Maintaining headroom with 2×4 rafters
Hi all. In the process of purchasing a 1905 home in KY (zone 4) with an attic we plan on finishing into a master suite. Currently, the attic floor joists are 2×6 (true 6″) with wood lath and plaster ceiling below, blown-in cellulose insulation filling the floor cavities, and rough 1x plank flooring. Rafters are 2×4 (3.75″) on 24″ center with gapped 1x plank sheathing, Tyvek underlayment, and 10-year-old shingles. Roof is hipped with a front dormer, we’ll be adding a dormer on the side for staircase headroom, and there are several skylights (one actuating).
My current plans for insulating are to remove the cellulose and use ccf to seal the roof deck, but I’m not sure what will be sufficient. I will be adding horizontal 2×4 furring on 16″ centers to hang drywall at the finished span of the rafters between knee walls and flat ceiling.
Will filling the 2×4 cavities with closed-cell spray foam be sufficient or do I need more? I would like to maintain as much headroom as possible. If more is needed, I could have them overspray the spans above and below the finished span, and in the finished span I could add 1.5″ rigid foam board between the furring strips, unless that creates a problem due to air gap between foam/foam.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Have you determined that the attic floor joist will support the new use? They may only be suitable for light storage.
I haven't. We're in the inspection period, I'm going back to look into spans. Might be possible to sister joists or introduce a beam below. Based on the insulation requirements, might be better to back out and look for something else at this point.
The master bedroom is one of the most important rooms in a house. Ground floor is the only way to go. Attics are for kids. I live in a similar house to there one you are looking at. Added a bedroom suite on the ground floor. Keep looking.
4" of closed cell spray foam is only going to give you a realistic R24 or so. That's not code, and not really all that much. 4" of polyiso on the exterior would bring you up to about R50, which would get you just over R49 required by code.
It's difficult to overfill and trip closed cell spray foam, so you'll probably end up with something less than a full fill. Depending on how your orient your 2x4 furring, you could potentially get another 3.5" of insulation in there, and then closed cell could be installed the "normal way" (under filled), with maybe around 6-7" or so. 7" would be abour R42, 6" around R36. Neither is current code, but it's enough that you won't be really low on R value in the roof and might be OK without doing anything on the exterior.
Bill
Removing the roofing to add exterior foam isn't really feasible. Looks like getting the slope walls to code isn't really doable without losing a large portion of finished area (around 15%, maybe a bit more, if we stand the furring on end to make 7.25" bays). I'll talk to the inspector before we back out and walk away, maybe he'll let us get away with a little less in the slope wall. If not, c'est la vie. Thanks for your help!
Check with your local building department on how much insulation you actually need.
Depending on the R value, what you can do is cross strap the roof with either 2x3 2x4 or 2x6 on edge perpendicular to your rafters. Spray foam the rafter cavity plus this extra space. Since spray foam now completely covers the rafters, it will reduce the thermal bridging and significantly improve your overall assembly R value. You can also install the strapping at 16" OC to simplify your drywalling.
Since the assembly R value is much higher in this case, you can comply based on U factor, chances are you can get away with only 5" of closed cell SPF with this type of roof.
Converting an unconditioned attic to finished space isn't necessarily simple. As others have noted, ceiling joists aren't the same as floor joists. I'd also worry about 2x4 rafters being able to take the additional weight of drywall. Plus insulation and air sealing can be tricky if the house wasn't built to accommodate them in the first place.
We renovated an old house with 2x4 rafters. As it turned out, they were undersized by at least two sizes. The solution was the remove all of the rafters and reframe the roof with 2x8 rafters on 16" centers, filled with FG insulation, and then 4" of rigid foam above the deck. Very expensive way to get conditioned attic space. It's still up on the third floor, and like Jonathan said (#7), attic are for kids. Picture yourself climbing all those stairs every day for the next xxx years. Yes, it can be done but there are probably better ways to spend your $$. Keep looking.