Roof insulation and cover board: installing the low-slope assembly

The insulation layer and the cover board, and what each one does

Roof insulation is the rigid board layer sandwiched between the structural deck and the membrane. It does three jobs at once. It carries the thermal resistance, the R-value the energy code is checking for. It gives the membrane a flat, sound base to bond to instead of a bare steel or concrete deck. And when it is cut tapered, it builds the slope that runs water to the drains. That last job belongs to a separate guide on crickets and tapered insulation, so this one stays on the flat side: the R-value, the board, the cover board, and how the stack gets attached.

The cover board is the thinner, denser board that goes on top of the insulation, directly under the membrane. It is there to take the abuse the soft insulation cannot. Hail, foot traffic, dropped tools, the cart wheels of the next trade, all of it lands on the cover board first, and the cover board gives the membrane a hard surface to bond to that will not crush under a heel.

Think of the assembly as a stack with a job at each layer. The deck holds the load. The vapor retarder, where the climate calls for one, keeps interior moisture out of the insulation. The insulation carries the R. The cover board protects the membrane and gives it something solid to grab. The membrane keeps the water out. Get the stack and its attachment right and the roof holds for thirty years. Get the attachment wrong and the wind takes it in the first big storm, R-value and all.

What does the roof insulation layer do?

The insulation layer carries the building's roof R-value, and on a low-slope commercial roof almost all of that R lives above the deck, in the board. There is no attic to fill. The continuous board over the deck is the thermal control layer, which is exactly why the energy code writes its roof requirement as a continuous-insulation R-value rather than a cavity number.

Past the R, the board is doing structural and constructability work that nobody credits until it is missing. It spans the flutes of a steel deck so the membrane is not bridging open ribs. It gives a fastener something to bite into and a bondable plane for adhesive. On a tapered package it builds the slope to drain, which the crickets-and-tapered-insulation guide covers in full. Strip the insulation out and you have a membrane lying on bare steel with no R, no slope, and nothing to bond to.

The mistake that follows from underrating the layer is treating insulation as a commodity you buy on price per board. The R-value, the compressive strength under traffic, the dimensional stability in heat, and the attachment all ride on which board you pick. A cheap board that loses R in the cold, crushes under a cart, or shrinks at the joints costs more over the life of the roof than the dollars it saved at delivery.

Roof insulation types and R per inch

Five rigid boards cover almost all low-slope work, and they trade R-value against cost, moisture tolerance, and fire performance. Polyiso is the default on commercial roofs because it gives the most R per inch and carries a good fire rating, which is why it took the largest share of the market. The polystyrenes, EPS and XPS, win on price and moisture tolerance. Mineral wool and cellular glass are the specialty boards you reach for when fire or vapor performance has to come first.

The R per inch numbers below are the rated values at standard 75°F mean temperature. Treat them as the starting point, not the design number. Polyiso in particular does not hold its rated R in the cold, which the next section covers, and the energy code wants the long-term design R, not the fresh-off-the-truck label.

Cost tracks roughly inverse to moisture tolerance here. Polyiso and EPS are the economical R, polyiso by the inch and EPS by the dollar. XPS costs more for its closed-cell water resistance. Mineral wool and cellular glass cost the most and buy you fire and vapor properties the foams cannot match. Pick the board for what the roof actually needs, then size the thickness to the code R after the cold de-rate, not before.

Does polyiso lose R-value in the cold?

Yes. Polyiso loses R-value as the temperature drops, and that is the single most important fact about the most common roof board. The blowing-agent gases that give polyiso its high R start to condense as the board gets cold, and a board rated around R-6 per inch at 75°F can fall to roughly R-4.5 per inch on a cold winter day. The drop is real, it is measurable, and a design that used the warm rated number is short on R exactly when the building needs heat most.

There are two corrections to know. The first is LTTR, long-term thermal resistance, the aged value that accounts for the slow loss of blowing agent over the first years of service. Manufacturers publish an LTTR per ASTM C1303, and it typically settles around R-4.3 to R-4.7 per inch. That is the number for the energy-code calculation, not the fresh label. The second is the cold de-rate. The NRCA has commonly recommended designing with about R-5.6 per inch in warm climates and around R-5.0 per inch in cold climates to account for the temperature effect. Confirm the current NRCA recommendation and the manufacturer's published values before you put a number on a submittal.

The field consequence is straightforward. In a cold climate you do not get the rated R out of polyiso, so a single-material polyiso package sized on the warm number underperforms in winter. The two fixes are to design to the cold value, which means more inches, or to hybridize the stack, putting a polystyrene or a different board where the cold performance matters. Either way, do not size cold-climate polyiso on the 75°F label. That is the rookie miss that shows up as a cold building and a heating bill the model never predicted.

What R-value does a roof need?

The required roof R-value is set by the energy code for the building's climate zone, and on commercial low-slope work that means ASHRAE 90.1 or the IECC, whichever the jurisdiction has adopted. Both define eight climate zones, and both write the above-deck roof requirement as a continuous-insulation R-value, because that is what a board roof delivers. Higher zone number, colder climate, more R.

As a rough map, commercial above-deck roof insulation commonly runs around R-20 in the warm zones 1 and 2, around R-25 in the middle zones 3 through 5, and around R-30 in the cold zones 6 through 8. These figures move between code editions and between 90.1 and the IECC, and they have generally tightened over recent cycles. Pull the requirement from the adopted edition and climate zone for the actual project, and check the envelope tables in 90.1 Section 5.5 or the IECC commercial provisions rather than carrying a number from the last job.

Two things trip people up converting the code R into board thickness. First, use the design R of the board after the cold de-rate, not the warm label, or the assembly comes up short on paper at plan check and short in service. Second, the metal fasteners through the insulation lower the effective R of the whole assembly, which the energy model may or may not credit. Size the inches to the design R per inch, then confirm the assembly meets the code by U-factor if the project is going that route.

Why install insulation in two layers?

Two or more layers with the joints staggered is the way to install roof insulation, and recent code editions now require it. The reason is the joint. Every gap between boards is a small thermal break and a path for air. Stack the layers so the joints line up and you have run a slot of low R straight through the assembly, top to bottom, repeated across the whole roof. Offset the joints between layers and the upper board covers the lower joint, so there is no continuous gap for heat or air to follow.

This has been good roofing practice for decades, and the energy code caught up to it. Recent IECC editions require insulation in not less than two layers with the edge joints of each layer staggered. Confirm the exact wording against the adopted edition, but the direction is fixed: single-layer aligned-joint insulation is no longer how a code-compliant low-slope roof gets built where two layers are required.

The second payoff is what the two-layer method does to the fasteners. When you fasten only the first layer to the deck and adhere the layers above it, the fastener heads and plates end up buried in the assembly instead of running to the membrane. That kills two birds. It breaks the thermal bridge the metal fastener would otherwise make through the full thickness, and it takes the fastener plates out from under the membrane where they can telegraph and wear. The staggered two-layer build is not a detail you add for extra credit. It is how the assembly hits its real R and its service life.

Why the cover board earns its cost

The cover board is the layer value engineering loves to delete, and deleting it is how you get a punctured membrane and a hail claim the insurer fights. The soft insulation under the membrane has very little resistance to a point load. A dropped tool, a hailstone, the corner of a condenser cart, all of it drives through the membrane and crushes the insulation, and the membrane has nothing firm behind it to resist the puncture. The cover board is the hard layer that spreads that load and protects both the membrane and the insulation under it.

It does more than block punctures. The cover board gives the membrane a smooth, dimensionally stable, bondable plane, which matters because the insulation below can shift and gap slightly with temperature. It improves the fire rating of the assembly. It can raise the wind-uplift rating. And in hail country it is often the difference between an assembly that passes the hail test and one that does not. The FM hail classification, FM 4473, separates assemblies that survive severe and very severe hail from those that do not, and a glass-mat gypsum cover board is commonly what gets an assembly into the very-severe-hail class. Confirm the specific tested assembly, because not every cover board carries the same rating.

Here is the blunt version. The cover board is a small fraction of the roof cost and it protects the most expensive and most failure-prone layer, the membrane and its seams. Skip it to save a few cents a foot and you have traded a known small cost for an unknown large one, paid later as punctures, hail damage, and a warranty argument. On any roof that will see traffic or hail, the cover board is not optional in practice even when the spec lets it be.

Cover board types

Four cover boards cover most jobs, and they trade impact and fire performance against R-value and cost. Glass-mat gypsum is the common pick on commercial roofs because it is hard, fire-resistant, and takes hail and traffic well. High-density polyiso, HD ISO, is lighter and adds a little R, but it does not protect against impact as well as gypsum and is generally not approved for the very-severe-hail class. Perlite and wood fiber are the older organic-based boards, still specified, often under hot asphalt and mod-bit.

Glass-mat gypsum, the DensDeck-type board, is the one to default to where hail and puncture resistance drive the call. Its hard surface absorbs impact energy and keeps the insulation below from crushing, and it carries the fire and hail classifications more roofs need. High-density polyiso runs lighter and gives roughly R-2.5 per half-inch, with a compressive strength in the 80 to 110 psi range, enough for light-to-moderate traffic but below gypsum for severe impact. Pick HD ISO when weight and a little extra R matter more than maximum hail protection.

Perlite and wood-fiber boards still have a place, particularly in built-up and modified-bitumen assemblies where their compatibility with hot asphalt is the point. They are more moisture-sensitive than gypsum, so they are a poor pick where the assembly may get wet. Match the cover board to the membrane, the attachment, and the hazards the roof faces, and confirm the choice against the tested assembly the warranty and the FM listing are written around.

Attaching the insulation: mechanical, adhered, or both

Insulation and cover board attach two ways: mechanically, with fasteners and stress plates driven into the deck, or fully adhered, set in a bead-applied or full-spread adhesive. Many real assemblies combine them, mechanically fastening the base layer and adhering the layers and cover board above it. The attachment is not a preference. It is engineered to the wind uplift the roof has to resist, per the wind calculation the edge-metal and wind guide covers in detail.

Mechanical attachment is fast and economical and bites hard into a steel deck, where it gives strong uplift and shear resistance. The cost is the fasteners themselves, which become thermal bridges, and the plates, which sit under the membrane on a single-layer build. Fully adhered systems spread the load across the whole board, which can ride better under foot and avoid the flutter a mechanically attached membrane shows in wind, but they need a clean, dry, primed substrate and they cost more in adhesive and labor.

The fastening pattern follows the wind zone. In the field of a roof, a common pattern lands somewhere around 8 to 12 fasteners per board or per 100 square feet, with the spacing set by the specific tested assembly and the calculated uplift, not by a number from memory. Pull the pattern from the manufacturer's approved assembly and the wind design, because the fastener count is what the FM or UL listing and the warranty are written around. Under-fasten the field and the membrane balloons and the boards work loose. The pattern is engineering, not habit.

Fasteners as thermal bridges

Every metal fastener that runs from the deck through the insulation is a thermal short. Steel conducts heat far better than foam, so each fastener and plate carries heat past the insulation it is supposed to be stopped by. One fastener is nothing. A roof has hundreds or thousands of them, and the cumulative effect is real: studies put the loss of effective assembly R in the range of roughly 10 to 30 percent depending on fastener density, insulation thickness, and climate, with many assemblies landing in the low-to-mid teens.

This is the hidden gap between the R-value on the spec and the R-value the building gets. The energy model may use the board's rated R and never account for the fasteners punching through it. The colder the climate and the denser the fastening, the bigger the miss, and it compounds with the polyiso cold de-rate, since both losses hit hardest in winter.

The fix is the same two-layer move that helps everywhere else: fasten the base layer to the deck and adhere the layers above, so the fastener heads stay buried below the upper insulation and the cover board instead of reaching the membrane. Burying the fasteners under a board or two dampens the thermal bridge sharply. A fully adhered assembly removes the through-fasteners entirely. When the energy target is tight, the attachment method is part of how you hit it, not just how you hold the roof down.

The deck and what goes over it

The deck under the insulation is usually steel, concrete, or wood, and the deck changes the attachment and what the first layer has to do. Steel deck is the common commercial case. Fasteners bite well into steel, and the insulation has to span the flutes so the membrane is not bridging open ribs. Concrete decks often go fully adhered, since you cannot drive a standard roofing fastener into structural concrete without pre-drilling, and the choice becomes adhesive or specialty fasteners. Wood decks take fasteners but need attention to the fastener type and embedment.

Over a steel deck, fire is the reason for the layer order. A combustible foam insulation laid straight on a steel deck can be a fire-spread concern, so assemblies commonly call for a thermal barrier, often a layer of gypsum board, between the steel deck and the foam to meet the fire-rating requirement of the tested assembly. That thermal barrier also gives a vapor retarder, where one is needed, a sound substrate, since a vapor retarder should not be laid directly on bare fluted steel.

Read the deck before you pick the attachment. A lightweight insulating concrete deck, a structural concrete deck, a steel deck, and a wood deck each lead to a different first-layer detail, and the FM or UL assembly is written for a specific deck. Installing a steel-deck assembly over concrete, or the reverse, is how you end up outside the tested listing the warranty depends on.

A vapor retarder belongs in this same layer order in cold climates and over high-humidity interiors. It keeps interior moisture from diffusing up and condensing inside the assembly, and it goes under the primary insulation near the warm side so the moisture is stopped before it reaches a cold surface. The rule that governs it is dew point: there has to be enough insulation above the vapor retarder to keep it warmer than the interior dew point, which ties the decision to the R-value split and makes it a design call, not a field improvisation. A natatorium, a food plant, a cold-storage building, any high-humidity interior in a cold zone is where it stops being optional. On a steel deck the vapor retarder needs a substrate, commonly the same gypsum board that serves as the thermal barrier, because you do not lay it on bare fluted steel. Confirm the need against the NRCA guidance and the project's building-science analysis, since a vapor retarder in the wrong place can trap more moisture than it stops.

Wind uplift, and why the perimeter and corners get more

Wind does not load a roof evenly, so the attachment is not uniform either. Uplift is highest at the corners, next highest along the perimeter, and lowest in the field, and the fastening pattern steps up to match. A field that holds at a wider spacing needs a denser pattern at the perimeter, often something like 6 inches on center along the seams in those zones, and denser still at the corners. The exact spacing comes from the wind calculation and the tested assembly, not a rule of thumb.

The zones and the pressures come from the wind design, commonly ASCE 7 for the loads and ANSI/SPRI WD-1 for applying them to the roofing assembly, with the fastening pattern pulled from the FM or UL approved assembly. The edge-metal and wind-uplift guide covers how those zones are laid out and how the edge termination, which is where most wind failures actually start, gets detailed. The insulation attachment and the membrane attachment both follow the same zone map.

The field failure to know is that wind peels a roof from the edge in, not from the middle out. An under-fastened perimeter is where the lift gets under the assembly and starts the progressive failure that takes the field with it. When the budget pressures the fastener count, the perimeter and corners are the last place to cut, not the first. That is where the storm finds the roof.

Keeping the insulation dry

Wet insulation is no insulation. Most roof insulation loses most of its R-value when it gets wet, and once water is in the assembly it does not just rob the R. It corrodes the steel deck, rots the organic boards, feeds mold, and adds dead weight, and it spreads laterally so a small leak wets a wide area of board. The membrane on top can look perfect while the assembly under it is soaked and dead.

The discipline is to install dry and to phase the work so insulation never sits open to weather overnight. You dry in what you lay each day, tying the membrane off to a watertight condition at the end of the shift, and you do not install over a deck or a layer that is wet. The detail calls for a dry substrate. On a re-roof you rarely get a perfectly dry deck, so you schedule around the weather and the dew point instead of pretending the drawing's assumption is true, and you check questionable areas with a moisture meter rather than your hand.

Insulation that got wet during construction and got covered anyway is a failure you built in and will not see until the energy bills run high or the deck rusts through. If a board got rained on and held water, it comes out. Drying a soaked organic or fibrous board in place is not a thing. Cut it out, let the deck dry, and replace it. The cost of replacing a few boards during construction is nothing against the cost of a wet roof discovered three winters later.

Once the membrane is on, wet insulation hides, so the way to find it before it eats the deck is a moisture survey: infrared, where a thermographer reads the roof after sunset and the wet areas show because they hold heat differently than the dry field, plus nuclear or capacitance meters that read moisture in the assembly directly. A survey done at the right conditions maps the wet zones so the repair is a targeted cut-and-replace, not a guess. The reason to survey rather than wait is that the wet area is almost always far larger than the leak by the time it shows as an interior stain. On a large or critical roof, tie a periodic survey to the inspection routine, walk the flashings and terminations first since that is where most leaks start, and record the results against the roof plan so the next inspection has a baseline.

Tying into the slope below and the membrane above

On a dead-flat deck, the same insulation layer also builds the slope to drain, by cutting the boards on a taper and laying them to fall toward the drains. That is a whole design and takeoff job of its own, with its own slopes, crickets, and overflow rules, and it lives in the crickets-and-tapered-insulation guide rather than here. A tapered system usually rides on a flat base layer that carries most of the R, with the tapered boards above building the fall, and the cover board and attachment sit on top of all of it. So everything in this guide, the board type, the cold de-rate, the cover board, the fastening to the wind, still applies on a tapered roof. The slope just adds one more constraint the same layer has to meet.

On top, the cover board is the surface the membrane actually bonds to or fastens through, so the membrane choice and the cover board choice are linked. A fully adhered membrane needs a clean, dry, compatible cover board the adhesive will grab, and the wrong surface or a dusty board is where adhesion fails months later. The membrane selection guide covers picking TPO, EPDM, or PVC for the building; this guide covers giving whichever one you picked a sound base. Match the cover board to the membrane and its attachment, because a hot-air-welded thermoplastic, a fully adhered membrane, and a torched mod-bit each need a specific tested combination. The manufacturer's approved assembly names the cover boards allowed under the warranty, so use one off that list, not whatever is on the truck.

The tested assembly: FM, UL, and the warranty

A roof assembly is approved as a whole, not as a pile of good parts. FM Global and UL test specific stack-ups, deck through membrane, for wind uplift, fire, and hail, and the approval applies to that exact combination of deck, fastener, insulation, cover board, adhesive, and membrane. Swap one component and you are technically outside the listing, even if the substitute is a fine product on its own.

This is why the submittal and the install have to match the approved assembly line for line. The FM approval gives a wind rating, a fire classification, and, with FM 4473, a hail class. UL listings cover uplift and fire by their own test methods. The membrane manufacturer's warranty is then written around an approved assembly, and the manufacturer can decline a claim if the roof that got built is not the roof that got listed. The inspector and the manufacturer's field rep both check the assembly against the listing, not against whether each part looks good.

The takeaway for the field is to treat the approved-assembly document as the build sheet. The fastener type and pattern, the cover board, the adhesive, the insulation, all of it comes off that sheet. When a supplier offers a substitution to save money or because something is on backorder, the question is not whether it is comparable. The question is whether it keeps the assembly inside the listing and the warranty. If it does not, the savings are not real.

Re-roof and recover: insulating over an existing roof

On a recover, you go over the existing roof instead of tearing it off, and the insulation question changes. You may add a layer of insulation to bring an old, underinsulated roof up toward the current energy code, and you almost always add a recover board, a cover board laid over the old membrane to give the new membrane a sound, smooth base and to separate it from the old surface. Going straight over a weathered, ridged old membrane without that board telegraphs every imperfection up into the new one.

The hard rule on a recover is that the existing assembly has to be dry. Cover over wet insulation and you have sealed the water in permanently, and now it is rotting and corroding under two roofs instead of one. A moisture survey of the existing roof comes first, the wet areas come out, and only a dry, sound substrate gets recovered. Code also limits how many roof layers a building can carry, commonly to two, so a building already on its second roof is a tear-off, not a recover.

The energy code can also force added insulation on a recover or re-roof, since bringing the roof into the project may trigger the current R-value requirement for the climate zone. Check whether the work scope pulls the roof up to current code, because that decision changes the insulation thickness, the height at the edges and curbs, and the whole takeoff. It is cheaper to find that out at estimating than at inspection.

Big roofs: warehouses, distribution, and data centers

On a large low-slope roof, the insulation and attachment decisions scale into real money and real risk, because the field is enormous and the perimeter and corner zones are long. A warehouse or distribution roof is mostly field, so the field fastening pattern and the cover board choice drive the material cost across hundreds of thousands of square feet, and a small per-foot decision multiplies fast. The temptation to value-engineer the cover board out is strongest here and the consequence is widest.

Data centers and other critical facilities push the other way, toward a more conservative assembly, because a roof leak over a server hall is not a maintenance ticket, it is downtime. On those roofs the cover board, the redundancy in the attachment, the hail rating, and the moisture control all get specified up rather than down, and the FM approval is often a hard requirement of the insurer, not a nice-to-have. The thermal performance also matters more where the building runs heavy cooling year-round, so the fastener thermal bridging and the polyiso cold behavior are worth the analysis.

Scale also changes the logistics of staying dry. You cannot dry in a ten-acre roof in a day, so the phasing plan, the night-seal details, and the weather windows become part of the means and methods, not an afterthought. The bigger the roof, the more the discipline of installing dry and tying off each day is what separates a clean job from a soaked assembly nobody can fully find later.

What to document

The assembly you installed is what the warranty, the energy compliance, and the next inspection are all measured against, so the record has to name every layer and how it was attached. The question this record answers later is whether the roof that is leaking, or running cold, or up for a hail claim, is the roof that was approved and specified. Without the record, that argument is unwinnable.

Capture the layers from the deck up, the insulation type and design R per inch and total R, the cover board, the attachment method and the fastener type, the field and perimeter and corner fastening patterns, the vapor retarder if any, the approved-assembly and warranty references, and the moisture or dry-in record for the deck. If the energy code was triggered on a re-roof, record the R-value the roof was brought to. Tie it to the FM or UL assembly number so a reviewer can check the build against the listing.

Common mistakes

• Installing a single layer with aligned joints, running a thermal break and air path straight through the assembly.
• Leaving the cover board out and letting the membrane get punctured by hail and traffic over soft insulation.
• Sizing cold-climate polyiso on the 75°F rated R instead of the cold design value, so the roof comes up short in winter.
• Running metal fasteners through the full thickness to the membrane and ignoring the effective-R loss from thermal bridging.
• Installing wet insulation, or covering an existing wet roof on a recover, and sealing the water into the assembly.
• Building under the climate-zone code R, or skipping the added insulation a re-roof triggers.
• Fastening the perimeter and corners at the field pattern instead of the denser pattern the wind zones require.
• Substituting a board or fastener that puts the assembly outside the FM/UL listing and the manufacturer warranty.

Field checklist

Standards and references

The energy code sets the R-value. On commercial low-slope work that is ASHRAE 90.1 or the IECC, whichever the jurisdiction has adopted, with the required above-deck continuous R-value set by climate zone in the envelope tables, commonly 90.1 Section 5.5 or the IECC commercial provisions. Recent IECC editions also require insulation in not less than two layers with staggered joints. The figures and the rules shift between editions and have tightened over recent cycles, so confirm them against the adopted edition and any local amendments before citing them.

FM Global and UL test and approve the assembly as a whole. FM approvals cover wind uplift and fire, and FM 4473 covers hail classification including the severe and very-severe-hail classes. UL covers uplift and fire by its own methods, such as the uplift and fire test standards. The wind loads themselves commonly come from ASCE 7, applied to roofing through ANSI/SPRI WD-1. The NRCA Roofing Manual is the practical reference for the insulation, vapor retarder, and cover board design, and it is the source for the polyiso cold-climate design recommendation.

The materials carry ASTM standards: ASTM C1289 for faced rigid polyiso, ASTM C578 for the polystyrenes, and the cover board and other boards under their own ASTM specifications. Above all of it sits the membrane manufacturer's approved assembly and warranty, which names the exact components and patterns and governs what you can install. Cite the standard that controls the point, and let the approved assembly and the project specification override any rule of thumb.

Units, terms, and conversions

Roof insulation carries a handful of terms that read differently across a spec, a manufacturer sheet, and the code, so the same property can look like two different numbers.

R-value is the thermal resistance, in hr·ft²·°F/Btu in US units, and its inverse at the assembly level is the U-factor the code may check instead. R per inch is the resistance of one inch of a board; total R is R per inch times thickness. Continuous insulation, written c.i., is insulation running uninterrupted across the structure, which is what an above-deck board roof provides. LTTR is long-term thermal resistance, the aged design R for polyiso. Compressive strength, in psi, is the cover board and insulation resistance to crushing under load.

R-value / R per inch

Thermal resistance of the assembly, and the resistance of a single inch of a given board

U-factor

The inverse of total R at the assembly level, the form the energy code may check

Continuous insulation (c.i.)

Insulation running uninterrupted across the structure, what an above-deck board roof provides

LTTR

Long-term thermal resistance, the aged design R-value for polyiso per ASTM C1303

Cover board

The dense board over the insulation that protects the membrane and gives it a bondable base

Vapor retarder

The layer under the insulation that limits interior moisture diffusion into the assembly

Compressive strength

Resistance to crushing under load, in psi, the cover board's protection against point loads

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