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Piggyback Attic Joists for Insulation Space

Posted by kevin1900 (My Page) on
Mon, Sep 27, 10 at 11:56

Our American Foursquare of 1904 has six-inch ceiling joists on the second floor, comprising an attic floor. There is no flooring there now. There is cellulose insulation to the joist tops in some spots, some bare areas showing in other spots.

I would like to add insulation to at least 12 inches depth, and maintain the ability to move around by piggybacking 2X6 joists on the existing, with OSB or plywood on top of that.

Is this a accepted practice, and is there a common process? May I simply glue and toenail the new joists atop the old, or should they be perpendicular?

Any other thoughts?

Thanks.


Follow-Up Postings:

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RE: Piggyback Attic Joists for Insulation Space

It depends on the structural conditions. How far do the attic joists span, what is the spacing, and will there be storage in the attic?


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RE: Piggyback Attic Joists for Insulation Space

Yes, it is common in older homes. You can put the 2x6 perpendicular to the existing framing and make your installation much easier.

However, if the house wasn't designed to use the attic as living space, the joist won't be designed to hold significant loads. If you are just going to toss some christmas decorations up there, I'm sure you'll be fine. If you plan to be up their frequently or store anything of significant weigh, then you will want to consult a professional to determine what will need to be done to make it livable space.


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RE: Piggyback Attic Joists for Insulation Space

I did just that in my house - ran them parallel, right on top of the existing joists, toe-nailed. 3/4" plywood over the top for storage. I removed the old cellulose and put in new batt insulation. Very stable. You could add some kind of cross bracing if the toenailing isn't strong enough for you.


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RE: Piggyback Attic Joists for Insulation Space

I would recommend the perpendicular route - for the reason bill1 suggests plus the fact that it will cut down on thermal transmission through the joists. In fact I would suggest laying down a piece of 3/4 styrofoam to separate the old from the new wherever they touch. Other than the need for longer nails and a small square (say 3"x3") of styrofoam at every joint, this will virtually eliminate thermal bridging for almost no cost.

Regarding strength of the floor, I think you will find a noticeable increase in stiffness, even with the perpendicular joists. This would be due to the plywood plus the original joists actually creating sort of a beam effect through their separation by the new 2x6's. Not enough to make it a living space, but it will seem more solid under foot.

I also agree with bill g web - at leas some cross bracing or blocking might be needed to keep the new joists securely upright.

I assume you are going to fill up the new joist elevation with more cellulose loose fill. Watch you don't inadvertently block off any eave vents. Create baffles with styrofoam to protect them if necessary.


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RE: Piggyback Attic Joists for Insulation Space

Because of the very small area of physical contact the degree of thermal bridging at cross members would be insignificant.


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RE: Piggyback Attic Joists for Insulation Space

I think I did what you are talking about in my old house. I installed a small (about 18" wide) catwalk to walk around in the attic without disturbing the insulation. It's been awhile, but I think I used some 2 x 10's or 12's screwed affixed to the truss be construction adhesive and plywood. The good think about running them parallel to the joists is that you can easily put insulation underneath.

I got the idea from a This Old House magazine issue. They have a description on their website.

Here is a link that might be useful: TOH Attic Catwalk.


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RE: Piggyback Attic Joists for Insulation Space

New owner of a 1922 foursquare Sears kit house. I'm in the process of deepening the attic joists for insulation also. Just replaced the roof and tossed out what scanty (old, sodden) insulation there was between the 2x4 joists. I plan to attach new 2x4's in parallel on top of the existing and screw on metal tie plates every 2 feet on each side of the joist (staggered), then roll out R25 and cover all with plywood for a storage surface. Putting bracing between the joists would be a problem with the insulation. I'm hoping the tie plates will hold the top 2x4 in place, but hadn't thought of gluing. Good idea - that construction adhesive is good stuff!


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RE: Piggyback Attic Joists for Insulation Space

It is usually best to start a new thread (and perhaps link to the original thread) so no one reads the entire thread only to find a new question.

The issue I would be concerned about is the strength of the 2x4 ceiling joists. They would nave been originally sized to support only the ceiling so it is unlikely that they are adequate to support even an attic storage load without excessive deflection.

The 2003 IRC limits the span of 2x4 SPF #2 ceiling joists supporting a 20 psf uninhabitable attic storage live load to 8'-7". Adding 2x4's on top will increase the dead load and add some resistance to deflection but it would be difficult to calculate how much. Construction adhesive might hold the two members together but it is difficult to know if it would provide adequate horizontal sheer strength near the supports so 5x7 tie plates might be a better idea (Simpson TPA57).


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