Yes, the R-value will add of your different layers. If you wear layer A with R=5 and layer B with R=2.5, the overall insulation value will be R=7.5.
To explain this a bit, we think of two layers or flat walls which interact only due to thermal conduction. This is just a model and in reality other effects will come in play. The Fourier Law for thermal conduction (relative to the surface) of one layer states:
heatFluxDensity = thermalConductivity / thickness * temperatureDifference
[W/m²] = [W/(mK)] / [m] * [K]
The R-value is stated as a material property as
R = thickness / thermalConductivity
with the unit [m²K/W] which leads from Fourier law to
R = temperatureDifference / heatFluxDensity
Adding several layers, we can use the electrical analogy and are able to add the resistances of each layer to an overall resistance.
Knowing your material properties (which might be difficult in the clothing industry), assuming a temperature gradient or a heat flux, you can then evaluate the other using this equation:
heatFluxDensity = (innerTemp - outerTemp) / (R1 + R2 + ... + Rn)
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