The long wavelength of the first instability in Rayleigh-Benard convection between nearly thermally insulating horizontal plates is typical of a variety of physical systems. The evolution of such an instability is described by the equation α t = αα − μ ▿ 2 α −▿ 4 α + κ ▿·‖▿ α ‖ 2 ▿ α + β ▿·▿ 2 α ▿ α − λ ▿· α ▿ α + σ ▿ 2 ‖▿ α‖ 2 , where α is the planform function, μ is the scaled Rayleigh number and κ = ±1. The quantities α,β,λ represent the effects of finite Biot number, asymmetry in the boundary conditions at top and bottom, and departures from the Boussinesq approximation, respectively. The quantity σ = β when the original problem is self-adjoint, but σ ≠ β otherwise. Planform selection is studied for α κ = +1 using equivariant bifurcation theory. On the square lattice both rolls and squares can be stable, depending on the parameters β, λ and σ. The possible secondary bifurcations located near various codimension-two singularities are analyzed. On the hexagonal lattice the primary bifurcation is always degenerate when β = λ = σ =0. Of the six primary solution branches possible in this case the hexagon branch is stable. When β − σ = λ =0, both rolls and hexagons bifurcate supercritically, but rolls are stable. Finally, when β ≠ σ and/or ψ ≠ 0 a hysteretic transition to H + or H - occurs depending on sgn( β + λ k 2 c −σ ); if ‖ β ‖, ‖ λ ‖, ‖ σ ‖ ⪡ stable H + , H - coexist at larger amplitudes, but if ‖β + λ k 2 c −σ‖ ⪡ ‖β‖, ‖λ‖, ‖σ‖ = 0(1) , a further hysteretic transition takes place with increasing amplitude in which the hexagons are replaced by stable rolls.