Materials and R-value
Blown-in Fiberglass
Loose glass fibre blown to depth across an attic floor. The most common way to raise coverage in an existing attic without opening anything up.
Thermal resistance
What the R-value actually is
Range
R-2.7 to R-3.6 per inch
Design value used
R-2.9
Natural Resources Canada, Keeping the Heat In, Section 3. Read the source. Consulted August 25, 2026.
This is a range, not a single number, and the range is the honest form. The value you actually get varies with the density it is installed at, the depth achieved, and whether it stays where it was placed.
The design value sits near the bottom of the range because installed density in a real attic is rarely the density of a laboratory sample.
R-value per inch is one property among several. Moisture behaviour, fire and sound are below, and the assembly decides which of them matters most. Compare the six on the Materials & R-Value guide.
Where it fits, and where it does not
Appropriate for
- Open attic floors where depth can be built up evenly
- Irregular framing and areas that are awkward to reach by hand
- Topping up an existing layer that is dry and in good condition
Not the right choice when
- Attics with an unresolved moisture source — the material is not the problem, the water is
- Where wind washing at the eaves would move it, unless baffles are in place first
- As a replacement for air sealing: it slows heat, it does not stop air
Beyond the number
Moisture, fire and sound
Moisture
Glass fibre does not absorb water the way an organic material does, but wet fibreglass loses its thermal value while it is wet and does not recover its position once compressed.
Fire
Naturally non-combustible — it is glass. Source: NAIMA Canada.
Sound
No independent measurement was found for this format, so no acoustic figure is published here.
Services that use it
Problems it helps correct
Deciding, rather than specifying
This page is the technical side — the numbers, the limits and where the material is the wrong choice. If you are weighing it up for your own house, the guide covers that:
About this material
Questions we get on this one.
Tap a question and the answer comes back. Anything specific to your project, we sort out on the call.
Eco-Therma
Usually replies the same day
Does blown-in fiberglass settle over time?
NAIMA Canada describes virtually no settling for blown fibreglass, which is one of the reasons the installed depth can be relied on later. What does move it is air — wind entering at the soffits can push it away from the eaves if baffles are missing.
Can it go on top of what is already there?
Only if what is already there is dry and sound. Adding depth over material that has been wet, compacted or contaminated hides the condition instead of correcting it, and the new layer ends up in the same state.
The attic decides which material, not the price list.
Send the address and roughly how big the attic is, and we come back with a range and the material it is built on.