4

I have 5.25" tall rafters, about 30" on center, with roof planks directly on top of them. I would like to keep the rafters visible, but add insulation between them. So I only have a few inches to work with. The work will not be inspected. I will finish with drywall for fire protection. There are no vents--just the central HVAC.

Is it better to fill all of the space with insulation, or put baffles on top and add soffit vents?

  • you should do all 3 tasks from your question: drill openings to allow air to circulate from eave to eave, add baffles (or keep the insulation away from roof bottom) and finally insulate (install so as to gain the highest R-value). – ojait Aug 26 '15 at 14:34
  • 1
    Thank you! I guess my question boils down to this--there must be a trade-off between an extra inch of insulation vs. an inch of ventilation. In a dry climate, using rigid foam insulation (which blocks moisture), would it be better to have an extra inch of insulation? – Phil Esra Aug 26 '15 at 20:42
  • House need to breathe one way or the other so as to dissipate moisture from inside to outside. Insulation (and it's proper location), vapor barriers and ventilation help control condensation and the problems associated with it. So unless your house is so tight that no air flow will occur you will need to leave an air space between the ceiling insulation and the roof to vent moisture. – ojait Aug 26 '15 at 21:23
2

For a similar situation, I'm considering an unvented cathedral ceiling (see references).

The main reason to vent an attic is to prevent warm moist indoor air from contacting the roof framing and decking. When the decking is below the dew point, the moisture condenses, the wood wets, and then an assortment of bad things happen.

There's another solution to keeping roofs dry: plug the air leaks between the house and the roof. If the warm moist air cannot contact the roof deck, it can't condense and wet the roofing. This is why pink insulation usually has a kraft paper backing (vapor retarder) that faces the inside the house.

When using the standard building materials of yesteryear, air and vapor sealing was all but impossible. In this brave new world of construction with Optimum Value Engineering, blower door testing, drywall sealing, taping OSB/plywood seams, and spray insulation, air sealing is no longer the stuff of science fiction.

Today, one can reliably air, vapor, and water seal a surface with closed cell spray polyurethane foam (ccSPF). The minimum depth required (by code) is based on the foam density and your climate zone. With you being situated in Climate Zone 3, I'd expect 1 inch (R-7) would suffice and more is shaving nickels off your electric bill. Since you have so little depth available, I'd consider filling the cavity with 3 inches of spray foam (R-21) in a two-step process:

  1. Spray foam 1" onto the decking as close to the soffits as possible. This first layer would be applied while the decking is exposed so you're likely to get uniform edge-to-edge coverage, a good air seal, and you won't have protrusions to trim off before you can drywall.
  2. One of:
    • For less than stuffed full, spray additional insulation to the desired depth and then cover with drywall.
    • To fill the cavity, hang a row of drywall, cover it with plywood, and then spray fill the cavity. The plywood will keep the expanding foam from popping your drywall fasteners. There are videos on YouTube showing a number of ways to do this.

Do read up on the building science behind this. In my jurisdiction, pulling a permit for this type of insulation retrofit is not required. It wouldn't hurt to verify that your local building code permits this. If not, showing them the relevant portions of the International Building Code might change their mind.

references

  • I did it. I removed the drop ceiling and filled the joist bays with ccSPF. Things worth mentioning: 1. The roof framing was in great shape. Ventilated roofs work, even in Seattle's climate. 2. There were many critters nests in the roofs and walls over the years. 3. I went from R-6 to R-35 and air sealed at the same time. 4. The spray foam GREATLY stiffens the roof. No more spring in that 1/2 decking on 24" centers. It's solid across the entire span. 5. Infrared photos reveal one corner where I achieved a less-than-great foam application. – Matt Simerson Dec 1 '17 at 0:18
1

Since it will be a conditioned space you will want to use as much insulation as possible to increase the R- value. If you intend to cover each bay (space between rafters) with drywall it would be wise to leave at least a 1 inch space between the bottom of the roof planks and the insulation to allow water vapor from the inside a way to dissipate to the outside. If moisture can not vent it will condensate; possibly enough to saturate drywall or promote mold growth. Venting can be as simple as drilling 2-3 two inch holes at the eaves and at the ridge board above. For insulation type I have used ridged board insulation with great success. It comes in varying thicknesses (1/2"- 4"), it is available with a metal foil skin that helps stop radiant heat from entering from the roof, fairly low cost and is easy to cut and install.

  • 2
    I would add that venting with some kind of screening is a must. With open holes, you're inviting critters and insects to move in. – nstenz Aug 26 '15 at 5:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.