How to Insulate a Tar Paper Covered Roof

The decision to insulate a roof covered with roofing felt is where expectations for thermal comfort, structural durability, and economic rationality converge. Unlike pitched tile roofs, where insulation naturally fits within the attic space, roofing felt typically covers flat or low-slope roofs—where every layer directly impacts waterproofing, load capacity, and moisture management.

The question isn’t whether to insulate—that’s already settled by building codes and common sense. The real question is: in what sequence, with which materials, and with what level of waterproofing accountability. A felt roof is a layered system where any modification affects existing layers and requires a carefully planned sequence of actions.

Decision Sequence Model: What to Establish Before Construction

Insulating a felt roof requires settling three things before any work begins—none can be skipped or postponed:

• Technical condition of the existing felt membrane—is it watertight, does it need repair, can additional layers be built on top. This isn’t a visual assessment—it requires professional verification.
• Insulation placement method—from above (on top of existing felt) or from below (from the interior side). This decision determines the technology, cost, and extent of structural intervention.
• Target insulation thickness—derived from thermal calculations and deck load capacity. It can’t be “adjusted during installation” because it affects parapet height, flashing details, and water drainage.

If any of these decisions is overlooked, you risk needing modifications—and on a flat roof, every modification compromises the waterproofing integrity of the entire system.

The Irreversibility Rule: What You Lose With Each Option

The choice of insulation method is economically irreversible. If you insulate from above, you forfeit the option of simple future upgrades without removing the felt. If you choose insulation from below, you sacrifice interior ceiling height and accept that the deck remains cold. Both options are technically sound—but each closes off different future pathways.

Decision Tree: Insulation from Above vs. from Below

The choice of insulation method determines the entire project logistics. Below I present the consequences of both approaches – not as an evaluation, but as a map of outcomes.

Insulation from Above (on Existing Felt)

Action Sequence: deck load capacity assessment → slope layer installation (if slopes are lacking) → thermal insulation placement (XPS, PIR, mineral wool in inverted system) → vapor barrier or separation layer → new felt membrane or roofing membrane.

Consequences for the Investor:

• Requires working at height and roof access – logistics, scaffolding, weather dependency.
• Increases deck load – structural verification necessary.
• Raises roof level – requires parapet modifications, flashings, sometimes chimney and roof installation elevation.
• Provides complete thermal insulation of the deck – no thermal bridges, warm deck from interior side.
• Allows felt replacement simultaneously – “two-in-one” investment.

Consequences for the Contractor: responsibility for new membrane waterproofing, coordination with sheet metal worker (flashings) required, continuous work necessary (cannot stop midway due to precipitation).

Insulation from Below (from Interior Side)

Action Sequence: framing or carrier profile installation → insulation placement between framing elements → vapor barrier layer → finishing board (drywall, OSB).

Consequences for the Investor:

• Interior work – weather independent, no scaffolding required.
• Loss of room height – typically 15–25 cm, depending on insulation thickness.
• Deck remains cold – risk of water vapor condensation in deck if vapor barrier isn’t perfect.
• No interference with existing felt – but also no opportunity for its assessment and repair.
• Lower cost – no high-altitude work, no felt replacement.

Consequences for the Contractor: responsibility for vapor barrier integrity (crucial), maintaining layer continuity required (every penetration is a potential moisture source).

Priority Matrix: How to Choose the Right Method for Your Situation

There’s no universal answer – only informed choice based on a hierarchy of priorities. The matrix below will help determine which method suits your situation:

See Also

Roof Structure Modification – Is a Permit Required

If the roofing membrane is old or leaking, the choice is obvious – top-side insulation allows you to fix two problems at once. If the membrane is in good condition and the budget is tight – bottom-side insulation is a rational path, provided the vapor barrier is executed flawlessly.

Common Pitfall: Confusing Savings with Risk Reduction

The most frequent mistake is choosing the cheaper method without considering moisture risk. Bottom-side insulation works correctly only when the vapor barrier is installed without compromise – and that requires an experienced contractor and scrutiny of every detail. If this condition isn’t met, the “savings” turn into a risk of ceiling moisture damage.

Practical Tools: Checklists for Contractor Conversations

Before making a decision and signing a contract, have a conversation with your contractor using the following control questions. Their purpose isn’t to test the contractor’s knowledge, but to obtain specific information that will help you assess the quality of the offer.

Questions before choosing an insulation method:

• Have you assessed the technical condition of the existing felt roofing? What is its condition and does it require repair?
• Have you checked the load-bearing capacity of the roof deck for the additional insulation weight?
• What insulation materials do you propose and why these specifically (lambda value, moisture resistance, durability)?
• How will you address the vapor barrier – what material, how will joints and service penetrations be executed?
• How will layer continuity be secured at parapets, chimneys, and roof drains?
• Does the price include flashings and parapet raising (if insulating from above)?
• What is the work schedule and how will you protect the building in case of precipitation during construction?

Questions about liability and warranty:

• Who is responsible for roof waterproofing after completion – do you have liability insurance?
• What warranty do you provide on the completed work and does it cover both materials and workmanship?
• Do you have documented references from similar projects (flat roof, insulation over existing felt)?

Answers to these questions will help you assess not only the contractor’s competence, but also their approach to responsibility. If any question receives an evasive answer – that’s a warning sign.

Investment Summary

Insulating a felt-covered roof is a decision that requires establishing the method before starting work – and awareness of the consequences of each choice. There’s no universal solution, just a choice based on priorities: thermal efficiency versus cost, maintaining interior height versus ability to replace felt, construction speed versus weather independence.

What’s crucial is understanding that with a flat roof, every layer matters for the integrity of the entire system. Therefore, what’s most important isn’t which method you choose, but whether the contractor understands their responsibility for each layer – from insulation, through vapor barrier, to the final coating.

The Rooffers philosophy is that the investor should know why they’re choosing a specific solution and what its consequences are – before paying for execution. Because a roof isn’t a surface – it’s a layered system that either works as a whole, or fails at the least expected moment.