Workshop and storage addition issues
JackPart
| Posted in Green Building Techniques on
I’m planning a small addition, 10 foot deep by 16 along the back wall of our house. Shed roof, about 4/12 pitch. There are several issues I’d like guidance on.
The rear of our house is at the bottom of a hillside, so there is sometimes a lot of rain and melt runoff on the ground. But there appears to be good drainage around the house, and a lot of sand in the flat strip where the addition will be. The half height basement is solid concrete and completely dry.
In order to be on the same floor level as the house I will have perhaps a foot of space under the 10 foot 2×8 floor joists, not enough to do any work under.
I’m on a limited, fixed budget, so economizing is important. One result of that is that I’d like to use cellulose insulation as much as possible.
I heat with wood, and my stove is good, but a little small during really cold times. So don’t want this room to put much additional load on the system. There will be one forced air duct to the addition. I plan to keep it quite cool except when I’m using it.
I’m in climate zone 6, so I’m shooting for R-49 in the roof, R-38 in the walls, and R-30 in the floor. I’m using R-5.5 for polyiso board and R-3.5 for dense pack cellulose.
So, first specific issue, the deck. I’m planning to screw 3″ T-strips (5/4 x 6 decking sliced in half) to the bottom of the 10 ft PT floor joists before dropping them into the joist hangers. I’m planning to use 2″ polyiso strips for the bottom layer between the joists, resting on the T-strips, then fill the cavities with cellulose, rolling it tight. This should give my my R-30. Do I need to worry about insects? And do I need to foam or caulk those strips for air tightness, or would it be better to use a layer of polyethylene under the sub-floor for air tightness and assume outward drying?
For the walls I plan to use a 10″ double wall and dense pack cellulose. 2x4s, 24″oc, staggered where possible. 1/2″ plywood sheathing with Tyvek and furring strips for as close as I can get to the plastic siding on the rest of the house. One external door, one door to the house, one window. Seems pretty straightforward.
For the sloped “cathedral” roof, after a lot of reading I had decided on external polyiso and cellulose between the 2×8 rafters. The 71/2 inches of the 2×8 rafters give R-26 at R-3.5 per inch for the cellulose. If I use 5″ of polyiso I’d have R-27 outside of the sheathing. However, since the issue of moisture and the sheathing really relates to the absolute temperature of the sheathing, I’m worried that my keeping the addition 10 or 20 degrees cooler than the rest of the house means I need a lot more of the insulation outside the sheathing. That bothers me, as to both cost and the fastening and trimming issues with a very thick stack of foam boards.
My other option would seem to be a vented roof, insulated mostly with cellulose. Perhaps using 2″ x 22 1/2″ polyiso strips as baffles below 1 1/2″ foam spacers for the air channels. That would leave me needing 49 – 11 = R-38, or 11 inches of dense pack cellulose. I would build an inner set of 2×4 rafters for this, with 3 1/2 inches between inner and outer rafters. I think I should not stagger these, in order to help with the settling issue. But I wonder about my ability to get a real dense pack and good contact with the polyiso. I haven’t yet found where to get a rental cellulose blower with the power needed. From what I’ve read I shouldn’t rely on the box store machines for the roof blow. Although the Eric Habagger posts suggest that with patience I might just be able to do this small room with a Cyclone blower(?) The other big issue is how to vent the upper ends of the air channels on a shed roof against a wall? Is there a practical way? Looks are not an issue, as hardly anyone will ever see it. Is the upper venting needed to avoid moisture issues in the sheathing?
So which way to go?
A related question. I’m planning to use 1/4 inch plywood on the inside of the walls and ceiling, as I’ll be doing most of the work without help. Any guidance on what thickness I need to hold dense pack cellulose with 24 inch spacing?
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Replies
Hi Jack,
As far as the floor system goes, I would suggest that you use canned spray foam to seam the rigid foam bottom layer, not just to keep insects out, but to air seal the assembly.
When it comes to the roof assembly, my first thought is that the fact that you plan to keep the space on the cooler side won't be an issue because the cooler air will by dryer air which will be less likely to cause moisture issues in you assembly, but I'm not confident in that answer, so I look forward to some other folks thoughts on this one.
That said, air sealing is your friend. So regardless of how you end up deciding to details the roof, make an air sealing plan before you start building so that your walls and ceiling are not leaking conditioned air into the roof assembly.
Thanks Brian. Interesting point about cooler, drier air. And your good tip about air sealing got me thinking that it's especially important for me. The furnace blower is on continuously in the winter to circulate the wood stove heat. So I probably have a small positive pressure in the above basement spaces. I guess my new room will be a lot tighter than the rest of the house. I can tell by the irregular snow on the roof that the builder was pretty sloppy with the batt insulation.
After thinking more about it... Would you still use a layer of polyethylene under the subfloor? And if so, I'm wondering why there would be any significant air flow in and out of the bottom of the assembly, other than atmospheric pressure changes? And with the foil faced bottoms, wouldn't you almost want that small leakage for drying outward?
You didn't really say what the foundation system would be for the addition, but assuming concrete, and the proximity to moist ground, I'd consider poly sheeting over the ground and the walls sealed at the top plate. I'd insulate the ground and walls and dry to the inside. My preference would be to keep the air and moisture from condensing on the cold flooring structure.
You're right, I should have specified. The foundation is a 16 foot ledger board on the house wall and a beam of tripple 2x8s on cement posts, with the 10 foot 2x8 joists on joist hangers. No enclosure below the deck, so free air flow underneath it. That's why I'm thinking dry to the outside.
I'm also wondering if the borate in the cellulose won't discourage any insects that might squeeze in past the foil facing of the foam boards.
Air, mosture, insects! I hate to give up any vertical insulation space, but you've now got me thinking about 3/8 plywood bottoms, caulked in for air seal but able to breath moisture. Then the polyiso with no foil.
A couple of small items. Unless the bottom of your addition is in direct ground contact, there is no need for PT floor joists, go with regular lumber or I joists. I like TJIs as they are lighter to handle and they have much less thermal bridging than 2x lumber.
Overall, it is never a good idea to leave foam unprotected on the outside of the house. It makes a good nesting bugs and critters. Get a layer of plywood/osb over it and cover the edges with flashing.
For the roof, going with 11 7/8 TJI with the vent baffle stapled to the underside of the flange. This gives you roughly 10 1/2" of space, with 1" polyiso vent baffle the rest dense packed you end up with an R42 whole roof R value, which is pretty decent and much simpler than the exterior rigid. For venting you can use above shingle vents near the wall (search for roof to wall vent).
For your interior, you still need a decent air and vapor barrier, painted 1/4 plywood with the seams taped will do it but will be ugly.
You can go with 8 mil poly and carefully tape and seal all the seams/edges/outlets, or it might be simpler to drywall. You can install the plywood over the drywall if you prefer that as the interior finish.
Everyone seems to have trouble building a floor system on piers that is quite close to the ground. All those parts - sheathing , joists, insulation, vapor barrier. Then how do you finish the underside?
If you're putting in concrete piers with wood beams on top why not use a SIP panel for the floor? It's structure, insulation and vapor barrier all in one. You could even attach a weather barrier and the finish layer to the underside of the panels before you lay them on top of the beams.
Build the rest with 2x materials (or maybe just the walls) but a SIP floor wouldn't be too expensive and would solve a lot of problems easily.
Jack,
You may find this discussion helpful. https://www.greenbuildingadvisor.com/article/insulate-raised-floor
Thanks to everyone for your comments. My thinking is definitely evolving on many issues. I'm still hoping for more input on the decision about external foam or all cellulose for the roof. But I'm most concerned now about the floor.
There's no getting around that the ground, from 12 to 16 inches below my beam and joists will be wet or damp for extended periods after every rain or snow melt. The outside of the addition is so close to a hillside that, short of building a retaining wall, there is no option for a swale. The concrete half basement of the house is surrounded by sand and a perforated drain, which I can see working on the far side of the house and lawn where it all slopes away from my addition side.
So I think that the beam, ledger board, and joists should all be PT, and I need a strong vapor barrier to the outside and virtually all drying of the floor assembly to the inside. So I'm now thinking 1/2 inch Advantech bottoms for the joist bays, with a bead of caulk all around before dropping them in, and caulking the one crosswise seam per bay. Is there a better material than Advantech to keep that ground moisture out and provide critter protection? Someone mentioned PT OSB. Would that provide a strong enough vapor barrier? How about nailing in the wood bottoms, then applying the bead of caulk all around before dropping in the polyiso with the foil side down? And I can caulk the crosswise seam before dropping in the second piece of poly for each bay. Why would that not keep out most or all of the ground moisture?
From reading the article you suggested, Malcolm, I like the idea of using mineral wool in the floor. For one thing, I will have to travel an hour to rent an insulation blower. The mineral wool will allow me to insulate the floor, get the addition framed in for the winter, and have only one blower rental for the walls and roof. I think I still need the cut and cobble 2"polyiso to get to a decent R value, in additon to the foil vapor barrier. With my floor level fixed by the floor of the adjacent room I don't have room for a layer of foam over the joists. But for an occasional use cool shop and storage I can live with the bridging. The only argument I can think of against the mineral wool might be that the cellulose would be better at absorbing or distributing any condensation or other moisture that might get in. What do people think about this?