Background:
Approx 1000 sq ft attic floor space and 12/12 pitch roof above. Attic floor has next to no insulation (approx 1/4 has poorly installed F/G). No soffit vents, no gable vents and no ridge vent. No collar ties. 5 foot tall 2″X4″s installed parallel to the soffit (presumably to decrease the rafter span). Gable ends have old (rotted) double hung windows. The entire floor is covered with 3/4″ plywood. Current use is storage only. I would rather not rip up the floor boards if possible.
In the short term, I would like to insulate the attic, replace the windows and install any necessary vents and vapor barrier. The goal here is to make the current living area (below) more energy efficient.
Long term, I would like to convert some of the space into a playroom (assuming the attic floor can take the additional load), add a few skylights and use the rest of the space as storage. The storage space would be in the low headroom areas. The goal here to make more usable floor space.
Based on short and long term goals, any thoughts as to what type of insulation and where to place it. Should I insulate the attic floor as well as the attic ceiling or just the attic ceiling. Any thoughts on vapor barriers and venting.
Replies
Im ny opinion, it's best to bring the entire attic space into the building envelope by insulating the attic roof.
"5 foot tall 2"X4"s installed parallel to the soffit (presumably to decrease the rafter span). "
Is that a knee wall you're describing? If so, bring that into the conditioned space as well. It's easier to effectively insulate the entire length of the rafter and bring the area behind the knee wall into the conditioned space (for easy future storage or to hide an air handler, etc) than it is to insulate and seal part of the attic floor, go up the knee wall, then continue up the rafters to the ridge.
First, you;ll be sealing up the attic and thus tightening up the house. Address any moisure problems in the basement/crawl space...either liquid or vapor. Use canned foam to seal every hole/crack/crevice in the basement ceiling that will allow air to move from the basement and either go up into the wall framing (balloon framing) or through the ceiling and ito the frst floor.
Address moisture generated by living. Cooking/bathing/laundry, etc.
Up in the attic, establish soffit vents and a ridge vent, as well as a channel for air flow through the rafter bays, from soffit to ridge.
I don't recall the depth of your rafters, but you need to figure your insulation. My favorite attic insulation is 4' by 8' sheets of 2" thick foil-faced polyisocyanate attached to the face of the rafters with a couple of buttons and nails. It's great in older homes where framing is not a uniform oc spacing, as the sheets don;t need to break over a rafter. They can break midspan. When you install the sheets, leave a 1/4-3/8" gap between the sheets. Once installed, then use canned foam to seal the gaps between the sheets. The attic roof will now be tight as a drum. The PI will prevent solar gain from the roof, which is the usual cause for hot summer attics. It'll also stop air flow dead, which is the usual cause for cold winter attics.
Do the same on the gable-end walls, using either 2" or 1" sheets of polyiso. Seal the corner between the gable wall and the roof with foam,
Run horizontal furring strips 16" oc across the attic roof. Screw through the furring strip, the 2" of foam, and into the rafter. The reason you only needed a few nails w buttons to originally hold the PI in place is because the furing strips will now lock it down.
You can furr the gable walls as well, or just screw the drywall through the foam and into the framing.
You can drill a few holes through the 2" of PI and blow insulation behind, filling the rafter bays from soffit to ridge. Cellulose would be preferred.
Repeat on the gable walls to insulate them.
Foam the drilled holes with canned foam, then cover with Al tape.
Drywall, screwing the drywall to the furring strips.
This attic will be tight as a drum. The 2" of PI yeilds about R12, even if you only get 5 or 6" of blown cells, the combined cells and PI, with the tightness of the installation, will outperform r-50 ceilings that are done in a more conventional way.
It's likely that when insulating, you'll need to blow insulation into th attic floor. Go from the rim joist to about 4' from the rim joist. Otherwise you'll have a cold floor and a cold attic.
As an aside, before you do all this, figure tou future use. Electrical/cable/cat-5/phone/etc. Do not compromise the 2" of polyiso with can liights of electrical boxes, though. That'll cause huge performance problems down the road. Use faux box beams surface mounted to the drywalled ceiling to hide what needs to be hidden.
One thing...muy importantay...after hanging the sheets of polyiso, go back and peek between the gaps to find the attic rafters. Use a sharpie marker to mark the centerline location of the rafters on the foil face fo the polyiso BEFORE you seal the gaps with foam. This will allow you to connect the tic marks later and easily locate the rafters when you try to screw the furring strips through the foam and into the framing.
The good thing about the furring strips? By screwing them, if the underside of the rafters isn;t quite in plane, you can back out a screw and shim under a furring strip to bring it out a bit. It'll help get the whole roof in plane for a better drywall job.
Or something like that...
I gotta go soak my finger in the jacuzzi...they're plumb worn out.
My original post hasn;t shwon yet...and the jacuzzi is still filling up...one more detail:
Obviously, by adding 1" or 2" RFBI to the gable walls you'll need to add a jamb extension to bring the jambs of the gable windows out flush with the drywall. It may sound tough, but it's simple, and will alow you to add additional detail to the trim.
Sounds like a good plan. Yes, the 5' 2"X4"s represent knee walls. I like the idea of using the 2" foam because my rafters are only 2"X8"... As far as the attic floor (2nd floor ceiling), how far apart do I need to bore holes to effectively blow in insulation? As far as the window jams go, I plan to replace the windows anyway so I should be able to order jam extensions to make up the additional 2" of foam and 3/4" of furring.
One other question: I do not plan to have the ceiling of the attic run all the way to the ridge. There will be a small triangular dead space above the new room. The soffit air will run up the rafter bays into this space. Hopefully I am explaining this OK. Would it be acceptable to use gable vents rather than a ridge vent?
Thanks for the great advice.
As far as the attic floor (2nd floor ceiling), how far apart do I need to bore holes to effectively blow in insulation?
To blow insulation into the attic floor to insulate the rim joist, you'll need to bore a hole in the floor for each attic floor joist bay...likely every 16". Drill about 4' back from the rim joist, then insert the hose in as close to the rim as you can get. Blow and slowly back out as the blower stalls, filling the joist bay with insulation.
As far as the window jams go, I plan to replace the windows anyway so I should be able to order jam extensions to make up the additional 2" of foam and 3/4" of furring.
Actually, it may be easier to just do these on your own after the windows are installed. Or, if you prefer the jamb extension KIT, and order the jambs to an over-sized depth...then once the wall is done and the window are in, you can rip the pieces from the jamb kit to the depth you need.
One other question: I do not plan to have the ceiling of the attic run all the way to the ridge. There will be a small triangular dead space above the new room. The soffit air will run up the rafter bays into this space. Hopefully I am explaining this OK. Would it be acceptable to use gable vents rather than a ridge vent?
I'll have to answer "no". With the volumes we're discussing here, it's very likely, almost totally assured, that the gable vents will not perform as desired in venting air from all of the attic rafter bays. The system will short curcuit, so to speak, when compared to the performance of a pure soffit-to-ridge system, or with just gable vents to vent a large, open, attic. Mixing the two really isn't a viable or workable option.
To blow insulation into the attic floor to insulate the rim joist, you'll need to bore a hole in the floor for each attic floor joist bay...likely every 16". Drill about 4' back from the rim joist, then insert the hose in as close to the rim as you can get. Blow and slowly back out as the blower stalls, filling the joist bay with insulation.
I understand that will will need to access each bay (16" centers) but about how many holes in each bay will I need to drill to cover the entire floor?
Regarding the windows, I will probably just make my own jam extensions as I did with several other windows in the house.
Thanks for your help!
Tharrett,
If you do the plan that Mongo offered, you will not need to insulate the entire floor of the attic, as the attic volume is wholly in the thermal envelope of the building. If you still wish to do so, however, then every 8' will do nicely, assuming no rows of solid blocking or similar obstructions.
Bugle
I've found that making your own jamb extensions is one of the easiest and most cost effective projects for a DIYr. All you need is a basic table saw to rip the stock pine or whatever to width, and then a miter saw or box to trim to length. If the new windows are pine, you can buy #2 1X_ inexpensively, and cut the imperfections out during the ripping and cutting to length.
For really wide jamb extensions (I did some ~ 8") use 3/4" veneer plywood, ripped and trimmed to size, and use the first lamination thickness as your reveal when you trim it out.