HullInterior

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Hull Interior

Summary

AAmpere (amp), SI unit of electrical current fibreglass hull doesn't need painting inside; whereas a steel hull must be finished with a complete paint system. In living spaces, a minimum of three inches of closed-cell polyurethane foam yields an insulating value of R-18.5. White deck paint, teak gratings or radiant barrier insulation in the ceilings significantly reduces air conditioning needs. Insulating cabin walls with mineral wool or aero gel provides privacy.

Paint System

In a steel hull all interior surfaces are painted with a complete epoxy and urethane paint system to protect the metal from corrosion.

Insulation

In living spaces, above the waterline, the hull should be insulated with closed-cell fire-retardant polyurethane foam or rubber foam like Armaflex. Armaflex sheet is a closed-cell, flexible elastomeric foam insulation. Aerogel, used in space missions, is also becoming more economical.

The primary purpose of insulating is to control condensation. Secondarily, it reduces heating and air-conditioning losses through conduction.

Heat is transferred by conduction (contact), convection (air circulation within an enclosed space) and radiation (infrared absorption). There is so much emphasis by governments and builders on insulation, which reduces conduction, that it is not well understood that most heat transfer is by radiation. Up to 90%percent of downward heat flow on the cabin top of a boat is by radiant energy.

Foam Insulation

Urethane foams are plastics having gas-filled bubbles. This makes them light weight and a good insulator. Urethane foam degrades in ultraviolet light. Exposed surfaces should be coated with a special protective coating.

Closed-cell foam has varying degrees of hardness and insulation value, depending on its density. It can be used for insulation or flotation devices. A normal, closed-cell insulation or flotation urethane is between 2 and 3 pounds per cubic foot. It is strong enough to walk on without major distortion. Polyurethane foam has an insulating value of R-5.6 to R-8.0 per inch.

Open-cell foam is soft, and has a poor insulating value. It can outgas. It absorbs water, and is not suited to marine applications.Foam rubber is lightweight, flexible (in its open-cell form) and buoyant. It can be used for thermal and acoustic insulation. Neoprene is a synthetic foam rubber.

Neoprene foam is a soft, flexible and durable form-fitting sponge rubber available in regular and high densities. It provides good thermal and moisture insulation and is resistant to many chemicals and petroleum derivatives. It can be used as an expansion joint filler and as a sound barrier in walls, ceilings or floors.

Aerogel

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas.

Aerogel has to date been used for insulating engine compartments, ovens, and rocket parts. It is also good at sound damping too. As the material becomes more common it is being used in building construction.

Radiant Barrier Insulation

Radiant barrier insulation (RBIRadiant barrier insulation), also called low-emittance or low-e, is used in building construction but it'sSecond not clear how this could benefit a boat, barring experimentation, because of the need for an air gap.

RBI in a building's attic yields significant benefits in hot climates by reducing air-conditioning costs. An air space between the RBI and the regular insulation is required for maximum efficiency. A minimum vented space of 3/4-in is recommended between the barrier and insulation.

RBI is a composite made of two layers of commercially fine aluminium (99.4% pure), bonded to both sides of a polypropylene insert. The polypropylene prevents the two aluminium foil surfaces from conducting heat. The surfaces reflect radiant energy. One of the first uses of this concept was in vacuum flasks, to keep liquids hot. The silvered surface inside reflects the radiant energy. The vacuum prevents convection. You’ve probably also seen the space-age emergency blankets in your local automotive or camping supply store.

On a fibreglass hull, the RBI can be bonded directly to the fibreglass.

Fitting it to a steel hull is tricky. You would have to use one-sided RBI, to prevent galvanic action between the steel and the aluminium foil. Any screw penetrating the foil would cause galvanic action. It’s possible that any galvanic action would be self-limiting as the foil is sacrificed around a contact point such as a screw. In any event, the foil is very thin.

White deck paint is almost as effective as RBI, and RBI has little value in cold climates. A teak grating is another posibility.

Interior Fittings

Inside the hull, non-watertight bulkheads and other interior fittings are usually screwed to hull stringers. This breaks the paint seal on steel hulls, and can lead to corrosion. Another way of doing this is to attach a neoprene rubber strip to the stringer, using waterproof contact cement. A marine-grade plywood furring strip is cemented to the rubber strip. Bulkheads, etc., are then screwed to the furring strip. This preserves the protective coating of the paint system, and flexibly mounts the interior which reduces noise conduction.

For a furring strip use flame-retardant closed-cell neoprene used in aircraft and trains for seat cushioning because of its fire resistance. Neoprene can be bonded to most surfaces such as metal, glass, wood, GRPGlass-reinforced plastic, rubber, fabrics and most plastics with super-strength neoprene contact adhesive, which is available with high flexibility, super strength, water proof (fresh and salt), and heat proof to 150 °Cdegree Celsius, SI unit of temperature.

Cabin walls are usually marine plywood with a veneer finish, which doesn’t reduce noise transmission much. Unless you want to listen to every noise from your neighbour or the adjacent head, make the cabin walls hollow, and insulate them with DFMDense fibre matting mineral wool or an aerogel sheet or use structural insulated panels (SIPStructural insulation panel). Aerogel is also an excellent thermal insulator, with a value of around R22 per inch.

Cabin soles are marine plywood or marine-grade SIP, covered with a hardwood or tile.

Structural Insulated Panels

A SIP consists of a solid foam core 4 to 8 inches thick (10.2 to 20.3 cmCentimetre) with structural sheathing glued to the foam core. SIPs are advantageous when you want an insulated floor. They have a high strength-to-weight ratio and a high R-value. SIPs are also called foam-core panels, sandwich panels, stress-skin panels and structural foam panels.

SIP cores are made of expanded polystyrene (EPS), polyurethane (PURPolyurethane) or polyisocyanurate (PIRPolyisocyanurate), a special formulation of polyurethane. PUR/PIR is water resistant, not water proof, so it will absorb some water (typically 5%) and encourage mildew. The most common types of sheathing materials for SIPs are oriented strand board (OSB) and plywood. Of course, marine-grade plywood is necessary in a boat.

Both polyurethane and polyisocyanurate outgas HCFC. New, they have an R-value of around 9 per inch but ageing reduces it to 6-7. EPS is R-4. The R-value of EPS remains fairly constant over time, and it is less expensive per inch of foam.

Polyurethane/isocyanurate panels, although almost twice as expensive, are more fire and water-vapour diffusion resistant than EPS. Panels made of PUR/PIR are typically 3.5 inches (89 mm) thick.

Because PUR/PIR does not bond easily, quality fabrication is especially important to ensure a long life and performance. The panels must be glued, pressed, and cured properly to ensure that they do not delaminate. Tough choice, isn’t it?

Comparing PUR/PIR & EPS
Factor	PUR/PIR	EPS
R Value/in	7	4
Bonding	Poor	Good
Outgassing	Yes	No
Water Resistant	Yes	Yes
Water Proof	No	Yes
Fire Resistant	Good	Acceptable
Mildew Resistant	No	Yes
Price	2x	1x

Tile Flooring

Tile flooring is not recommended in a small fibreglass hull, due to the twisting and flexing. It could work in a 50-ftFoot or larger steel boat. While in a home tile can be bedded in a mortar over plywood, plywood contracts and expands too much in a boat to provide a firm footing. This, plus vibration, can easily crack mortar and tiles.

In a larger boat, cover the plywood (minMinute 3/4-in) or SIP sole with a rubber membrane and then 1/2-in of impermeable cement board such as the original Wonderboard. Leave 1/8-in expansion space between sheets and caulk between them. Attach the tile with a multi-duty acrylic-modified thinset. Consider means of isolating tiled sections from each other with wooden expansion strips, and from steel stringers with neoprene.