Roofing Felt and Vapour Barriers: Avoiding Specification Confusion
Water resistance and water-vapour control are different requirements. Do not describe paper-based roofing felt as a tested vapour barrier simply because the sheet contains asphalt.
By Merily Waterproofing · Bulk supply for importers, distributors and building-material wholesalers
Ask which moisture movement is being controlled
A project may be concerned with liquid water, water vapour or the role of a layer within an assembly. Those are not interchangeable questions.
Obtain the actual requirement from the responsible designer rather than selecting by a broad word such as “waterproof”.
Request the relevant evidence
If vapour performance is required, identify the applicable test method, value and product scope. A roll weight, label photograph or unrelated report cannot establish it.
Merily does not publish an untested vapour-permeance value for the paper-based felt described here.
Keep the purchasing conversation useful
Record the intended layer and performance requirement. If the available product evidence does not address it, leave suitability unresolved instead of inventing a reassuring answer.
The buyer can then obtain technical confirmation before the product is offered for that use.
Put it in your buying record
| Requirement | Evidence needed | Not enough |
|---|---|---|
| Liquid-water role | Specified assembly and relevant data | A generic product name |
| Vapour control | Applicable method and product value | Roll mass alone |
| System compatibility | Design/product confirmation | Visual similarity |
| Purchase approval | Exact item and evidence | Another material’s report |
Questions buyers ask
Is every asphalt felt a vapour barrier?
No such blanket claim should be made without the relevant product evidence.
Can a waterproofing report answer vapour performance?
Only if it actually covers the required property and offered product.
Who should confirm the assembly?
The responsible project technical party should review the requirement and evidence.