Ensuring Rockwool ComfortBatt Insulation is Tight to Underside of Unvented Roof Assembly
We’ve just completed the exterior work of an unvented roof assembly and are moving on to installing the rafter bay insulation. The house is a 1923 bungalow with the typical 1 1/2 stories. There was an addition from the 80’s with a 2/12 shed roof. We are in zone 4a. Assembly from the outside in: three tab asphalt shingles with standing seam on the shed roof, synthetic underlayment, 1/2 in CDX plywood, 3 inches of XPS (sorry, didn’t understand problem with greenhouse gas potential at the time), another layer of synthetic underlayment, 3/4 inch mix of CDX and solid wood, 2 x6 rafters sistered to actual 7 1/4″, R30 Comfortbatt. The living spaces will be finished with drywall, the shed roof will be exposed ComfortBatt. Rafters are 24″ on center.
While trying to insulate the very hard to access portion of the shed roof during the roofing process we discovered the Comfortbatt insulation did not completely fill the rafter bay and would leave a gap up to 1 inch from the under side of the roof decking. As a result, because we needed to move on with the roofing portion of the work, we removed the furring strips reducing the rafter depth to a nominal 2×6 and used strapping perpendicular to the rafters to hold up the batt. This seems to have derated the R30 insulation to R23 but maybe even worse than that if the material is too dense.
The Rockwool installation instructions are a bit thin with unvented roof assemblies and, despite searching off and on for weeks, I have not found this question answered on GBA.
What is the correct way to support the batt insulation while ensuring the rafter bay is full and there is no gap between the roof decking and the top of the batt insulation? Please, any help will be greatly appreciated.
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Replies
Keeping the furring strips would have been helpful to achieve full depth. I know the codes are explicit on "direct contact with the roof deck" but in reality there's little physics to support that requirement if we are to believe Joe:
https://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance
The 1" air gap would not significantly alter performance, assuming an airtight cavity.
But to answer your question, I've seen housewrap or roofing felt used to hold batts in place.
Thanks Jason, from reading the link it would seem at 7.25" there won't be much airflow at least due to stack effect in the rafter bay. I'm not sure the 1" gap is in an airtight space as the historical 1x material used in roof decking has maybe a 1/4" to 3/8" gap between boards. There is a synthetic underlayment applied under the 3" XPS and the XPS joints were taped so that assembly is airtight. The perimeter of the roof was replaced with 3/4" CDX and caulked with PU caulk so there shouldn't be any wind pushing air around. I'll restore the 1.75" furring strips and fluff up the R30 comfortbatt.
You can use "insulation wires" to support batts. These are just cut pieces of steel wire a little longer than the gap between rafters. You push the wires up into the bottom of the batt and they bite into the rafter on either side to provide some support. This is usually all you need, just be careful during installation that the wires don't pop out and poke you. Wear safety glasses!
I've seen batts supported with thin plywood strips that are pressed between rafters too. All you need is a little spring force to hold things together. If you really wanted to go all out, you could nail up some 1x2 (or plywood strips) on either side of the rafter and use those to support slats that would support the batts.
You can buy pre-cut insulation wires that are stiffer than normal suspension wire which is what I would use. You can also cut your own wires from rolls of something like 9 gauge steel support wire (commonly used for drop ceiling grid support, which is why it's called "suspension" wire).
Bill
Hi Bill, I saw the insulation support wires and was not impressed but your endorsement has me reconsidering. I can order a few different lengths to support the varying widths of old house rafter bays.
Richard,
The crew installing my batt insulation used wire spikes that pushed through the batts. A push on metal washer/cap went on each wire to hold the batts up. Each wire has a small metal base onto which they put a dab of construction adhesive to hold it to the decking. The wires were put in place a day before the other crew members set the batts.
I wasn't entirely convinced the wires and glue would hold over time, so I stapled leftover banding from bundles of materials across the batts at 24" intervals. These were just fiberglass batts, which I think are quite a bit lighter than Comfortbatts, so I would guess that extra support might be a good idea.
I also used the wires that span between joists or rafters and found the biggest annoyance being the tendency for them to "flop" bow side down. I had to go back and set a number of them to make sure the bow went up and pushed on the batts. Do keep some heavy duty clippers or sidecutting pliers on hand when set the wires on less than standard width. Too much bow makes them even more twitchy.
An electric stapler will allow you to place the plastic banding from bundles of whatever pretty quickly. If new material is the only choice, the webbing used for lawn chairs would be cheap and provide lots of support.
Hi Roger, I think I'm going to use a combination of strapping and the wire supports. I ordered a 2000 foot roll (smallest available) of box strapping material before starting the project. The electric stapler will let me drive in 1/2" staples in a confined space. I was worried about the long term survival of the strapping but feel better when considering the longer length. Maybe a combination of the spring wires to push up the insulation where needed and strapping for broader support will work.
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