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Attic catwalk with minimal thermal bridging

I'm planning to add 12" of Rockwool batts over 2x4 ceiling joists (there's already cellulose between the joists), and looking for some creative ideas for building a catwalk with minimal thermal bridging. The usual design involves a lot of 2x4s sitting on top of the joists and sticking up to support the walkway, creating numerous thermal bridges through the ceiling.

I'm wondering if the 12" of Rockwool batts would be enough to spread the load of 5/8" plywood simply laid over it (plus a person walking) without crushing the batts. There is also an interior partition wall that runs directly under the center of where the walkway would be. If not, perhaps Foamglas instead of Rockwool under the plywood?

Thanks.

Asked by Adam Liberman
Posted Sep 19, 2015 12:49 PM ET

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10 Answers

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1.

Could you run joists from roof rafter to roof rafter? These would be above your new insulation so no worry with thermal break. I've done this in Cape style houses in Maine. If the roof rafters are too far apart, perhaps you could drop verticals off the rafters and suspend the new joists off of these. When I used the first recommendation I put the joists ( 2x4's) at 4' o.c. and used 2x12's as the crawl surface. The 2x4's flex but won't break- at least over the 6' span that I had. It's nice to be able to move through the attic after it's insulated. It will be appreciated in the future. Good luck with your project.

Answered by KEVIN ZORSKI
Posted Sep 19, 2015 1:59 PM ET

2.

Thanks for your suggestions. I though of the verticals from the roof, but it is a 115 yo house with 2x4 roof braced with vertical planks. I don't know that I'd want to add that load on the roof. Another thought I had was to do the conventional method (5/8" plywood with 2x4 runners), but support it only every 4' or so, like you say. It seems like support every 16" is more than is necessary.

Answered by Adam Liberman
Posted Sep 19, 2015 2:30 PM ET

3.

If you run the supports as collar ties you don't really have to worry about the load - the ties will keep the rafters from pulling apart. Also, the load is trivial. Like Adam, we frequently do something similar when we're trying to get above insulation.

And why are you adding Roxul instead of just adding more cellulose? The latter would be cheaper and easier.

Answered by Dan Kolbert
Posted Sep 19, 2015 4:04 PM ET

4.

Adam, I wouldn't worry so much about thermal bridging from the relatively few pieces of catwalk supports you'd need to support a walkway intended for just one person's weight. Wood has an R value of 1 to 1.2/inch, so that 12" of wood still has R-12 to 14 of insulation value, with a cross-sectional area that will be a small fraction of the attic floor area. You wouldn't take much of a hit. I built a similar set of catwalks above the R-60 attic floor of my house, and the best numbers I've backed out of energy costs for a heating season give me a total envelope heat loss rate even lower than my spreadsheet model predicted, without any such thermal bridging included. I do agree with Dan, that just dumping more cellulose over what's there likely will be cheaper and easier than Roxul

Answered by Dick Russell
Posted Sep 19, 2015 8:01 PM ET

5.

I'm thinking the Roxul for a few reasons:
1) Makes a fire barrier between the attic and the house even with loss of the ceiling
2) I need to use at least some around the fire sprinklers we installed to keep insulation out of them
3) If I want to alter or add any wiring in the future, I presume it would be easier to pull out batts or cut through it, than to plow aside 12" of loose fill
4) The stiff batts will transfer at least some of the load to the joists they are resting on, lessening the load on the plaster ceiling.

Answered by Adam Liberman
Posted Sep 19, 2015 8:31 PM ET

6.

I think it is easier to wade through cellulose than it is to move batts and then move them back. You can just rake the cellulose back in place, but replacing batts that have been moved around is pretty difficult. Also Roxul batts don't hold together very well when they are moved. They break into pieces.

I love Roxul, but for the attic in this situation, I think cellulose is the way to go. I am also installing 2' x 8' pieces of plywood so I can get around my attic in the future, then I'll blow in 10-12" of cellulose. I've got to mark where light fixtures, electrical stuff is with flags of some sort before I do that.

Answered by Lucy Foxworth
Posted Sep 19, 2015 10:05 PM ET
Edited Sep 20, 2015 8:28 PM ET.

7.

Someone once confessed how much more work it was to lift all these bags of Roxul in the attic and that it is way easier and efficient to use a blowing machine to do the work. Being sponsored by Roxul was a good reason to do it but I doubt it's your case.

Answered by Marc Labrie
Posted Sep 19, 2015 11:18 PM ET

8.

While 115 year old houses are one of the reasons we have building codes, the weight of a intermittent use catwalk is insignificant to the loading of the roof, and as mentioned, if designed properly, may help support it.

Answered by Keith Gustafson
Posted Sep 20, 2015 8:46 AM ET

9.

The thermal bridging from a catwalk is minimal, only a consideration in truly high-performance houses. A good reason to raise the catwalk, though, is to make it easier to install the insulation continuously and to facilitate future work at the ceiling plane. As Dan said, the loading on even undersized rafters isn't an issue.

Answered by Michael Maines
Posted Sep 20, 2015 9:05 AM ET

10.

Thanks all for your suggestions! I won't worry about the thermal bridging, and I'll ponder the insulation, and probably go with cellulose after doing the necessary blocking around the sprinklers with the Roxul.

Answered by Adam Liberman
Posted Sep 20, 2015 4:19 PM ET

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