HYBRIS Or PIR: What Works Inside A Skeiling?

Once a skeiling needs insulating, one of the practical questions is what material should actually go inside the sloping roof construction.

HYBRIS and PIR can both be used within suitable pitched-roof applications, but they are very different products to work with.

That does not mean one is automatically better.

Start with the roof, not the product

Asking whether HYBRIS or PIR is better before looking at the roof is a little like choosing the tool before knowing the job.

The installer first needs to understand:

• rafter depth
• rafter spacing
• whether the framing is regular
• what roof underlay or membrane is present
• what ventilation arrangement the roof requires
• how the skeiling can be accessed
• where the insulation layer needs to continue beyond the slope
• what overall thermal performance the completed build-up needs to achieve

Once those things are known, the product comparison becomes far more meaningful.

ACTIS lists HYBRIS for pitched roofs and ceilings and describes it as flexible and friction fitting. PIR manufacturers likewise publish pitched-roof solutions using rigid boards around rafter-level constructions.

HYBRIS behaves differently because it is flexible

HYBRIS uses a honeycomb structure rather than forming a solid rigid board.

That flexibility can be useful within timber framing because the material can be fitted into the available bays and accommodate some variation within the structure.

For existing roofs where rafters are not perfectly regular, that characteristic can make detailing around the timber more practical.

The product still needs to be installed as part of the correct roof build-up rather than simply pushed into any available void.

Our HYBRIS insulation service uses the product where the construction makes its characteristics appropriate.

PIR takes the rigid-board approach

PIR is supplied as rigid board and is cut to suit the intended area.

In pitched roofs, manufacturers provide systems where rigid insulation is installed between rafters and combined with additional insulation above or below them depending on the roof design.

Its rigid format can work extremely well where:

• the framing can be measured accurately
• boards can be cut closely to the required dimensions
• the intended roof build-up suits rigid insulation
• available thickness is an important constraint

The fitting accuracy becomes especially important when boards sit between existing timbers.

Irregular rafters can change the practical decision

Older roofs do not always contain perfectly straight timbers at perfectly repeated spacings.

Individual bays can vary.

With rigid board, those variations need to be accounted for when each piece is measured and cut.

A flexible product can sometimes accommodate irregular framing differently.

That does not make flexible insulation inherently superior.

If the roof framing is regular and the board can be detailed accurately, rigidity can be entirely appropriate.

Available depth matters

Skeilings are often restricted by the depth of the existing rafters.

That naturally makes thermal performance within the available build-up an important consideration.

PIR products are frequently chosen where strong thermal resistance is required within limited thickness.

HYBRIS is available in a range of thicknesses, with ACTIS publishing core thermal-resistance figures and specific roof build-ups for the product.

But comparing product thickness alone still does not tell you which completed roof will perform better.

The whole build-up has to be calculated.

What about thermal bridging through the rafters?

If either product is installed only between rafters, the rafters themselves continue through the construction.

That repeating timber affects the overall U-value.

This is why pitched-roof systems may include additional insulation above or below the framing rather than relying only on the material installed between individual rafter bays. PIR manufacturers explicitly publish between-and-over or between-and-under rafter arrangements for this reason.

The exact solution in an existing skeiling depends on what can realistically be installed without unnecessarily rebuilding the entire roof.

Installation quality can outweigh a theoretical advantage

Imagine Product A looks slightly stronger on paper but is badly fitted around irregular rafters.

Now imagine Product B produces a slightly different theoretical build-up but can be installed continuously and neatly across the actual construction.

The product specification matters, but so does the finished installation.

This is why we should be wary of reducing the comparison to a single thermal-conductivity figure.

Ventilation cannot be ignored with either product

Neither HYBRIS nor PIR gives permission to ignore how the roof manages moisture.

The correct arrangement depends on the roof membrane, ventilation strategy and wider build-up.

Some traditional pitched-roof configurations require a ventilation gap above between-rafter insulation; manufacturers’ installation guidance varies according to the specific construction and roof system.

That is why an installer should identify the roof build-up before deciding how much of the available rafter depth can actually be occupied.

What about vapour control and airtightness?

Warm, moisture-containing indoor air behaves differently from cold outdoor air.

The way vapour is controlled through a pitched roof therefore forms part of the design.

The exact solution depends on the complete roof system rather than one generic rule applied to every skeiling.

Insulation should sit within that moisture strategy, not replace it.

HYBRIS can be attractive in awkward framed spaces

A skeiling with variable timber spacing, awkward access or framing that is difficult to cut rigid boards around may play to the practical strengths of a flexible friction-fit product.

That is particularly relevant in retrofit work where installers are dealing with an existing roof rather than designing perfect rafter centres from scratch.

PIR can be attractive where thickness is tight

A rigid PIR build-up can be a strong option where available depth is restricted and accurate cutting and fitting can be achieved.

Rigid-board systems are well established for pitched roofs, with manufacturers providing specific guidance for applications around rafters.

Again, the advantage depends on being able to achieve the intended installation on the actual roof.

There is no meaningful winner without seeing the roof

This is ultimately the problem with asking whether HYBRIS or PIR is “better”.

Better where?

Better between straight new rafters?

Better around irregular nineteenth-century timber?

Better where headroom is extremely restricted?

Better where access is only available through a small eaves void?

Those are different jobs.

The product that makes the most sense is the one that can form the most appropriate complete thermal system for the construction in front of the installer.

Sometimes the solution may involve more than one layer

Pitched-roof insulation does not have to mean choosing one material between the rafters and stopping there.

Depending on the system, additional layers can help improve continuity across the framing and reduce the influence of repeating thermal bridges.

The design therefore needs to consider the complete roof section rather than treating the rafter bay as the entire job.

What should an installer establish first?

Choose around the construction

HYBRIS and PIR are not two competing answers to exactly the same question.

They are two different ways of creating thermal resistance within a roof construction.

HYBRIS may offer practical advantages where a flexible system suits the framing and access.

PIR may offer practical advantages where a rigid, accurately cut system and strong performance within a restricted thickness suit the build-up.

The right answer is the one that works with the skeiling rather than forcing the skeiling to suit the product.

Our skeilings insulation approach therefore starts with the roof construction before the final material is specified.

Frequently asked questions