The R value of an insulating material is a measure of its resistamce to conduct
heat R = (T1-T2)/Qa. This is the temperature differential across an
insulator devided by the heat flux or heat transfer per unit time.
Qa is the quantity of heat.Fiberglass insulation has an R factor of about 3.
heat would be transfered through a squar foot of fiberglass insulation 1"
thick if the temp on one side of the insulation were 60*F and the
temperature on the other side of the insulation were 30*F
ANS: Qa =
(60-30)/3 = 10 BTU/hr.
Fiberglass insulation is inexpensive and commonly
available, but if it's compressed or if it gets damp the thermal resistance
Vapor Barriers prevent moisture
inside the house from condensing on the cold fiberglass surface. Many
insulating materials are paper faced or foil faced. This helps, but it is
best to cover the fiberglass with polyethylene film. The extra cost and
extra time it takes to staple plastic on the studs
is worth the effort.
Attis normally require twice as much insulation as walls BUT where in the attic
should the insulation be placed?
Some people place attic insulation
on the underside of the roof to keep heat inside in winter and outside in
Unfortunately blocking the flow of air against the insided of
the roof could cause an overheating problem and damage the roof shingles.
The best place for insulation in an attic is between the floor joists. Fiberglass or celulose
or even foam insulations could be used.
Most contractors prefer blowing cellulose insulation between attic floor joists rather than stuffing
fiberglass batts in that tight area. Celulose insulation may be layered to increase the
R factor, BUT if the home owner wishes to place a floor over the floor
joists the layer of celulose should not be excessive.
The R factor of
cellulose insulation per inch is only slightly higher than the R factor of
fiberglass but it's easier to work with in most cases.
Foam insulation covers most of the voids between studs and it has about twice as much thermal resistance as fiberglass...
It's what they call a super insulator..
BUT it is expensive. Super insulated houses conserve heat energy but they
also intefear with the normal air infiltration that most houses require.
contact for more information