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Attic Storage

Drake Oswald | Posted in General Questions on

Hi All,

Thank you in advance for your help and expertise. I’m cleaning out and re-insulating a formerly rat-infested attic. I’ve air sealed most penetrations, seams, and top-plates. Now I’m making a storage platform – just 8×10. Due to the former rat problem, I’d like to use polyiso panels between joists with plywood running perpendicular and seal all gaps (if the rats ever get back in, my thinking is to minimize the “dens” I create for them).

Here are my questions:
Can I use panels with foil on both side and layer them up to the height of the joist?

Can I use expanding foam to seal the edges and seams of the polyiso?

Can I use polyiso to make ‘boxes’ to go over some recessed lights that are not IC rated?

Thanks again,
San Diego

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  1. Andrew C | | #1

    Several thoughts –

    Good for you to air seal your attic. Seems to be the biggest bang for the buck in many cases.

    Can you fit the required insulation (e.g., R49) underneath the platform, below the plywood?

    If you want to keep the rodents out and you build your platform as proposed, I’d use a wood perimeter so they can’t get directly at the insulation. I’d probably seal that instead of the foam seams and edges.

    Most attic critters seem to enter where there are gaps in the trim, like at the roof/wall/soffit intersection if you have a dormer-type feature on your roof. Use hardware cloth, roll it and bunch it up and pressfit it into place, and then use a UV-resistant black foam to seal it all up. In my area, it’s red squirrels that seem to cause the most problems, although birds will use the same holes that have previously been left open (either by the original carpenter or the red squirrels).

    I’m uncomfortable with foam boxes above recessed lights. I’ve used the mineral wool recessed light covers from Tenmat instead. It’s much harder to burn or melt rocks.

    The bigger ask is, do you REALLY need the storage space? Can you possibly simplify/reduce/store your inventory at the store? I’m terrible at this myself, and I’m busy looking after parents with the same packrat habits. Almost everyone’s life can be improved by jettisoning more flotsam. I know it’s hard and may not feasible, but it’s the best solution to your storage problem. By far.

  2. Drake Oswald | | #2


    Thanks for your replay and wisdom. I’m researching the mineral wool from tenmat now.

    I won’t be at the required r-value, but I’ve been with poor/no insulation for a few years. I figure an 8x10 platform is a compromise I can always go back on later and remove the ply and add more insulation - just don’t tell the wife I said so ;). The items are small antique furniture, so I can let the clutter-anxiety go for now.

    I like the idea for wood around the perimeter.

    Follow up question - Do the seams inside need any kind of seal/tape?
    Is the foil okay on both sides of the polyiso?

    Thank you!

  3. Andrew C | | #3

    RE sealing the rigid foam seams in this case -
    In this case, the foam isn't acting as an air barrier (that's probably your drywall ceiling), and it doesn't have to shed water like a WRB, and for this small area it probably doesn't matter what it's permeability is - it's just acting as insulation. I don't think there's any reason for using foil sided foam, especially if you end up with it inside a wood box. Someone with more building science cred may have different ideas regarding permeability, but that's my take, fwiw.

    Good luck,

  4. GBA Editor
    Brian Pontolilo | | #4

    Hi Drake,

    I'm with Andrew on taking steps to keep the critters out where they are getting into the attic instead of just trying to keep them out of your storage area. Also, I agree with him that with all of the air sealing that you have done, you won't need to tape the seams of the rigid insulation.

    And yes, you can make air-tight boxes for your recessed lights from rigid foam. Here's a good article from FHB on that topic:

    I am curious how you are insulating the rest of the attic floor and wonder if there is a way to continue that insulation under the storage platform. At just 8x10, it wouldn't be expensive or difficult to add to the height of the joists to make more space beneath them (framing lumber is a lot less expensive than polyiso). Without being able to see the situation though, it's hard to say. Perhaps you could share some photos.

  5. Expert Member
    Zephyr7 | | #5

    You're thinking about insulating, but have you thought about the structure? If you're going to be storing a fair amount of weight in that attic, you need to be sure the structure can handle it. Previous owners in my house stored too much in the attic over the garage which resulting in some deformation to the trusses which I now have to repair, and it's a complex project.

    If you have a stick built attic space, with real rafters, you're probably OK. If you have trusses, be careful, especially if they only have 2x4 chords.

    I agree with everyone else about the pests. I've had rats, bats, and squirrels. Make sure all your flashing is secure (not just against wind and rain, but against little critters too), and use hardware cloth to block entry through any ventilation openings. Make sure whatever barrier materials you use are chew proof, which typically means they have to be made of metal. Critters can be pretty determined when they're looking for a warm shelter in bad weather.


  6. Andrew C | | #6

    Truss loading -
    I was going to mention that. It's easy to add a lot of weight, a little bit at a time, without realizing the weight of the overall load. Plenty of people have permanently bent their car/truck/SUV suspension components by putting bags of (shingles/landscaping materials/whatever) in the back, one bag at a time. (Raises hand - but I was in high school at the time, if that's an excuse.)

  7. Drake Oswald | | #7

    Thank you Andrew, Brian, and Zephyr.

    I had considered the structure, but after hearing this I’m going to stencil the plywood: “Max Total Load: 350lbs.” That’s a made up number, but I have a wall running almost beneath the middle of the proposed platform. Adding height isn’t an option due to the overall height being too low.

    I’m going to do the polyiso for now, unfaced and with wood blocks on the ends that are sealed. I’ve already tightened the framing, flashing and roofing up to keep the critters out, but will stay vigilant.

    Thank you all again!

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