopped a lot of stack loss from the house.

 

Future insulation projects to finish this job are:

-additional attic insulation and sealing, as well as basement rim joist foaming.
-Excavation around the uninsulated block walls, and at least 6" of extruded foam at least 4 ft down.
-Larsen truss insulation on the lower floors.
-New windows downstairs.

Bill

 

 

 

 

Gary January 11, 2009