Moderating temperature swings in an unconditioned attic
I have a decent-size walk-in attic accessible through a weatherized door. The air barrier and 15 inches of cellulose insulation are installed in the cavity between the attic subfloor and the dropped ceiling below. Thus, the attic is outside the thermal envelope of the house.
We actively use the attic space for storage, but my wife dislikes the large temperature swings of the attic. In zone 5, the temperatures can range from well-below freezing to squelching hot. My wife worries about the impact on some of her more sensitive belongings.
I really don’t want to bring the attic into the conditioned space. That would mean moving the air barrier and insulation to the roof deck, either above or below. Aside from the expense and work involved, the total heat load for the house would go up substantially.
My question: Can modify the attic space to make it “moderately conditioned,” with temperature swings from say 50 to 80 degrees or so. I thought about sealing up the vents and installing fiberglass bats between the rafters and wall studs. The primary thermal and air barrier will remain at the attic floor.
Am I creating a dangerous situation?
I also thought about installing reclaimed polyiso insulation to the undersides of the rafters. This would allow the roof sheathing to continue to be vented.
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There is no such a thing as “moderately conditioned" attic; is either conditioned or not, and yes, its a bad idea the way you describe it.
IMO, you would need to install 8" of reclaimed polyiso under the rafters for an R49 min. in CZ5, and then would become a conditioned attic. As long as you make sure the rafter bays are ventilated, it should work. You need to tape and seal the polyiso and make sure the attic has an air supply (hence "conditioned").
In most of zone 5 with R50+ insulation between the attic and conditioned space there's no way to keep it from dropping below freezing in winter or going north of 110F degrees in summer. Installing perforated radiant barrier on the underside of the rafters can probably knock a few 10s of degrees off the peak summertime temperature and a few degrees off the peak winter low, but probably not as much as you'd like.
Was that "...50 to 80 degrees or so..." the attic temperature limits- a 30F range of temperatures from 50F to 80F in the attic, or the difference between the wintertime low and summertime high, say 20F in winter to 100F in summer, which is an 80 degree range?
Putting polyiso on the underside of the roof to achieve those swings could work, but it would take a LOT of polyiso. As long as there is at least 6" of polyiso the risk of creating condensation issues in the stored goods is low, but without venting it to the drier outdoor air it's an issue.
I'll modify the temperature range to a low of 40 degrees in mid-winter to 90-degrees in mid-summer. We just wanted to eliminate freezing temperatures and excessive heat.
What are you storing in the attic that can be damaged by high or low temperature? My garage attic is where we keep stuff we rarely use and the temperature gets well below zero F in winter and 100+ in summer. The skis, lobster cooking pot, suitcases, Christmas decorations, etc. don't seem to mind. If necessary, a conditioned storage unit would be cheaper than insulation.
Oh, things like candles and old photographs come to mind. The steam cleaner has water in it, which can damage the cleaner if it freezes. Since we don't have a full basement, this space is our general storage area. So, things like seldom used cleaning fluids may be stored, but we don't since it freezes in there.
Scan your old photos.
Become a minimalist or install shelving or downsize or all the above.
Ignoring code:
You can add some insulation to the roof deck and leave the attic vented - moderating temperature extremes somewhat. Ie, a vented attic with less exterior heat gain/loss than most.
You can add insulation to the roof deck, seal the attic and then actively condition it (temp and humidity) to anything you want, including less than your interior. Ie, a sealed conditioned attic with turned-down heating/cooling.
You could also turn just a portion of your attic into conditioned space.
I second this idea. Build a small room inside the attic, insulate and air seal as required, install a small heater. That addresses the freezing. The excess heat is another matter. Maybe you could run a duct and fan to the exterior. That would make a big difference.
The heater wouldn't keep it cool (or dry) in summer, but a half-ton PTHP in an insulated storage room should be able to do both.