diff --git a/src/item_factory.cpp b/src/item_factory.cpp
index 767250d3ab7fe..4db394cfcbb87 100644
--- a/src/item_factory.cpp
+++ b/src/item_factory.cpp
@@ -2479,24 +2479,37 @@ static void apply_optional( T &value, const cata::optional<T> &applied )
     }
 }
 
+// Gets around the issue that cata::optional doesn't support
+// the *= and += operators required for "proportional" and "relative".
+template<typename T>
+static void get_optional( const JsonObject &jo, bool was_loaded, const std::string &member,
+                          cata::optional<T> &value )
+{
+    T tmp;
+    if( value ) {
+        tmp = *value;
+    }
+    optional( jo, was_loaded, member, tmp );
+    if( jo.has_member( member ) ) {
+        value = tmp;
+    }
+}
+
 void islot_armor::load( const JsonObject &jo )
 {
     optional( jo, was_loaded, "armor", sub_data );
 
-    cata::optional<float> thickness;
-    cata::optional<int> env_resist;
-    cata::optional<int> env_resist_w_filter;
     cata::optional<body_part_set> covers;
 
     assign_coverage_from_json( jo, "covers", covers );
-    optional( jo, false, "material_thickness", thickness, cata::nullopt );
-    optional( jo, false, "environmental_protection", env_resist, cata::nullopt );
-    optional( jo, false, "environmental_protection_with_filter", env_resist_w_filter, cata::nullopt );
+    get_optional( jo, was_loaded, "material_thickness", _material_thickness );
+    get_optional( jo, was_loaded, "environmental_protection", _env_resist );
+    get_optional( jo, was_loaded, "environmental_protection_with_filter", _env_resist_w_filter );
 
     for( armor_portion_data &armor : sub_data ) {
-        apply_optional( armor.avg_thickness, thickness );
-        apply_optional( armor.env_resist, env_resist );
-        apply_optional( armor.env_resist_w_filter, env_resist_w_filter );
+        apply_optional( armor.avg_thickness, _material_thickness );
+        apply_optional( armor.env_resist, _env_resist );
+        apply_optional( armor.env_resist_w_filter, _env_resist_w_filter );
         if( covers ) {
             armor.covers = covers;
         }
diff --git a/src/itype.h b/src/itype.h
index 235f73d3ad051..ebb0a730b7d05 100644
--- a/src/itype.h
+++ b/src/itype.h
@@ -309,77 +309,84 @@ struct armor_portion_data {
 };
 
 struct islot_armor {
-    /**
-    * Whether this item can be worn on either side of the body
-    */
-    bool sided = false;
-    /**
-     * The Non-Functional variant of this item. Currently only applies to ablative plates
-     */
-    itype_id non_functional;
-    /**
-     * How much warmth this item provides.
-     */
-    int warmth = 0;
-    /**
-    * Factor modifying weight capacity
-    */
-    float weight_capacity_modifier = 1.0f;
-    /**
-    * Bonus to weight capacity
-    */
-    units::mass weight_capacity_bonus = 0_gram;
-    /**
-     * Whether this is a power armor item.
-     */
-    bool power_armor = false;
-    /**
-     * Whether this item has ablative pockets
-     */
-    bool ablative = false;
-    /**
-     * Whether this item has pockets that generate additional encumbrance
-     */
-    bool additional_pocket_enc = false;
-    /**
-     * Whether this item has pockets that can be ripped off
-     */
-    bool ripoff_chance = false;
-    /**
-     * Whether this item has pockets that are noisy
-     */
-    bool noisy = false;
-    /**
-     * Whitelisted clothing mods.
-     * Restricted clothing mods must be listed here by id to be compatible.
-     */
-    std::vector<std::string> valid_mods;
+    public:
+        /**
+        * Whether this item can be worn on either side of the body
+        */
+        bool sided = false;
+        /**
+         * The Non-Functional variant of this item. Currently only applies to ablative plates
+         */
+        itype_id non_functional;
+        /**
+         * How much warmth this item provides.
+         */
+        int warmth = 0;
+        /**
+        * Factor modifying weight capacity
+        */
+        float weight_capacity_modifier = 1.0f;
+        /**
+        * Bonus to weight capacity
+        */
+        units::mass weight_capacity_bonus = 0_gram;
+        /**
+         * Whether this is a power armor item.
+         */
+        bool power_armor = false;
+        /**
+         * Whether this item has ablative pockets
+         */
+        bool ablative = false;
+        /**
+         * Whether this item has pockets that generate additional encumbrance
+         */
+        bool additional_pocket_enc = false;
+        /**
+         * Whether this item has pockets that can be ripped off
+         */
+        bool ripoff_chance = false;
+        /**
+         * Whether this item has pockets that are noisy
+         */
+        bool noisy = false;
+        /**
+         * Whitelisted clothing mods.
+         * Restricted clothing mods must be listed here by id to be compatible.
+         */
+        std::vector<std::string> valid_mods;
 
-    /**
-     * If the item in question has any sub coverage when testing for encumberance
-     */
-    bool has_sub_coverage = false;
+        /**
+         * If the item in question has any sub coverage when testing for encumberance
+         */
+        bool has_sub_coverage = false;
 
-    // Layer, encumbrance and coverage information for each body part.
-    // This isn't directly loaded in but is instead generated from the loaded in
-    // sub_data vector
-    std::vector<armor_portion_data> data;
+        // Layer, encumbrance and coverage information for each body part.
+        // This isn't directly loaded in but is instead generated from the loaded in
+        // sub_data vector
+        std::vector<armor_portion_data> data;
 
-    // Layer, encumbrance and coverage information for each sub body part.
-    // This vector can have duplicates for body parts themselves.
-    std::vector<armor_portion_data> sub_data;
+        // Layer, encumbrance and coverage information for each sub body part.
+        // This vector can have duplicates for body parts themselves.
+        std::vector<armor_portion_data> sub_data;
 
-    // all of the layers this item is involved in
-    std::vector<layer_level> all_layers;
+        // all of the layers this item is involved in
+        std::vector<layer_level> all_layers;
 
-    bool was_loaded = false;
+        bool was_loaded = false;
 
-    int avg_env_resist() const;
-    int avg_env_resist_w_filter() const;
-    float avg_thickness() const;
+        int avg_env_resist() const;
+        int avg_env_resist_w_filter() const;
+        float avg_thickness() const;
 
-    void load( const JsonObject &jo );
-    void deserialize( const JsonObject &jo );
+        void load( const JsonObject &jo );
+        void deserialize( const JsonObject &jo );
+
+    private:
+        // Base material thickness, used to derive thickness in sub_data
+        cata::optional<float> _material_thickness = 0.0f;
+        cata::optional<int> _env_resist = 0;
+        cata::optional<int> _env_resist_w_filter = 0;
 };
 
 struct islot_pet_armor {