DEAP includes a calculation based on IS EN 13790: 2004 to assess a dwelling's capacity to store heat within its structure, represented by internal heat capacity. Internal heat capacity will tend to have two opposing effects in relation to net space heat demand calculated by DEAP:
Disadvantageous: Under normal intermittent heating conditions, a higher internal heat capacity (heavier) structure will tend, in heating up and cooling down, to respond more slowly than a lighter structure and hence will maintain a higher internal temperature during heating off periods. This will result in a higher daily mean internal temperature in the dwelling and, since gross daily demand for heat is reflected in the difference (temperature lift” between internal and external temperature, will increase the gross demand for space heating.
Advantageous: A higher internal heat capacity offers a higher potential to store heat from free heat sources (internal gains and solar gains). Such free heat is irregular in its pattern of availability, and thus not necessarily useful in contributing to the scheduled heating requirements of the dwelling. By storing a higher proportion of such irregularly received heat, a higher internal heat capacity structure allows that heat to be retained and released at times when it can make a useful contribution to the scheduled heating needs. This is reflected in the DEAP procedure in the form of a higher utilisation factor being applied to the free heat gains.
Whether or not the net effect of internal heat capacity is beneficial thus depends on the relative extent of these two effects (see heating systems in section 4).
The position of insulation affects the internal heat capacity of a construction. If a masonry component is insulated internally, the masonry will not contribute internal heat capacity, but if insulated externally, it will. The internal heat capacity of a building element (wall, roof, floor, internal partition, etc.) is determined primarily by the properties of the layers adjacent to the living space. Thermal capacity deep within an element (say 10 cm or more) contributes little to heat storage under a typical daily internal temperature cycle.
The index of internal heat capacity required is the thermal mass category of the dwelling. The five categories are: low, medium-low, medium, medium-high, or high. The category is determined using one of the following options:
For existing dwelling assessments, the thermal mass category may be derived using Table 2.43 or Table 2.41 (optionally combined with the calculation method outlined below following EN ISO 13786).
For new final or new provisional dwelling assessments, the thermal mass category may be derived using Table 2.41 (optionally combined with the calculation method outlined below following EN ISO 13786). Table 2.43 is not used for new dwelling assessments.
As an alternative way of determining whether an element is thermally light or heavy, the internal heat capacity of the element may be calculated as described in EN ISO 13786:2017 using a time period of 24 hours and with surface resistances included. A result of 38 kJ/m2K or greater is considered to have high thermal mass, and a lower result is considered thermally light. The resulting figures are then used as per Table 2.41 to determine the Am/Af ratio and the resulting thermal mass category.
Thermal mass also impacts the cooling energy demand for dwellings with fixed cooling systems installed.
¶Calculating the Thermal mass category of a dwelling
First, each opaque element type (walls, ceilings, floors, both external/ exposed and internal) are classed as either low, medium, or heavy mass, by selecting the class of the construction from Table 11a most closely matching the construction in question.
If an element appears borderline between two of light/medium/heavy from Table 2.42, then assume half of its area belongs to each of the two applicable mass categories.
The area of the heavy thermal mass elements is calculated as “Aheavy“
The area of the medium thermal mass elements is calculated as “Amedium“ and is multiplied by 0.5.
Derive Am = Area of thermally heavy elements = Aheavy + (Amedium x0.5). It does not include area of low thermal mass elements
Divide Am by the total floor area of the dwelling to obtain the Am/Af value.
Select the dwelling thermal mass category to be input into DEAP using Table 2.41 here, based on Am/Af
¶Table 2.41 (formerly Table 11) - Thermal mass categories for the dwelling
Thermal Mass Category
AmAf ranges
Heat capacity per unit floor area [MJ/m2K]
Low
High
UF
IH
Low
<0.25
0.25
0.07
0.07
Medium - low
0.26
0.75
0.14
0.09
Medium
0.76
1.50
0.20
0.11
Medium - high
1.51
2.75
0.32
0.15
High
2.76
>2.76
0.50
0.20
Notes
Am = area of thermally heavy elements
Af = Total dwelling floor area as calculated by DEAP and shown on Results tab
AmAf = Area of thermally heavy elements divided by total floor area. As an example, any AmAf value between 0.26 and 0.75 results in the dwelling’s thermal mass category being Medium-low in DEAP.
UF: for calculation of utilisation factor
IH: for calculation of effect of intermittent heating
a) Where an element has two sides exposed to the heated space, e.g., internal partitions or intermediate floor, both sides of the element are included in the area calculation above.
b) Where an element includes an opening, e.g., a door or window, the calculated area is net of that opening.
c) The Table 2.41 methodology can be used for existing or new dwellings. Alternatively, Table 2.43 may be used for existing dwellings.
d) Windows and doors are categorised as light and are not included in the Am/Af calculation.
This table is used to identify the mass for common floor and wall types. It is used when following the thermal mass derivation methods in Table 2.43 or the method above in Table 2.41.
Table 2.42 (formerly Table 11a) - Thermal Mass of construction elements
Element type & Description
Thermal Mass
Ground floor
Suspended timber floor
Light
Solid floor
Medium
Suspended steel frame floor
Light
Suspended beam and block floor
Medium
Suspended concrete beam floor
Medium
Suspended concrete plank floor
Medium
External wall
Timber/steel frame
Light
Masonry cavity fill with plasterboard on dabs
Medium
Masonry externally insulated with plasterboard on dabs
Medium
Masonry internally insulated
Light
Masonry cavity fill with dense plaster
Heavy
Masonry externally insulated with dense plaster
Heavy
Curtain walling
Light
Aerated concrete blockwork with plasterboard on dabs
Light
Separating wall
Masonry with plasterboard on dabs
Medium
Masonry with dense plaster
Heavy
Timber/steel frame
Light
Internal partition
Plasterboard on timber/steel stud
Light
Masonry with plasterboard on dabs
Medium
Masonry with dense plaster
Heavy
Party Floors
Concrete floor slab
Medium
Timber I-joints
Light
Precast concrete planks floor, screed
Light
Chipboard floor, plasterboard ceiling
Light
Notes
Ground floor: Can be bare, laminated, tiled, carpeted, or with wood laid directly over.
Masonry external walls includes all types of masonry, such as brick, concrete block, hollow concrete block, cast in situ concrete. The only exception is aerated concrete block which has a specific mass assignment if in combination with plasterboard on dabs as shown above.
Most ceilings will have a light mass. Where ceilings are present with heavy mass such as a concrete intermediate floor in a dwelling or concrete floor of apartment above is the ceiling, the Table 2.43 approach cannot be used for existing dwellings. Where this is the case, Table 2.41 is used.
The derivation of the thermal mass category also requires knowledge of the thermal mass of internal partitions (as recorded during survey or review of dwelling plans).
The thermal mass category may be based on the following defaults for existing dwellings. Alternatively, the method in Table 2.41 may be used.
To derive the thermal mass category to be used in DEAP using this method, first identify each of the predominant element mass values for each of the four element types (ground floor, external walls, separating walls to adjoining premises and internal partitions) using Table 2.42. For example, Table 2.42 defines the mass of a ’Suspended timber floor‘ as ’Light‘ and that of a separating ’Masonry with dense plaster‘ wall as ’heavy‘. See Table 2.42 for a detailed list of thermal mass of different element types. Count the number of each type identified according to light, medium and heavy. Use Table 2.43 here to identify the DEAP thermal mass category based on the applicable count.
¶Table 2.43 (formerly Table S10) -Thermal mass category defaults
Number of light elements
Number of medium elements
Number of heavy elements
Thermal mass category
4
0
0
Low
3
1
0
Low
3
0
1
Medium-Low
2
2
0
Medium-Low
1
3
0
Medium-Low
2
1
1
Medium
2
0
2
Medium
1
1
2
Medium
0
4
0
Medium
1
2
1
Medium
1
0
3
Medium-High
0
3
1
Medium-High
0
2
2
Medium-High
0
1
3
High
0
0
4
High
Notes:
When using Table 2.43, there are fourelement types to be considered: ground floors, external walls, internal partitions, and separating walls between the dwelling and adjoining premises.
As an example, if a dwelling has a ’medium‘ ground floor, ’light‘ internal partitions, ’heavy‘ external walls and ’heavy‘ separating walls, it will have 1 x medium, 1 x light and 2 x heavy. Therefore, from Table 2.43 above, the thermal mass category in DEAP is ’MEDIUM’.
For detached dwellings, apply the ’external wall‘ condition to the ’separating wall‘ column.
Where an element is a mixture of thermal masses for different sections, for example light and/or medium and/or heavy, the element mass occupying the largest area for that element type applies when identifying the mass for that element for use in Table 2.43. For example, if a dwelling has mostly plaster on timber frame internal partitions (‘light‘ from Table 2.42) and a smaller section of internal partition with masonry with plasterboard on dabs (‘medium‘ from Table 2.42), then the internal partitions are assumed to be ’light’.
Where ceilings are present with heavy mass, such as concrete intermediate floor in a dwelling or concrete floor of apartment above is the ceiling, the Table 2.43 default approach cannot be used. In this case, Table 2.41 must be used.
Several issues can arise when assessing Thermal Mass of a dwelling for a DEAP assessment, such as:
Error 1: Element descriptions / thermal mass data inadequately recorded or not recorded during survey: The information required to identify the thermal mass category of an element must be recorded during the survey. This can be on the survey form, sketches and/or additional notes. As with all survey supporting documentation, ensure the records are clear, complete, and transparent. While elements such as ground floors, walls and ceilings tend to be recorded during the site survey where the SEAI survey form is used, elements such as internal partitions and separating (party) walls are sometimes overlooked.
Error 2: Building elements assigned the incorrect thermal mass categorisation, e.g. external wall categorised as light when it should be heavy based on site observations. Ensure elements are correctly categorised before assigning an overall thermal mass category using Table 2.43 or Table 2.41 for the dwelling.
Error 3: Incorrect interpretation of the difference between separating walls and internal partitions. The key differences are identified below:
Separating walls: the walls between the dwelling in question and an adjoining heated or enclosed unheated space, e.g. garage in a house or corridor in an apartment block. Typically, a separating wall will be a party wall between the dwelling in question and an adjoining dwelling. For example, a semi-detached house will have a separating (party) wall. When using Table 2.43 and there is no separating wall, e.g. a detached house without adjoining garage, assume the separating wall has the same characteristics as the predominant external wall.
Internal Partitions: are the walls dividing the spaces internally within the dwelling. They are not heat loss walls. As stated above, it is common for these partitions to be overlooked when recording the dwelling characteristics during the site survey.
Error 4: Both sides of internal partitions not accounted when using the Table 2.41 AmAf methodology. As both sides of internal partitions absorb/release heat to the dwelling, the area of both sides must be accounted when using the Table 2.41methodology.
Error 5: Openings not deducted from element areas when carrying out a Table 2.41AmAf calculation. Windows and doors are categorised as light thermal mass when carrying out an AmAf calculation in accordance with Table 2.42 above. The area of thermally massive elements is therefore net of the area of windows and doors. Note that when dealing with internal partitions the opening area is deducted from both sides of the internal partition area calculation.