Materials and R-value
Fiberglass Batts
Pre-cut glass fibre sections fitted between framing. Precise where the cavity is regular, and unforgiving where it is not.
Thermal resistance
What the R-value actually is
Range
R-3.1 to R-4.3 per inch
Design value used
R-3.2
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 product density chosen, and above all whether the batt is compressed during installation.
A batt squeezed into a cavity smaller than its thickness loses part of its value. The number on the package is the number for an uncompressed batt.
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
- Regular, accessible cavities where a batt can be fitted without compression
- Attic floors with consistent joist spacing and few obstructions
Not the right choice when
- Irregular attic geometry, where the gaps at the edges of each batt add up
- Any assembly where the batt would have to be compressed to fit
- As an air barrier — a fitted batt still allows air through the gaps around it
Beyond the number
Moisture, fire and sound
Moisture
Same behaviour as blown glass fibre: it does not absorb the way an organic material does, but it does not perform while wet.
Fire
Naturally non-combustible. Source: NAIMA Canada.
Sound
Commonly used in interior partitions for sound, but no independent figure is published here for this format.
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
Why would batts be chosen over blown-in for an attic?
Where the attic floor is regular and accessible, batts give a predictable thickness that can be inspected by eye afterwards. Where it is not regular, blown material follows the shape better and leaves fewer edge gaps.
What happens if a batt is compressed to fit?
A compressed batt loses part of the value printed on the package. The rating assumes the batt sits at its full loft, and the trapped air is what does the insulating, so a batt squeezed into a shallower cavity performs closer to the depth it actually occupies than to the number on the label. Where the cavity is not the standard depth, the honest answer is a different product rather than a compressed one.
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.