DIY Cellulose Insulation for Finished Attic
My husband and I would like some advice on installation of cellulose insulation for a finished attic renovation. We are finishing off our attic (3rd floor) as a master bedroom suite. We are concerned about sound and energy efficiency (especially in the summer – we are in the Memphis, TN area). The 3rd floor living space will have 5′ knee walls surrounding it except where dormers are located. Our home is approximately 7 years old and has blown-in fiberglass between the 2×8 joists (R-19????). We plan to seal – with the appropriate spray foam – around any perforations for piping and lights and replace all non IC rated cans with IC rated ones. We would leave the existing blown-in fiberglass in the floor of the attic but add cellulose for sound deadening. In the floors of the knee wall areas we need R-30 to meet code which would seem impossible with 2×8 joists and just cellulose (we are looking at Green Fiber Loose Fill which requires 9.3″ at installation (8.4″ after settling)) unless we overfill the joists. Our finished ceilings (no vaults) are required to be R-30 which is the same issue as previously stated – can we overfill the joists? Walls must be R-13 which seems like it might be possible with 2×4 construction with drywall installed but the Greenfill requires 4.1″ at installation to get R-13 but it does not take into account the R-value of the drywall. Do we need to use foam board insulation in this area to help meet the requirement? We assume we leave the roof alone in the knee wall attic area and the area above the ceilings and that the cellulose previously mentioned would we sufficient. We have soffits and would ensure that they are not blocked when the cellulose is installed. We know some will suggest using foam in lieu of cellulose but we have already had that discussion! 🙂 Your feedback on our questions and comments on anything we may have missed would be appreciated.
Replies
You have to know how to write in paragraphs before using or asking about cellulose insulation.
My old eyes and brain just don't do big blocks of text.
Whenever you get into an attic remodel the questions aren't simple (and the answers aren't cheap, or necessarily the ones you'd like to hear).
I gather you don't want to hear that foam is the only viable option?
DIY Cellulose...and DIY Foam???
In a perfect world we would love to use spray foam for everything...but cost is prohibitive. We are now strongly considering spray foam for the roof. With R-30 cellulose in the attic, spray foam between the rafters would be for comfort during the hot and humid summers here in Memphis and not required by code. Do we use open or closed cell foam - I hear arguments for both. If open what do you suggest for ventilation? What would be the minimum number of inches to help with the sweltering summer heat?
Thanks!
Look down exactly one inch... I already told you what you needed!
Foam is the only way for you to get to R30 values. You don't need to do full depth fill, but you need it as at least part of your equasion.
With R30, you would need
2" of foil faced polyurathane (R6.5 per inch) = R13
plus
5.5" of Cellulose (R3.5 per inch) = R19
This gives you R30 within the limitations of your 2x8 construction.
2" of closed cell spray foam against the underside of the deck is the fastest way to do this, it seals best too. If you actually called a local foam contractor, you might find it similarly priced to what it would cost to cut 4x8 sheets and stick them into your bays, then seal the edges with gun foam.
the floor of a knee wall???
I think the answer is that you will add insulating foam panels as needed to achieve the R value required. You haven't metioned a plastic sheet air barrier or how you plan to ventilate the space once you've made it wheather tight.
You have done alot of research however, it might be worth it the hire an architect or contractor who has done this before and pay them to detail the plan and materials. The fee might be as high as $1500 bucks but it would be worth it if you know the job was done properly.
Good luck and keep us posted!
Something of a metadiscussion: Something I don't understand is why one would decide to insulate part of the ceiling, the knee wall and the floor in the knee wall area, rather than just doing the entire ceiling, bringing the knee area inside the "envelope". Seems to me you're adding two hard-to-insulate joints you don't need, plus amplifying the warm roof/cold eave situation with regard to ice dams. I could maybe see that you wanted to use the knee area as a sort of plenum for roof ventilation, but I don't see how that's going to improve things in general.
Something of a
I think I see the pattern in NoelN's descirption, it is a situation DIY (and professionals on thier first few attempts) fall into. This is basicaly characterized by searching for work arounds where materials cost are low and installation is uncomplicated manual work. The problem is that it is time consuming and in this case detailed work that needs to be done right.
Given thier DIY approach they are the best workers for the job, even a good foreman has to pay alot of attention to what the crew is doing in a cut, tape, fill, leave no gap job like this. It is hard to get a job like that on time and under a spray foam budget, not to mention done properly. Which is why a contractor trades off and goes for a simple, better and more expensive job because for us the labor isn't free and getting it wrong hurts dearly. DanH, I would do as you say but I'm a contractor.
What can a DIY take from that? Insulation done in a proffessional manner has resale value, a sniff of DIY in any home Improvement and the new Buyer won't give you credit for it. That's why I say to NoelN find a pro to consult and at least put eyes on your work.
Noel...
you can do it with cellulose, but it involves some extra work
<<<< In the floors of the knee wall areas we need R-30 to meet code which would seem impossible with 2x8 joists and just cellulose (we are looking at Green Fiber Loose Fill which requires 9.3" at installation (8.4" after settling)) unless we overfill the joists. Our finished ceilings (no vaults) are required to be R-30 which is the same issue as previously stated - can we overfill the >>>>>
so in the joists behind the kneewalls... we would normally use 20" of cellulose
if all you want is the R30.. and you don't need to use the space behind the kneewall.. then just overfill it
if you need the space... frame a 2x4 false joist accross the existing 2x8 ( perpinducular ) blow the space.. 7 1/4 plus 3 1/2
and lay a plywood subfloor over the 2x4
in the slant ceiling area.. i guess you have 2x8 rafters also... so same deal... cross the rafters with 2x4 and you'll wind up with a 10 3/4 " space... even with the ventilation baffles you will still have 9"
in the flat attic area just blow the depth you need