Cellulose insulation & vapor barriers?
Several insulation contractors have recommended loose fill cellulose attic insulation for my 1970’s era ranch in northeastern CT. The house is heated with oil via a hydronic baseboard system. Central AC was installed 12 years ago, with equip. located in the attic. There have been no problems with the AC to date. Here is my question:
Would it be a good idea to install some sort of semi-permeable vapor barrier on the attic floor before adding the cellulose? I ask this because I know that cellulose can retain water. FYI, there is only about R-5 insulation in the attic now. I think it is fiberglass. There have been no roof problems & no leaks to date, but there are 2 small gable vents plus a ridge vent. I’m concerned that wind-driven rain could get into the attic and dampen the cellulose. Is this an unfounded fear? By the way, I want to stay away from SPF, which is why I thought the cellulose might be a good choice. Many thanks in advance for your thoughts.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Cellulose is fine- the BEST choice in fact, and a few drops of wind-driven rain through the vents aren't going to cause an issue, even if you have zero over hangs on the rake of the roof cape-style. When you mist-spray the top of cellulose it causes a small amount of localized settling, but binds the flakes of paper together, making a thin weak but wind-resistant crust to keep it from blowing around.
In a vented attic no vapor retarder is necessary, but with the ducts & air handler in the attic it might make more sense (albeit much more expensive) to insulate at the roof deck with a combination of SPF and fiber, and insulate & seal the gable-ends. (But it'll be at least a 5x cost multiplier unless most of it is DIY.)
The absolute critical thing to do before adding any insulation is to air seal the ceiling, paying close attention to the duct penetrations, and all electrical, plumbing, and flue penetrations. Flues & chimneys need sheet-metal air-barriers, and clearance to combustibles. Strict adherence to code would require 2" clearance from any chimney or flue, without filling the gap with insulation of any type, but it's pretty common to separate masonry chimneys from cellulose with a wrap of R15 rock wool batts tied in place with stainless or galvanized steel wire (or fencing like chicken wire, etc.). Neglecting to air seal presents a much higher risk of moisture problems in the attic than the gable vents.
Air sealing the ducts and air handler are also right up there in importance, since air-handler driven leakage creates pressure differences between attic & conditioned space orders of magnitude higher than stack effect pressures, driving infiltrations sky high.
I'm constantly amazed/annoyed at HVAC contractors installing cooling equipment & ducts in attics, especially in one story homes that have basements. Putting it up there adds at least ton of direct cooling load due to the hot attic environment, and by putting it outside the pressure boundary of the house it pokes holes that leak air 24/365 and creates another half-ton of cooling load due to the higher infiltration when the thing is running.
In our neighborhood (I'm probably less than an hour drive away from you in MA) anyone heating with oil should be considering heating at least partially with ductless mini-split air source heat pumps. Even at 20 cents/kwh residential-delivered electricity, at $3.50/gallon oil the mini-split would pay for itself in three heating seasons on the difference in operating cost. And a mini-split offers super-efficiency cooling that doesn't drive air infiltration to boot. Download and dig into this short policy piece published earlier this year:
http://www.rmi.org/Knowledge-Center/Library/2013-05_HeatPumps (The cost & efficiency numbers in that document aren't 100% accurate IMHO, but close enough.)
Of course you should still be tightening up & insulating that attic (and any other places where it's deficient. Very few 1970s ranch houses have foundation insulation, and once your attic is up to snuff the basement could easily end up being 20% or more of your heat load, from both leakage at the foundation sill & band joist, and conducted heat through the ~R1 poured concrete foundation. Air sealing & insulating basements are probably the most overlooked under-treated heating/cooling problem in older homes in this area.
Marian,
I agree with Dana. If you decide to go ahead with the plan to install cellulose on the attic floor, you don't need a vapor barrier. But you do need to perform air sealing work before beginning the insulation job. This article explains the necessary work: Air Sealing an Attic.
Dana is also correct that it's always a bad idea to include an air handler and ductwork in a vented unconditioned attic. Once you have HVAC equipment up there, the insulation belongs at the sloping roofline -- so that you create an unvented conditioned attic. This article explains what you need to know: Creating a Conditioned Attic.
Thank you, Dana & Martin! Hindsight is certainly 20/20. Over the past several years, I have learned that putting the central AC equip. in the attic isn't a good idea, but I did not know that 13 yrs ago. I have also learned that making the attic part of the insulated space by insulating the attic side of the roof is probably the best way to handle this, so I was surprised that the energy auditors did not recommend that. Also, they did do a little spot air sealing in the basement on top of foundation walls where fiberglass was barely there. They added a door sweep on the door separating the 1st floor from the basement. They did not say that they did any air sealing in the attic during the audit. They also did not do anything to the garage ceiling, even though there is a very visible hole the size of a hand ball. The garage is under the house (1/2 of the basement is actually the garage), so it's hard for me to believe there is no significant air leakage from the garage.
Dana mentioned the importance of insulating the basement sill & band joist. Interestingly, even though there is no heat in the basement now aside from the furnace, it remains warm enough to work down there (wearing a sweater) during the winter. I had always thought that being partially built into a hill (3 sides) was the reason.
So, with the minimal air sealing they did + door sweep, difference in their blower door readings pre & post was only 616 at 50 Pa.(Pre=3318; Post=2702 @ Pa50). This was a surprise, as the house feels very drafty when cold weather hits. I go through a ridiculous amount of oil in the winter with my 1400 sq ft ranch, which is kept at or below 66°F. Lastly, I should mention that I do have a fireplace with newly installed damper door that seals very well. The whole fireplace was taken down to the roof line & rebuilt, with new clay liners and cap after Winter storm Alfred. Glass doors were removed & have not been replaced yet. The auditors did not cover the fireplace with cardboard or plastic during the blower door tests.
Now, here's my dilemma:
1. Should I go with the auditor's recommendation of loose fill cellulose on the attic floor, or go for the attic side of the roof insulation?
2. Is there a way to do the latter without using SPF? I don't mind using foam for air sealing, but do not want to spray the whole roof.
3. If there is a way to insulate the roof, does this mean that ridge vent will be closed off?
4. Will the whole house fan (used in temperate times of the year) need to be abandoned?
Thank you again for your thoughts! I am most appreciative!
The fact that it's warm enough down in the basement without explicitly heating it means you have a HUGE fraction of your heating bill going out the ~R1 of above-grade foundation and ~R2 band joist. A square foot of exposed foundation is losing about as much heat as 10 square feet of insulated 2x4 wall, so if you have 9' walls and 18" of above grade exposure, the foundation is losing about as much as your framed wall.
Your minimal air sealing job was worthwhile- reducing the air leakage by ~20% but 2700cfm/50 isn't a very tight house- there is likely to be plenty of low-hanging fruit there. A $50 chimney balloon , a $100 gasketed top-sealing chimney damper (or a $5000- air-tight wood burning insert in the fireplace with ducted outside combustion air) is probably worth at least another 100cfm/50, maybe even more- depends on just how well sealed that new damper really is.
It's dramatically more expensive to insulate at the roof deck which is probably why it wasn't recommended. To do it without SPF you would need to put at least R20 (3.5" of rigid polyiso would be the thinnest) above the roof deck and install new roofing, then you could put cheap fiber insulation between the rafters to bring up the R. There are a few ways to do it with SPF, but none of them are cheap.
You may not really NEED the whole house fan if you air seal and insulate the attic floor. When you have only R5 in the attic you get a huge heat gain through the ceilings, but if you bump that to even R25 the effect on cooling loads is huge. The other major heat gains are typically from west-facing windows, which can be shielded from afternoon gains with exterior roll down shades. I suspect you have multiple grilles/registers in the ceilings to be able to use that whole house heat fan? If that's the case getting rid of the whole house fan and sealing them up air-tight would likely reduce your cfm/50 numbers by a significant fraction.
Even though you have existing heating & cooling equipment it's still worth considering adding a ductless mini-split heat pump to heat and cool a large zone, which will pay for itself in very short years on operating cost savings. Sized correctly for heating it would also likely be 3x as efficient in cooling mode as your centralized ducted system (and maybe more efficient than your whole-house fan) despite being somewhat oversized for your cooling loads, since it modulates and doesn't drive infiltration, and doesn't suck in humid outdoor air (adding to the latent cooling load) like the whole house fan does.
Marian,
Q. "Should I go with the auditor's recommendation of loose fill cellulose on the attic floor, or go for the attic side of the roof insulation?"
A. If you can afford the work, it would be better to install the insulation to follow the sloping roofline, as I advised in my earlier answer.
Q. "Is there a way to do the latter without using SPF?"
A. Yes. I explained the different approaches in the article that I advised you to read in my earlier response: Creating a Conditioned Attic.
Q. "If there is a way to insulate the roof, does this mean that ridge vent will be closed off?"
A. Yes.
Q. "Will the whole house fan (used in temperate times of the year) need to be abandoned?"
A. If you create an unvented conditioned attic, then you can't use a whole-house fan that is mounted in your attic floor. However, if you want to have an attic fan, you can buy a whole-house fan that is designed to be installed in your sloped roofing assembly.
Thank you Dana and Martin, once again! I have lots to ponder. I am grateful for your input!