Ask most rigid-foam buyers what defines their polyol and they will name the supplier and maybe the viscosity. Ask what OH value and functionality they run and you often get a pause. Yet those two numbers, more than any others, determine whether your insulation is dimensionally stable, thermally efficient, strong and fire-compliant. They also drive how much MDI you consume and therefore your cost. This guide explains hydroxyl value and functionality for rigid-foam producers, how to select rigid polyols, and how the polyether-versus-polyester choice shapes your product.
The two numbers that define a rigid polyol
Hydroxyl (OH) value, reported in mg KOH/g, measures how many reactive hydroxyl groups the polyol offers per unit mass. It is inversely related to molecular weight per reactive site: a high OH value means short segments between reaction points. Rigid-foam polyols carry high OH values — frequently in the hundreds — which is precisely what produces a tight, hard network. Flexible-foam polyols, by contrast, have low OH values and long chains, giving softness and elongation. The OH value essentially sets the "hardness DNA" of the foam.
Functionality is the average number of reactive hydroxyl groups per polyol molecule. Flexible foam uses triols (functionality ~3) with long arms. Rigid foam uses higher-functionality polyols — often from about 3 up to 6 or more — because more arms per molecule means more crosslinks and a denser three-dimensional network. High functionality plus high OH value is the combination that makes rigid foam rigid.
Why crosslink density is everything in rigid foam
Rigid insulation must hold its shape and strength under thermal cycling, load and time. That comes from crosslink density — the number of network junctions per unit volume. The higher it is (within limits), the more the foam resists deformation, creep and shrinkage. OH value and functionality are your two dials for crosslink density: raise either and you tighten the network. This is why rigid systems look so different on paper from flexible ones — they are engineered for the opposite property.
Polyether vs polyester rigid polyols
Rigid foam uses two families of polyol, and the choice shapes performance, fire behaviour and cost:
Polyether rigid polyols
Made by adding alkylene oxides to high-functionality starters (such as sugars, glycerine or amine-based initiators), polyether rigid polyols are versatile, relatively moisture-tolerant, and widely used in PUR insulation, spray foam and appliance foam. They offer a broad range of OH values and functionalities and are the default for much rigid production.
Polyester (aromatic) polyols
Aromatic polyester polyols bring thermal stability and fire performance thanks to their aromatic backbone, and they are highly compatible with the high-index PIR chemistry used in fire-rated boards. That is why polyester polyols feature so prominently in PIR insulation. They can be more moisture-sensitive and viscous, so systems are engineered around those traits. Many commercial rigid systems blend polyether and polyester polyols to balance processing, thermal performance and fire rating.
OH value drives your MDI stoichiometry — and your cost
Here is a commercial point buyers often miss: the OH value of your polyol blend directly determines how much MDI you need. Isocyanate demand is calculated from the total reactive groups in the formulation — the polyols' OH value, plus water and any other isocyanate-reactive components — and your target index. A higher-OH blend needs more MDI to reach the same index. So when you compare polyols, you are not just comparing polyol price; you are comparing the whole system cost, because a different OH value shifts your MDI consumption. Accurate, consistent OH values on the CoA are therefore essential both for on-spec foam and for costing.
How the numbers map to performance
- Dimensional stability: driven by adequate functionality and OH value. Under-crosslinked foam shrinks; correctly specified high-functionality polyols hold shape.
- Compressive strength: rises with crosslink density and density; the polyol sets the crosslink contribution.
- Thermal performance (lambda): a system outcome — cell structure and blowing agent dominate, but the polyol governs the cell-wall polymer and closed-cell content that trap the gas.
- Fire behaviour: favoured by aromatic (polyester) content and PIR chemistry; the polyol choice is central to meeting fire classifications.
- Friability: very high crosslink density can make foam brittle and friable; there is an optimum, not a maximum.
Selecting a rigid polyol: a practical sequence
- Start from the application. Fire-rated board (PIR, polyester-rich), appliance foam (polyether, tight lambda), spray foam (polyether, processing-friendly), sandwich panel (PUR/PIR per fire spec).
- Set functionality and OH value to the crosslink density that gives dimensional stability at your target density without excessive friability.
- Choose polyether/polyester balance for the fire and thermal requirement.
- Build the system — blowing agent, catalysts (including trimerisation for PIR), surfactant, flame retardant — around the polyol.
- Confirm stoichiometry — recompute MDI demand from the blend's OH value and your index.
Then trial on your equipment, because processing (mixing, mould/line temperature, demould time) interacts with all of it.
Consistency: the quiet requirement
A rigid polyol that drifts in OH value or functionality between batches quietly moves your index, your dimensional stability and your fire performance — the last thing you want in a certified insulation product. Insist on consistent, documented polyols with a Certificate of Analysis on every shipment. Consistency is what lets you certify a board once and keep making it to that certification, rather than re-qualifying every time a batch shifts.
How Ambizent helps you specify and source
We supply rigid-foam producers with polyether and polyester rigid polyols across the OH-value and functionality range, polymeric MDI, trimerisation and gel catalysts, surfactants and flame retardants — as matched systems, with a Certificate of Analysis on every shipment. Tell us your application, density and performance targets (dimensional stability, lambda, fire class), and we will recommend a polyol blend and PMDI grade and quote the system. Read our companion pieces on polymeric MDI for rigid foam and dimensional stability and shrinkage, browse our polyurethane range, and get in touch to source it.
Worked example: OH value and MDI demand
To see why OH value is a commercial number, not just a technical one, consider two rigid polyol blends targeting the same board. Blend A has a higher average OH value than Blend B. Because isocyanate demand is driven by the total reactive groups in the formulation, Blend A requires more polymeric MDI to reach the same index as Blend B. If MDI is your largest cost — as it often is in PIR — then the polyol with the "better" price per tonne can produce the more expensive board once its higher MDI consumption is counted. The lesson: always convert a polyol comparison into a whole-system cost at your target index, using the blend's OH value to compute MDI demand. A supplier who quotes systems rather than components does this for you.
Handling and storing rigid polyols
Rigid polyols — especially polyester and system polyols — reward correct handling:
- Moisture control: polyester polyols in particular can be moisture-sensitive; sealed storage protects them. Absorbed water shifts your effective index (water reacts with isocyanate) and can cause processing and foam defects.
- Temperature: viscosity rises sharply as polyols cool, affecting mixing and metering; keep them within the recommended range and allow drums to condition before use.
- Homogeneity of system polyols: formulated B-sides contain blowing agent and additives that can stratify; follow the supplier's rolling/mixing guidance before use so every draw is representative.
- Shelf life: system polyols with volatile blowing agents and reactive catalysts have a finite life; rotate stock and use within it.
Common rigid-polyol mistakes
- Comparing polyols on price alone, ignoring the OH-value-driven MDI consumption that changes system cost.
- Under-specifying functionality, producing an under-crosslinked foam that shrinks and fails dimensional testing.
- Ignoring the polyether/polyester balance and then struggling to meet a fire classification.
- Accepting drifting OH values, which quietly move your index, dimensional stability and fire performance batch to batch.
Consistency underpins certification
Insulation products are frequently certified — for fire, for thermal performance, for building compliance — and a certification is only as stable as the raw materials behind it. A polyol whose OH value or functionality wanders forces you to either re-qualify or risk shipping off-certification product. That is why documented, consistent polyols with a Certificate of Analysis on every shipment are not optional for a serious rigid-foam producer; they are what let you certify once and keep making to that certification. Tell us your application and performance targets and we will match a consistent rigid polyol and PMDI system and quote it. Get in touch to source.
Key takeaways
- OH value and functionality set crosslink density — the property that makes rigid foam stiff and dimensionally stable.
- OH value also drives MDI demand: always compare polyols as whole systems at your target index, not on drum price.
- Polyether polyols are versatile; aromatic polyester polyols bring the fire and thermal performance PIR needs — many systems blend both.
- Under-specified functionality or OH value causes shrinkage and dimensional failure.
- Consistent, documented polyols let you certify once and keep making to that certification.
Specify from the application back — fire class, lambda, density, dimensional stability — choose functionality, OH value and the polyether/polyester balance to suit, then confirm stoichiometry and trial on your line. Share your targets and we will match a consistent rigid polyol and PMDI system with a CoA on every shipment.
The bottom line
OH value and functionality look like technical footnotes and are actually two of the most consequential numbers in your whole operation — they set whether your foam holds its shape, meets its fire class, hits its lambda, and what it costs to make once MDI consumption is counted. Buyers who understand and control them specify better, cost more accurately and suffer fewer dimensional failures. Pair that understanding with consistent, documented polyols and you have a rigid product you can certify once and reproduce indefinitely.
One practical closing habit: keep a simple record of each polyol's OH value and functionality alongside the MDI demand and index it implies, so that whenever you evaluate a new grade you can slot it straight into a whole-system cost and performance comparison. That small discipline stops the most common and most expensive rigid-polyol mistake — judging a polyol on its drum price while ignoring how it changes your MDI consumption and your dimensional performance. Buy the system, not the component, and your costing and your foam both improve.