Insulation reference
Compressed insulation: can you still use the label R-value?
Find the information needed to assess compressed insulation. Understand why scaling label R by the thickness ratio is not a general correction method.
Short answer
Do not assume that a batt compressed into a shallower space retains its full label R-value. Do not assume that R changes in direct proportion to thickness either. Use product-specific performance for the installed thickness, or record that the installed R-value is unknown.
On this page
Prepare your inputs
- Exact product identity, labelled R unit and original thickness.
- Measured installed thickness and the location of gaps, folds or voids.
- Performance evidence applicable to that product in its installed condition.
Compression changes more than the thickness number
For a homogeneous layer at fixed conductivity, R = thickness / conductivity. Compressing a fibrous product can also change density and the air spaces that influence its thermal performance. Holding conductivity constant is therefore an assumption that needs evidence, not an automatic correction.
A hypothetical 150 mm product fitted into a 100 mm space has a thickness ratio of two-thirds. That ratio alone does not establish that its installed R is two-thirds of the label, nor that the original label value remains valid.
Identify the product and the installed condition
Record the manufacturer, exact product, label thickness and label R unit. Measure the available cavity depth and installed thickness at representative locations. Gaps, folds, voids and an uneven fill are separate defects; one average thickness does not describe all of them.
Look for a manufacturer table, test report or technical assessment that actually covers this product at the compressed thickness. A table for another density or product family is not an interchangeable correction factor. Preserve the document's conditions and scope with the result.
Enter a supported installed value, not an invented correction
When a suitable installed R is documented, select its SI or US unit in the relevant calculation. When only conductivity applicable to the installed condition is available, use it with the actual thickness. These are alternative input routes, not two resistances to add for the same material.
If the evidence is missing, label the thermal result as unresolved. You can compare explicitly hypothetical inputs to see sensitivity, but such a comparison does not establish the product's real installed R. Thermal bridging and air movement still need their own assessment.
Reference table
Choosing evidence for an installed insulation layer
| Information available | What it supports | What remains unresolved |
|---|---|---|
| Label at original thickness only | Identification of the uncompressed product | R at a different installed thickness |
| Product table at installed thickness | The stated performance within the table's conditions | Conditions outside that table |
| Applicable installed conductivity and measured thickness | A layer calculation under those stated assumptions | Gaps, parallel paths and air leakage |
| Thickness ratio only | A geometric comparison | A verified thermal correction |
Common mistakes
- Scaling R solely by the thickness ratio while silently holding conductivity constant.
- Using another product's compression table as a universal correction.
Check before using the result
- Check that the evidence covers the installed thickness and density.
- Preserve an unknown result when installed performance is not documented.
- Assess thermal bridges and air-barrier continuity separately.
Method limits
No universal compression factor, installation recommendation or compliance decision is provided. The cited historical installation guide establishes the issue, not the present performance of a named product.
Questions and answers
Can I scale the label R by the thickness ratio?
Only if the required assumptions are supported by applicable product data. The thickness ratio by itself does not establish a thermal correction because compression can also change material properties.
Can I use another insulation product's compression chart?
Do not assume it applies. Verify the product family, density, thickness and test conditions or retain an unresolved installed R-value.
Sources and scope
- ENERGY STAR — Thermal Bypass Checklist Guide, 2007
Historical installation guidance on compression, gaps and air barriers; not a compression-factor table or current compliance checklist.
- NIST SP 811, Appendix B.9 — thermal conductivity and insulance units
Unit definitions and conversion factors; no product performance or required insulation thickness.