Adding new insulation on top of old
Rigel
| Posted in Energy, Heating & Insulation on
I would like to add more attic insulation. What I have now is about 8 inches of what looks like very finely shredded bark, loosely filled. It is not vermiculite. The house was built (here in Idaho, where there are lots of lumber mills) in the late 70’s. What can I put on top of the existing insulation? Also, is there a way to calculate the effect of adding more insulation to what is existing? Thanks.
Replies
The shredded bark would have an R value of 1.5-2.5 per inch so you have R12-20, depending on how finely shredded and its density. Today's attics in your area should have at least R40-R50 in attics if space allows.
You should consider blowing in celulose on top of what you have.
You can add virtually any type of insulation that you wish, on top of the existing insulation. The thermal resistance of the new insulation is directly additive to that of the existing; e.g.. if you presently have an R of 20, and you add another 20, you'll end with an R of 40.
What insulating materials you might consider is largely dependant on your ceiling/roof framing system. If its roof trusses, you're very much stuck with a blown-in medium, whereas if its conventional joists and rafters, you've got a choice of either blown, batt, or board. As the joist/rafter system is open and free of truss verticals and diagonals, it better lends itself to DIY. For ease of installation and subsequent attic space access, I have used 4x24x48" expanded polystyrene boards laid across the tops of joists, and as additive to batt or blown insulation between joists; these slabs can be readily slid or "thrown" in the direction needed as the work progresses, they are capable of bridging my weight between joists and thus allow me ease of movement within this space, and there is no problem with the release of aiborne fibers. My 2nd choice for DIY installation is "Roxul", a rockwool batt which is nonflamable and does not suffer loss of physical properties when wet, (as with fiberglass and celulose).
Forget about the foam boards over loose fills. This is an area in which the "Law of Diminishing Returns" is really working against you (meaning its payback will be longer down the road, maybe 15-30 years since foam is our most expensive insulation). Blow the cellulose (one of my favourites) where the payback will probably be under 10 years. The cellulose has a feature the others don't: as it settles (about 15%) and becomes denser, it restricts/slows air leakage, heat loss and moisture movement from below; the others aren't as good at this!. Any attic today show have a blown insulation installed unless there's a real good reason for otherwise (I can't think of any now except that you're going to remove batts for use elsewhere later???)
"My 2nd choice for DIY installation is "Roxul", a rockwool batt which is nonflamable and does not suffer loss of physical properties when wet, (as with fiberglass and celulose)."
Fiberglass is inflammable and cellulose is tested for flame spread rating so it is fire resistant. Rockwool will hold moisture also and lose R value as will the others.
Side note: There must be a huge urban myth floating around out there about insulation and moisture that goes something like this: "Put insulation in your house and it surely will get wet!" This has to be since about 50%+ of posters are talking about the moisture and how one insulation is better than others when wet ,or it won't get wet. or..........
I have been working in and around the insulation/energy industry for 28 years now (In the home inspection/consulting field now- http://www.ahi-ns.ca ). I have seen thousands of attics and only 3 or 4 had any amount of moisture in the insulation. These were all houses that had other interior moisture problems such as unvented dryers/ kitchens /bathrooms, had wet/damp firewood stored in the basement, had wet/damp basements from poor drainage or no moisture barriers over soil floors, had house full of plants/ aquariums, etc.
LONG STOREY: By chance on Thursday, I had 2 complaint calls from unsatisfied customers of insulators (one a blown fiber and the other a spray foam product). For the first time in years, I saw water in insulation- it was coming through the holes drilled for sidewall installation of cellulose. These were under cedar shingles that had been removed and re-installed after filling the cavities. Before the shingles were re-installed, the 2+1/2" access holes were filled with low expansion foam.
The problem was with the re-installation of the shingles. For removal, the shingles to be removed are cut along the upper overlap with an upward bevel cut using a sharp utility knife. A little leverage from a wonder bar snaps them off. The re-installation is simply to slip the top edge in under the bevel cut, tap it upwards for a tight fit, and then nail the lower corners about 3/4" - 1" in from edges. I've done this for years with no moisture problems ever reported.
In the last few days we've had two high wind rain storms with 60-80 mph gusts and horizontal rains (at different times over 3 days, I lost power, internet, phone service and maybe a computer-this is a new one!!!). I've investigated wind driven rain entry before where water has actually moved upward over flashing due to wind. So I thought due to wind, the water may be moving in from the shingle side edges to the holes. Yesterday I investigated by removing shingles and found that the water was coming from the top edges that had been cut; the side edges were essentially watertight since they had been painted after the insulation job. This company was not tapping the removed shingle tightly up to butt against the cut edge at the shingle overlap. In some places, I found about 1/16 or so gap- enough for water to move through driven by high winds. Here it ran down to the foam filled hole and into the cellulose filled wall cavity in instances where the foam had not "sealed" the holes.
The owners had aleady removed plaster to expose a couple of cellulose filled wall cavities. The water had wetted or dampened about 1-1+1/2" of cellulose at the sheathing for up to 12-15" down at the worst. The solution; seal the tops of all cut shingles with caulking and let the walls, where damp, dry natually. Heat will slowly be moving outward all winter and drying the cellulose. The borax will prevent mould in the cellulose and since it's soluble, some will be transfered to the wood to prevent rot- a nice quality.
If this were a foam, fiberglass or rockwool retrofit, I'd be a little concerned about the water causing mould or beginning rot before fully drying. None of these have mould fighting additives and if water got into the wood next to them what would happen? Maybe nothing as all will dry before anything happens but I feel better with the treated cellulose!