Tag Archives: R-19 insulation

Blown-In Fiberglass Attic Insulation

Blown-In Fiberglass Attic Insulation

In Climate Zones 3 and higher blown-in fiberglass attic insulation is extremely popular due to lower investment cost and high performance.

Looking for a best solution for your barndominium or post frame attic? It is inevitable an insulation contractor will warn you away from blown-in fiberglass due to a dated study proving fiberglass insulation loses up to half its R-value due to internal convection. It is important to understand this study’s results.

Titled Thermal Performance of Fiberglass and Cellulose Attic Insulations, this paper describes research done by Kenneth E. Wilkes and Phillip W Childs at Oak Ridge National Laboratory 30 years ago.

Wilkes and Childs set up an attic test module simulating temperature differences across an insulated attic floor.  Basically they put a whole roof and attic assembly into big chamber and measured R-values of three insulation types:

  • Loose-fill fiberglass
  • Fiberglass batts
  • Loose-fill cellulose


They kept temperature below the ceiling drywall at 70° F and varied the exterior temperature from 45° F down to -18° F.   Here’s what they found:

  • Fiberglass batts and loose-fill cellulose performed as expected at a whole range of temperature differences. 
  • Loose-fill fiberglass showed a significant reduction in R-value as the attic got colder and the temperature difference got larger. 

Loose-fill fiberglass lost 35% to 50% of its resistance to heat flow at temperature differences of 70° F to 76° F.  This loss of R-value started at a temperature difference of about 32° F.  With temperature below ceiling drywall held at 70° F, R-value started dropping when attic temperature was reduced to 38° F and had lost 35-50% when attic temperature got to 0° F and below.

In looking at this data, researchers saw a pattern leading them to suspect convection within insulation as the culprit.  They did some calculations and further experimentation and concluded this was indeed what was occurring.  Further experimentation they did was to put a covering layer over the loose-fill fiberglass top.  They tried both a polyethylene film and fiberglass blanket combination and (2) R-19 fiberglass batts.  Both eliminated convection and reduction in R-value.

If our story ended here, the lesson learned would be to avoid loose-fill fiberglass for attic insulation or use it with a covering layer.  But there is a Paul Harvey….

If you read this paper and think about what they did and what they found, a couple questions might occur to you.

Why would loose-fill fiberglass and fiberglass batts behave differently in an attic?  They’re made with the same material and were of similar density.

Is fiberglass made and installed now the same as it was back 30 years ago when they did this research?

This Oak Ridge paper doesn’t say what brand of loose-fill fiberglass insulation they used, but at least two fiberglass insulation manufacturers have written technical bulletins about their product and shown data about measured R-values under conditions similar to those studied. 

Density of fibrous insulation materials is certainly an important factor.  But, fiberglass batts and loose-fill insulation used were of similar density.  Batts were 0.46 to 0.48 pounds per cubic foot (pcf) and loose-fill ranged from 0.40 to 0.56 pcf.  So density doesn’t explain any discrepancy.

What does explain it, according to Owens Corning and Johns Manville, is chunk size.  Fiberglass batt or blanket insulation is one large chunk with a lot of glass fibers bonded together.  Thirty years ago, Owens Corning loose-fill fiberglass was made by taking their fiberglass blanket insulation and cutting it into little cubes. Johns Manville doesn’t say how they were making loose-fill fiberglass then, but they do say they used these research results to establish design specifications for all of Johns Manville’s loose-fill fiberglass attic insulations to improve winter thermal performance.  This led Johns Manville to maintain an appropriate nodule or tuft size, decreasing installed insulation air permeability. 

 

As explained in Owens Corning’s bulletin, “The bonded cubes did not nest well, leaving voids of relatively large air spaces and allowing R-value depleting convection to occur.”  This is why older loose-fill insulation had a problem with convective loops.  And it’s why modern loose-fill fiberglass product doesn’t.  They now use smaller chunks, nesting well together.

In summary, researchers at Oak Ridge National Lab found loose-fill fiberglass insulation 30 years ago had a problem.  As the attic temperature dropped, so did R-value.  It happened only with loose-fill fiberglass insulation they tested, though.  As a result, fiberglass insulation manufacturers took a good look at their product and found by using unbonded material in smaller chunks, this problem went away.

Sometimes people (usually those who sell other types of insulation) will refer to this Oak Ridge study as proof fiberglass doesn’t work at all, ever, in any circumstances.  This has always been an exaggeration because a flaw was found only in loose-fill fiberglass used in horizontal installations on an attic floor.

Manufacturers say they have eliminated this problem altogether by improving their product and their research proves they’ve gotten rid of this problem.  A lot of people know about this Oak Ridge study from something they heard from someone who heard it from someone else who heard it from their boss who talked to someone who learned about this at a conference in 1994 (Hmmm – sounds like social media).

Is your new barndominium in Climate Zones three and greater and have a dead attic space? If so, then loose-fill fiberglass insulation is going to deliver results you can depend upon.

Should Poly Plastic Barrier be Used on Interior of Walls and Ceiling?

Reader JUSTIN in MONROE writes: “Hello. Hopefully an easily answered question? I have built a 52×30 post frame, steel siding and roof. Walls have Tyvek between steel and girts. Roof is steel directly on purlins with no barrier of any kind. It has a concrete slab and I plan to periodically heat it during winter months. I’d like to insulate but not sure of best method with my situation and climate. I plan to use R-19 for walls and possibly ceiling. Or blow in for ceiling. Also I have 50% soffit ventilation with 18″ overhang as well as 40 ft of ridge vent. Should I use poly plastic on interior of walls and ceiling? I’m concerned I will create a moisture problem. I’m open to doing things whichever way is best. Things are always easier and cheaper to do it correctly the first time. Any advice would be greatly appreciated. Thanks”

Dear Justin,

housewrapI agree things are always best when done correctly the first time around. While it is not always less of an investment, when the long term problems arise and things have to be corrected, it makes it nearly not as fun and cheap becomes expensive. Usually in a quick hurry.

If the roof trusses are not designed for at least a five pounds per square foot bottom chord dead load, you are sunk on adding a ceiling without an engineered truss repair. This would be the place to start, as it will dictate the solution.

I will approach the building as if it is my own and from where it is now.

On the floor – I am hoping you have a vapor barrier beneath the concrete slab. If not, use a high quality sealer on top of the floor.

A penetrating concrete floor sealer is likely the best bet to protect and maintain a concrete floor. These concrete floor sealers penetrate deep into the concrete’s pores coming into contact with the alkali and calcium ions, forming a gel.

This gel expands filling the pores and hairline cracks inside the concrete, turning the concrete into a solid mass. This process will prevent moisture and vapor migration up through the concrete floor, as well as down into it.

Look for a penetrating concrete floor sealer which is water based and says silicate penetrating solution on the specifications. These sealers can be applied with a pump up sprayer.

On the Walls-you did good with the Tyvek. Kudos! If your building has girts flat on the outside of the columns, you can add another set to the inside of the columns. If you have 6×6 columns, your post frame building will now have an 8.5 inches thick insulation cavity. I would use BIBs (read about BIBs here: https://www.hansenpolebuildings.com/2011/11/bibs/) for my wall insulation, and would have a deep enough cavity to get around R-35. There does need to be a vapor barrier on the inside (heated) side of the wall, under the gypsum wallboard.

Roof– the underside of the roof steel needs to be isolated from any warm moist air which would enter the attic. Use closed cell spray foam directly sprayed directly onto the underside of the roof steel. Assuming your building’s roof trusses are strong enough to support a ceiling, blown in insulation is going to be your most economical. Hopefully you (or your builder) had the foresight to order roof trusses with a raised heel so the insulation will remain full thickness from wall to wall. If not you may want to have closed cell spray foam insulation on the “cold” side of the ceiling in the area with a couple of feet from the sidewalls. Make sure to allow a provision for air in the overhangs to not be blocked from venting the attic.

Do not put a vapor barrier between the trusses and the ceiling. You want the warm moist air inside your building to be able to rise into the attic and be vented out through the ridge. And if you are going to insulate your ceiling, R-19 is really not near enough. At a minimum I’d think about R-38 or 45 blown in.

Thank you for allowing me to share some insight into insulation.

Mike the Pole Barn Guru

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