Replies

1. 
   user-974013 | Oct 26, 2011 01:43pm | #1

   Cellulose yes, Spray foam no. i will explain just what you want to do. First about me, our company won a Century Club Award last year (Complete over 100 Home Performance with Energy Star jobs with in a year), and i have done (guestimate) about 200 houses in the last 2 years this way.

   You can install cellulose over fiberglass batts, and as a contractor I would not remove insulation to add more. This would cost you more money and honestly is not needed unless you have water damage or rodent problems in your attic (they tend to use fiberglass as a rest room!!)
   Here is the thing, when you read about cellulose you will see that it draft blocks pretty well (top plates in your attic) BUT you should never insulate (with any type of insulation except spary foam) without air sealing your top plates first. Some blow and go companies will tell you that you do not need it....you do and it can save you 5% off your utilities or more, plus the safety and indoor air quality perks you receive from air sealing. When there are fiberglass batts in your attic the cellulose will not draft block the top plates. It will however fill in all the gaps that fiberglass batts leave along side of your ceiling joists ("if you see wood its no good"), and add the to your R factor (cellulose is second best with spray 2 part high density spray foam as first in my book).
   Air sealing can be done by using single part spray foam at about a 1/4 inch bead (caulk and silicone can be used as well, but i stick with 1 part foam), any chases should be done with intumescent caulk and tin around flue pipes (and a dam to prevent the insulation from touching the flue go with 12 inches to be safe), stairway slants use owens corning fan fold and foam (wood, cardboard,etc).
   Do not forget to add baffle vents to your soffit area, do 100% so insulation does not touch your roof deck!! If you have proper soffit venting it most likely is vented 100% so you will need to install the baffles anyway!
   Good Luck!
   Frank Bovio

2. 
   user-974013 | Oct 26, 2011 01:46pm | #2

   Sorry, also if you want to do spray foam do 3 inches (or more) of 2 part high density spray foam on your roof deck. Stop any venting from your soffit and make sure to foam the soffit area as well. Than is the only time you want to remove insulation. If you have access to a supply and return duct from your HVAC install 1 to 2 of each in the area. Out of experiance do not waste the money on the spray foam by doing your attic flat!! Do the roof deck and gable walls. Make sure to disconnect the attic completely from the outdoors (gable vents, soffit vents, attic fans, and ridge vents must be encasplated!!)
   Frank Bovio

3. 
   user-723121 | Oct 26, 2011 07:22pm | #3

   Good advice from Frank, now is the time to address the attic bypasses, you will notice the increased comfort this winter.

4. 
   jtlloyd | Oct 26, 2011 09:35pm | #4

   You can easily install cellulose over fiberglass blankets. I would avoid the use of spray foam, you don't want to change the location of your insulation. And like Frank said, air seal the attic first. Seal up those open soffits, vent stacks and wire penetrations. You want to make the ceiling as air tight as possible. This work could potentially save you more money on your heating and cooling bills then adding the extra insulation because you are trapping the air that you have already conditioned in the home. Remember, fiberglass and cellulose is only an air filter, it slows air movement it does not stop it.

   If you are going to do the work yourself, go to the Energy Star website and look for the DIY to insulating and air sealing your attic guide. Great information on how to air seal and what to look for.

5. 
   boykob | Oct 27, 2011 07:24am | #5

   I'm making an assumption that this is cold climate.

   Another critical reason for good sealing of the attic pressure plane is moisture vapor transport. I've done many investigations of moisture damage and mold on the roof deck after adding insulation without addressing air selaing of the ceiling plane. The added insulation actually works thermally and makes the roof deck and framing cooler than in the past. Just a few degrees difference can cause phase change from vapor to liquid. The added cellulose, although adding value thermally, is still very vapor permeable and will quickly transport interior humidity via infiltration across the pressure plane into the attic space.

   The good news is that you can easily identify exfiltration points that need air sealing in the attic plane by lifting the fiberglass batts, if they are unfaced. The exfiltration points will be obvious by the dark areas on the batt where the exfiltrating air has been filtered.

   The message is: always air seal when you have the opportunity.