Replies

1. 
   paul42 | Aug 03, 2006 12:53am | #1

   Except for panes of glass, building materials either reflect or absorb radiant heat.  It turns out that fiberglass insulation absorbs it.  Because it absorbs the radiant heat, the temperature inside a fiberglass bat in your attic can actually be higher than the attic temperature.

1. 
   experienced | Aug 03, 2006 01:19am | #2

   But wouldn't blown cellulose or foam also absorb the heat since they don't have reflective surfaces?

2. 
   Piffin | Aug 03, 2006 01:46am | #3

   I'll adress the convection issue. It is covered and documented in "Walls that work" fromn Corbond. They have specificly tested the issue with independent labs.

   Typical R-values are established for insulated walls by measuring differential temps and heat lost with differences of only about 20°F . But when insul;ation is needed is when the differential is more like 50-70 degrees. With such a wide variation between inside and outside, a convection loop sets up within each studbay. The heat contained in the outer portion cools to the outside by radiation and the cooler air sinks to the bottom by virtue of it's hnow higher density. Air nearer the interior wall is heated and rises. The loop is established and heat is lost to the outside.

   Faom or dens packed cellulose or blown FG are much more efficient at fighting this action.

    

    

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1. 
   experienced | Aug 03, 2006 06:40am | #4

   Do you have that paper?

   I just looked at their website and find that they too appear to be "stretching the envelope of truth" or have asked that a specific wall that they "designed to fail" be tested against theirs. When a company sponsors a private test to promote a product, I'm always quite sceptical! This is what the P2000 insulation folks did.

   From personal experience since mid-May:

   The P2000 (1" type 1 beadboard with foil = R27!!!!) folks have just had a complaint lodged against them with the federal government here in Canada. Their ads that were running locally and claiming the above R27 value have now been changed to claim no R value but just refer people to their website. This is after they had used Intertek to test their product to their own company specs but not to  tests like ASTM 518 that have insulation industry consensus acceptance.

   Now Intertek is no small cookie - 556 offices worldwide and 307 labs-  but they have allowed a private letter from them to a competing insulation manufacturer to be used to help the case against P2000 since the non standard testing was not to be released to the public or nor used to promote P2000 products.

   I worked with my old gov't dept and the chief building code official here on developing the case against P2000.  As well, had a few e-mails and discussions with the editor of Energy Design Update on this issue. That's why I would like to see the corbond versus fiberglass tech paper on the batts.

   If a fiberglass batt performs so poorly in extreme conditions, why are our low energy R2000 homes (a government program here and no relation to P2000- the similarity a coincidence?.....I don't think so!!!) performing so well in the great white north with temps down to -25 to -40F???? The Build America program in which Loe Lstiburek's Building Science Corp is a major partner still allows batts in their homes. Are these knowledgeable people all duped by the glass companies and should look at the Corbond site??

    

1. 
   moltenmetal | Aug 03, 2006 07:30pm | #5

   When you do the heatloss calculations, the major losses remain air infiltration and glazing.   Frittering away huge amounts of effort and energy and money to obtain better basic R values for the wall and ceiling units themselves without addressing infiltration is basically a waste. 

   Unless you address air infiltration very carefully during construction, batt insulated assemblies will suffer greatly in REAL heat loss performance terms, and it would be costly to retrofit to fix these sorts of mistakes after the fact.  Sprayed foam and blown cellulose are superior from that perspective- they give you tighter construction from an infiltration point of view even if you do a bad job with your vapour and air barriers.  The surety of infiltration reduction alone may pay for the higher installed cost between batts and the other methods of insulating.  Or not.  It depends how badly the job is done in the first place.

   As to the presence of natural convection currents inside conventional fibreglass batt-insulated stud bays, there is definitely a theoretical basis for this phenomenon.   Real wall assemblies commonly do not meet the requirement to have continuous contact between the batt and the sheathing, so that certainly leaves an air gap for free convection to occur.   Whether sufficient evidence of significant thermal performance degradation due to natural convection within the batts themselves, based on valid tests of realistic wall assemblies, is a matter of debate.  Personally, what I've seen has convinced me of the significance of this effect with relation to ordinary fibreglass batts, especially at high differential temperaures.   I'm also sure that this effect would be greatly reduced by using a denser product such as Roxul mineral fibre batts.  

2. 
   Piffin | Aug 04, 2006 12:19am | #6

   Could be.have you ever lived a winter in a foamed house? The experience will make a believer out of youContact Corbond, they have the papers available for distribution. Another area they have been helpful in is convincing AHJs that vented attics are not necessary when insulated properly with a Urethene faoam 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!