Want to add more attic insulation

[burleymike]

My house has 5.5" of 52 year old rockwool and about 6" of fiberglass on top of that. In many places the fiberglass is quite compacted from people working up there. I would like to add more to bring it up to R-50.

I have thought about getting one of those vacuum trucks that does duct cleaning to come out and remove all the old insulation since I don't know how well the old rockwool insulates. We just had it tested for asbestos because it has a lot of suspicious debris in it. The lab said it is 10% wood chips, 2% mica and 88% rockwool, I thought the mica could be vermiculite.

From what I have read fiberglass will not stop air movement but rockwool will. Would it be advisable to blow more rockwool on top of the fiberglass or would it's weight just compact the fiberglass making the whole thing an exercise in futility?

[Gary Swart]

Here's a lesson I learned about insulation. I built a new garage/shop several years ago and insulated the walls with 6" batts. I thought I would buy bags of insulation from the local Big Box Store because the provided a blower free when you bought the insulation from them. But, before doing this, I talked to a local insulation company. They provided two men, one to feed the blower and one to direct the insulation in the attic, and the insulation for a little less than it would have cost for the Big Box deal, plus I didn't have to get someone to feed the blower and I didn't have to get covered with fiberglass. I question the need to remove the old stuff, it is likely doing some good so just beef it up. At least check it out with a professional insulation company.

[Dana]

My house has 5.5" of 52 year old rockwool and about 6" of fiberglass on top of that. In many places the fiberglass is quite compacted from people working up there. I would like to add more to bring it up to R-50.

I have thought about getting one of those vacuum trucks that does duct cleaning to come out and remove all the old insulation since I don't know how well the old rockwool insulates. We just had it tested for asbestos because it has a lot of suspicious debris in it. The lab said it is 10% wood chips, 2% mica and 88% rockwool, I thought the mica could be vermiculite.

From what I have read fiberglass will not stop air movement but rockwool will. Would it be advisable to blow more rockwool on top of the fiberglass or would it's weight just compact the fiberglass making the whole thing an exercise in futility?

Putting more insulation on top may compress the goods below, but that doesn't take it's R-value to zero- the compressed layer will have about the same R value per-inch, just slighly fewer inches. It's the settled/compressed depth that determines the R-value of the layer, but it'll still have the same R/inch. A 6" layer of blown low density fiberglass is worth ~ R18-19, but compressing it to 4" still yields ~R12-13. It won't start going negative on a per-inch basis until it's an inch or so (which would take quite a bit of weight.)

Rock wool is higher density, and does outperform fiberglass on convection losses, but still undeperforms cellulose in that regard. No fiber insulations "stop air movement", but on a good-better-best in open-blown goods, cellulose wins in the the air-retardency category. Blowing as little as 3" of cellulose over low-density fiberglass is enough to "restore" it's convection-related losses in R-value. If you use "stabilized cellulose", it contains an adhesive that keeps it from settling, or being moved around by high-winds through the soffit venting as well. JM Spider super-fine fiberglass has a similar adhesive, but needs to be installed at 1.8lb/cubic-foot density

A combined 10-12" of rock-wool + fiberglass yields something on the order of R30-R35. Blowing 6" of cellulose, or whatever it takes to get a uniform depth of ~17-18" would get you to R-50. Both rock wool and old-skool blown fiberglass yield ~ R2.5-R3.5/inch of depth, cellulose will give you at least R3/inch in a low-density open-blow (more like R3.5/inch or more, in wall-cavities at higher density.) At 18" of total depth, you're there, with a little bit of margin for settling/compressing over time.

Many installers will use a series of cardboard gages distributed around the attic as a guide, then rake it all smooth at the end.

It is true that the pros will often install it cheaper than a DIY, but there are good/better/best installers, and "fluffing" fiberglass for higher depth/lower density was an all too common practice in some regions 20 years ago (don't hear too much about it today). When they do that the total R is lower, and the convection losses even higher- packing that down with a higher-density overblow IMPROVES the performance of a fluff-blow by quite a bit. It's a little harder for them to cheat that way with cellose or rock wool, but not impossible. Unlike fiberglass, fluffed cellulose doesn't take a very big initial performance hit, in R/inch but will lose considerable loft- like 25-30% in it's first 5 years of service, as opposed to 5-10% over 2 decades (or centuries) for proper-density goods. But stabilized-cellulose (aka "wet sprayed" ) doesn't have that issue- stays pretty much at it's initial installed depth unless you walk on it or something.

Before insulating it's always a good idea to air-seal the attic floor/conditoined-space interface, with spray foam/caulk/boxes over recessed lights, etc, since the as-installed performance of the insulation depends on it. You may need to pressurize/depressurize the house to find & fix them all, but you can count on there being quite a bit of air-flow into the attic on a previously-untreated house. Beyond the ubiquitouse nightmare leaks like recessed lighting cans, concentrate on plumbing, vent & flue penetrations, electrical penetrations, un-plated partition wall framing, etc. Hiring pros to air-seal the whole house with before & after blower-door verification of their work is probably more cost-effective than adding 6" of insulation in your situation.

[burleymike]

Thank you very much for all the detailed information that is just what I was wanting to know. I have been slowly gutting one room at a time and re finishing over the last 4 years. 80% of the exterior walls are not insulated with r-13 and a sealed polyethylene vapor/air barrier installed. As I have run new wiring I always sealed up every hole I made. I also used mastic to seal all the duct work in the attic and insulated it.

It is interesting to read about "fluffing" of fiberglass because that is what I would guess they did. The fiberglass up there is very light and just dragging an empty cardboard box over it will compress it in half. Once I get the rest of the wiring done I think I will go with cellulose. I will get a few bids from the two local insulation contractors and see if doing it myself will have any significant savings.

I need to put a boardwalk to the air handler so future service activities will not disturb the new insulation. Thanks for all the replies

[Dana]

Fluffed fiberglass is nearly worthless for R-value, and could take a century or more to fully settle. Compressing it with a denser-fiber overblow is the best thing you could do for it! If it drops to ~3" it'll still be worth R10-ish if it has a 6-9" cellulose overcoat.

To sweeten the pot you might ask the contractors to bid doing the remaining un-insulated walls as separate line-item. With dry-blown goods there is almost always a way to get it in there with minimal post-insulation repair to the siding or interior walls. Even if the walls are dry-blown at low density vs. "dense packed" (which would cost more- it's more labor + ~50% more material) insist on "borate only" or "sulfate free" goods. Sulfated fire retardents are corrosive to metals, should it ever get wet. Stabilized cellulose is ALWAYS a borate-only fire retardent, and can be dry-blown.

In cellulose insulated walls in your climate zone you should NEVER use a poly vapor-barrier. Cellulose is hygroscopic, and can buffer a lot of moisture without damage or loss of performance, but it needs to be able to release that moisture, and any incidental leak-moisture that gets into the walls. A sheet of poly cuts the drying capacity of the wall by about half. The most vapor-retardent interior finish you would need is latex paint, unless you live above 5000' in ID, in which case a modestly vapor retardent latex "vapor barrier" primer might be called for. If you have vinyl siding or have a rainscreen-gap under the siding, standard latex would do just about anywhere in ID. With rock wool or fiberglass batts in the wall a poly vapor retarder is overkill for ID, but if you didn't have in there, and alkyd paint or vapor-barrier latex primer could be used.

But FAR more important than vapor barriers, the interior wall surface needs to be air-tight into the stud cavity. A square inch of air leak will transport more moisure than an entire wall's worth of vapor diffussion would through even un-painted sheet rock. Caulking under the trim boards and foam-sealing the elecrical boxes to the poly, and myriad other details are necessary to get it fully "right". Blower-door testing after all the obvious stuff is taken care of is also a good idea. With the poly there it has zero drying capacity toward the interior, and air leaks in winter can potentiall put moisture in faster than it can dry toward the exterior- it depends on the size of the air leaks, and the type of siding. Rainscreened siding can cure a LOT of ills by maximizing the exterior-drying capacity. Moisture can get into a wall by any number of methods, but leaves primarily via vapor diffusion, which makes true vapor barriers like poly or foil a double-edged sword- if they leak air, they can bring moisture in to condense, but are still pretty effective at keeping it from escaping. Kraft facers on batts (or vapor-retardent latex primers) have about 10x the drying capacity of 6mil poly, but only allow ~1/5 the moisture in via vapor-diffusion as standard latex paint.

Dry-blown cellulose in walls at low density (aka "2-hole method") blocks convection currents and infiltration leaks by about 90% compared to R13 fiberglass batting, but if dense-packed to greater than 3lbs/ft^3 it's nearly (but not quite perfectly) air-tight for most practical purposes, and will yield a slightly higher R-value. On walls that don't have other air-barriers in place it can be worth the additional expense. If you went with a new-skool fiberglass blowing wool like Certainteed Optima or JM Spider, packing it to 1.8lbs density or higher works from a convection & infiltration stopping point of view, and you'd get ~15% more R out of it than with cellulose (~ R15 vs ~ R13 in a 2x4 stud cavity). But since fiberglass can't buffer the moisture you'd still have to put a vapor-retardent paint on the interior. At ~1.0lbs density those fiberglass wools allow considerable convection in the cavity, and deliver ~R13, but still outperform batts, since there are essentially zero compressions & gaps. Compressions & gaps are inevitable flaws in all real-world batt installations reducing performance by ~15-20% in a "typical" wall, making a full rated R13 batt perform closer to R10-11 in practical terms. Blown or sprayed goods usually come close to hitting their numbers in the real world.

Good call on sealing & insulating the ducts. Taping all seams on the air handler with FSK tape (the 2" aluminum tape used on metal ducts and foil-facers of rigid insulation, etc.) helps too. If the ducts get buried or partially-buried with the new insulation, so much the better!