I recently had blown-in cellulose put in my attic. Can I install batt insulation over it to increase the R-value?
Ranch house built in 1954 -- original batts still in, but shrank to under 3". Decided to blow in the cellulose over it (in 2008). I'm not 100% convinced that I have a high enough R-value still. I believe that 8" of cellulose was blown in.
In Rhode Island, you probably want a minimum of R-38 in your attic. (Check this Energy Star website for their recommended R-value in your area.)
- If you have fiberglass batt that has been compressed to under 3", you are not getting much value out of that.
- Blown-in cellulose typically has an R-value of R-3 per inch -- so with 8" you only have an R-24 plus whatever the fiberglass may add to it, so you are on the low side for sure.
- You can install batts on top of the cellulose or blow more cellulose on top of what you have -- either option will help your performance.
One important thing to do prior to installing more insulation is to do some air sealing in your attic at the ceiling level.
- Look for all joints at wall intersections, pipe and wire penetrations, gaps around chases, chimnies and chases and soffits in the home and seal these with spray foam.
- If your insulation contractor does not understand what you mean by "air sealing," find a home foam insulation installer with some building performance training from BPI (Building Performance Institute).
For more information:
Read Jesse Terzi's Q&A "Should attic insulation extend into the eave soffits?"