Best approach to renovate vaulted ceiling
I have a 1-story ranch in zone 5, Colorado mountains. The great-room half of the house has a vaulted, drywall ceiling about 11 feet high at the peak. The roof structure has 2×12 rafters filled with FG batts measuring 7 1/2 -in, with cardboard baffles above resting on applied ledgers leaving an air channel below the OSB sheathing. Metal roof, soffit and ridge vents. The batts are loosely fitted. The wind blows a lot and pressurizes the insulation space. Cold air blows out of the few ceiling electrical boxes. I want to air seal the insulation cavity from the vent channels and increase the insulation R from the present 24 up to 48 if possible. I would like advice on the best plan. One approach I am considering would be to strip the roof off, eliminate the baffles and vents, add additional R13 batt, re-sheath and add exterior foam. Another would be to strip the interior drywall, seal the baffles to the rafters (maybe replace them with foam board), refit batts, and then add foam underneath. The latter approach would steal a lot of ceiling height I really don’t want to lose. Advice?
An additional question relating to the metal roof, if I choose the option requiring exterior foam, is the best approach to providing a better attachment for the metal roof panels. Currently they are screwed down to the OSB sheathing, and in tightening the screws I have found that maybe 1/4 or more of the screws had been overtightened or overstressed and stripped out the hole in the sheathing. They have been replaced with oversize screws, but it seems that the OSB is not very robust. The wind blows a lot, and it seems to stress the roof panels. The neighbor had a new roof installed and 2×4 crosswise nailers were added over the new membrane, screwed through into the roof trusses/rafters to provide a much better attachment for the metal panels. It looked good, but leaves the panels unsupported underneath. My thought would be to combine this with adding surface foam. Any downside? Or would using ZIP R panels for the roof foam be as good or better?
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Replies
There is nothing wrong with a vented roof, if the wind blows through device boxes, your low hanging fruit is to air seal those.
Over insulation is mostly worth it to do when you need to re-roof. If you roof is in decent shape (after your fixes), it is hard to justify to pull the whole thing off.
You would be much better off to install a layer of rigid insulation under the drywall and panel over it some nice T&G. About 3" of polyiso would almost double your roof R value there. You can either extend the electrical boxes or remount them further in.
If you want to insulate above your roof, keep in mind that you need to maintain a ratio of exterior foam VS interior fluffy for your climate (40% of overall R value as rigid), so the more fluffy insulation you put the more rigid you need. This get pretty expensive much above 4". I would limit the fluffy to R24/R30 batts against the roof deck and install 2.5" to 3.5" of polyiso above the deck.
It is best to split the exterior rigid insulation up so that the final layer is 1.5" thick, this way you can embed a 2x3 on flat into it to screw down your metal roof.
If you go with 2.5" rigid, you can cross strap the roof with 2x3 on edge and rip the insulation to fit in between. This is ever so slightly less R value but a very quick install.
In zone 5 there really not much saved in terms of energy cost by having more than an R30 effective roof (taking into account the thermal bridging of the rafters), there is very little payback by going crazy with rigid insulation.
Hi WMF.
If you are willing to open up the roof, from either the inside or the outside, you have some options. This article explains them all well: How to Build an Insulated Cathedral Ceiling.
Though this is not an idea application for ZIP R-sheathing, Huber does provide a detail for using it on a roof. You can find it towards the end of this article: Working with ZIP R-sheathing.
Akos, thanks for the perspective. I am trying to find the best balance between cost and resulting improvement with several different competing considerations. Adding interior foam and a T&G ceiling would be really reasonable, and a fix for the popcorn ceiling too! I have been into the ceiling several times to remove a can light and fluorescent fixtures, and was concerned about the loose fit of the FG batts (do they shrink?) and leakage of wind from the vent channels past the baffles - very noticeable. There is also a 4ft long section between 2 rafters that often melts snow faster. I have sealed around the remaining two boxes, and probably just need to seal the wire entry from inside the boxes - canned foam? But doesn't the wind circulation within the insulation greatly reduce it's effectiveness? The loose screws fastening the metal roofing also has me a little spooked. I have been told to tighten them every year or two because vibration from the wind loosens them - OSB holds them poorly.
Low density fiberglass batts don't have much structure, it is a lot of work to get them to fill the cavity properly. You rarely see a nice install with these.
The bay where snow melts faster either has a large interior air leak or the insulation is in really bad shape. If it is an easy fix to get in there and take a look, go for it. I think it is more worth you time to install an extra inch of rigid insulation and ignore everything above the rafters.
If you do have that strong of a wind, you probably need to block off some portion of your eave and ridge vents on the windy side. You don't want to block these all the way as you still need more ventilation, but it does sound like you have way too much right now.
For the interior foam, try to stick to less than 2.25" of foam so you can hang the strapping with standard 4" construction screws from the box stores. If you want to go thicker, you can get pretty long deck screws from roofing places but it get challenging hitting the rafters through very thick foam.
Make sure to tape the seams of the foam as it should be your primary air barrier. Try to move the junction boxes to inside bellow the foam.
With 1/2" strapping (you'll need this to nail the T&G) and the T&G itself you have almost 1 1/4" of depth. You can carve a bit of the foam to make a pocket for a 1.5" octagonal box and then you would only have the wire running through the foam which is much easier to air seal.
Your lowest cost option for foam is reclaimed roofing polyiso. If you can't find this locally, buy it new is it is generally the lowest cost in terms of R value /$.
If you use thick foam on the inside, there is an easy way to mount the boxes:
Assume 2.5” thick foam. Use 4” square boxes, which are 1.5” depth for the standard depth ones. Cut a hole to clear the box with a tight fit. Press a piece of 1” thick foam into the back of that hole, again with a tight fit. You now have a perfect size hole to fit your 1.5” square box. Punch a hole for the wire AND CLAMP, and inject the hole with canned foam after installation to air seal around the wire. If you use a foam gun, the small snap-on plastic tip can get around the cable in the clamp to inject the foam. You can probably fit the straw for non-gun canned foam too, but I’ve never tried that way myself.
Finish off the box off with a mud ring to give you whatever size opening you need. Mud rings are available for 4” square boxes that provide single gang, double gang, and round (like you’d get with an octagon box) openings. Use the right depth mud ring for the thickness of drywall you’re using.
This is much easier to do that trying to carve a hole for an octagon box that is the proper depth.
Bill
Yes, air blowing through insulation greatly reduces its insulating value. The best way to deal with that problem is probably to seal the vent baffles. Depending on what you have now, you might find the easiest solution is to build new vent baffles with 1/2” polyiso, using scraps of polyiso or 1x2s to form the channel with the 1/2” polyiso forming the baffle. Seal the edges with canned foam to complete the seal. This will ensure an airtight vent baffle. You can then use fiberglass or mineral wool batts to insulate the remainder of the cavity. Rigid foam on the inside behind the drywall will help with thermal bridging.
Canned foam isn’t a good option to air seal electrical boxes from the front because it tends to expand into the box which you want to avoid. I prefer to use fire rated silicone (the red silicone fire caulk, NOT the intumescent fire stop caulk) if I have to seal from the inside because the caulk doesn’t expand.
Bill
Thank you both for your comments and information. Now to see what I can afford to do.