Insulating a Vented Roof
Hello all,
I’d like to formulate a solid approach to my upcoming project. I currently live in Zone 5 (Wisconsin). My house is roughly 30 x 30′ two story with attic. It has a failing tile roof and I am going to be removing the roof and replacing with a metal roof. Reasons for metal would be lighter, less maintenance, better in snowy environment-not interested in asphalt. The roof is hipped with only a 3-4′ ridge. There is no ridge or upper attic venting. There are some soffit vents, but I don’t know the extent of their surface area and function.
We like to keep doors and windows open in the warmer months and don’t like to use AC unless it’s very warm in the house.
Currently the attic has floorboards, no surface insulation on the floor or rafters and I haven’t pulled up a board to see if there’s anything underneath. We use it to store fairly light items that would take up no more than a 12×12 area. There is a pull down staircase for access that is not sealed in any way.
There is a bathroom exhaust fan that is vented out a soffit and it is going to be replaced with a continuous fan system-likely venting through a roof although that would entail a few 45 degree elbows so the vent stack is somewhat out of sight (not in the front of the house).
My question revolves around the best way to insulate the area, leaving area for storage and to still allow a ventilated roof. I did have someone recommend spray foam, but I have reservations as I would be unable to detect a roof leak and I am unsure if that makes sense given our attic door access and preference to have windows open resulting in a humid environment. I could live with slightly reduced r value and would possibly consider adding some foam board to the underside of the rafters, but I don’t know if that would make sense either.
I am leaning toward blown dense-packed cellulose under the floorboards, soffit vents and either using the 3-4′ ridge as a vent and/or a box vent.
The space will never be finished while I live there.
Metal roof would be on top of Grace with ice/water shield, enkamat 7010, I am going back and forth with the roofer re: need for purlins under the metal for more under-metal breathing and water escape.
What would you do in this situation?
Thanks in advance for your insight-
Dave
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Replies
I would clean out the attic floor completely, pulling boards out and all. Use a vacuum to get all you can, and make sure all soffits are clean too, so you can install baffles.
Once your entire attic is clean, swept and vacuum, cover with solid materials all large holes, like plywood or thermoply, and seal all edges and penetrations. Froth-Pak foam sealant is a great product for that. Install baffles around the attic perimeter.
In CZ5, with the new 2021 Code, you need R60 or about 17”-18” min. of blown cellulose or fiberglass. You may need to build a platform, above the insulation level, if you intent to continue to use your attic as storage.
Build/install a raised attic hatch with a good insulated attic door.
Edit to add: The 2021 IRC Code requires attic ventilation at 1/150 or 1 square foot of attic vents area for every 150 square feet of attic floor space. See R806 Roof Ventilation.
Armando is right on. Whether or not your jurisdiction requires you to meet current code levels of insulation, it is still a good target. You can certainly use the 4' or so of ridge for venting. It doesn't meet the specifics of the code, but it's better than no high venting at all. I wouldn't supplement it with box vents. They're ugly and often leak. If you can get reasonably wide soffit vents, you can probably meet code targets that way with some supplement from the ridge. For the attic floor, an approach I've used in the past, similar to Armando's recommendation is to cover all holes and gaps you can see with foam, plywood, cardboard, just about anything. If you have recessed lights in the attic floor, either replace them with "IC" rated lights or install insulation boxes around them. Once the floor is well treated, install a flash coating of spray foam, only an inch or so. This seals up all of the little holes and leaks without breaking the bank. Then dump as much cellulose on top as you can manage.
Armando and Peter-thank you for your replies.
Vent calcs using the 1/150 and 1/300 rule state I'll need about 2-3 square feet of venting at the top and at the bottom roughly 250 in2. A 4' ridge vent would proved approx. 50 in2 total, leaving me a bit short for exhaust but I can get that amount along the soffits, I think I'm ok with at least some ventilation. Currently, there's none and all the timbers look great (1935 home).
To clarify some of your points: Remove floorboards and clean everything. Replace exhaust fan and any lights with IC rated fixtures-seal any gaps around fixtures, sewer stack, with Froth-pak.
Peter: flash spray with Froth-pak (?) between joists or the joist timbers too(?), but nothing on the rafters or around the soffits(?).
Place preformed baffles between rafters that correspond to the soffit vents but nowhere else. Should I perform any blocking with foam board in between the rafters just above the soffits that are not supplying intake?
Finally-the thermoply-I think you mean for me to use that as a hole-filler, and not as a new flooring material(?)
Thanks again for your time and expertise-I've asked 3 people in the trades who haven't given me clear direction other than "yeah, let's spray foam and you'll be fine."
telebiker,
Just t0 add to the sound advice Armando and Peter have given you: Search around for ridge vents. Some like this one are more effective and less prone to clogging than the ones that rely on mesh closure strips.
https://www.theroofduck.com/ridge-vents-simple-effective-concept/
Also on a steeper roof, you don't want full Ice and Water.
Besides costing you more money is not gaining you any extra durability plus it is next to impossible to remove without tearing the sheathing off. A steep slope snap lack metal roof is liquid tight, no water should make it in until it rusts through.
No need for extra space for venting bellow the metal panels. Most metal panels have enough gaps from the striations and the snap lock feature to allow for a small amount of air flow. This is more than enough to deal with the night time condensation on the underside of the metal panels.
With a vented roof, using a permeable synthetic underlayment can also provide a bit of extra drying for minimal extra cost.
Agreed.