How to Calculate Heat Loss in a House

If you’re considering a new heating system for your home, it’s a good idea to conduct a heat loss assessment of the property to ensure that your chosen heating system will keep the space sufficiently warm. In this expert guide, we’ll walk you through the benefits of this kind of energy audit, providing you with a step-by-step walkthrough of how to perform a heat load calculation. You’ll discover: 

• How to calculate the design temperature of a space 
• How to work out surface heat loss 
• How to calculate air infiltration heat loss levels 
• If radiant floor heating will provide enough heat for a property 
• The benefits of working with a professional contractor on energy audits 

How Can Heat Loss Affect a Property’s Heating System? 

‘Heat loss’ describes the amount of heat that escapes a property, typically through its walls, floors, windows and other building elements. So, before installing a new heating system it’s always advisable to conduct a heat loss assessment as part of an overall energy audit to pinpoint areas in your home where such heat loss is occurring so that you can specify the right heating system for your needs. A room with very high levels of heat loss will require a heating system with a much higher heat output than a well-insulated room, for instance – something which can result in inefficient energy usage and in turn, higher running costs. 

Looking for more tips on reducing heat loss in your home? Read our informative article to discover everything you need to know. 

6 Steps for Calculating the Heat Loss of a Property 

In order to specify the right floor heating system for a property, you will need to know how many BTUs (British Thermal Units) are required to replace the heat that is escaping your home through its walls and other surfaces. Below, you can find a simple 6-step guide on how to perform a heat load calculation for a typical room in a house: the total value is determined by calculating and adding up the heat loss levels of each of these surfaces alongside the heat loss created due to air infiltration. 

1. Determine Design Temperature 

The first step is to find the difference between the ideal temperature inside the property and the lowest average outdoor temperature for your location in winter. 

This difference is called ΔT (Delta T). 

An example of this could be: 

• Indoor design temperature: 68°F 
• Average outdoor winter temperature: 40°F 
• ΔT = 68 – 40 = 28°F 

2. Calculate Surface Area

Next, you need to measure the total external wall area of a space by measuring each wall and then subtracting the area of its windows and doors. 

An example of this for one wall would be: 

• Wall length: 25 ft  
• Wall height: 8 ft  
• Area (A) = 25 × 8 = 200ft² 
• Windows and doors: 36ft² 
• Subtraction of windows/doors from total area: 200 – 36= 164ft² 

3. Determine U-Value 

U-value (measured as BTU/hr·ft²·°F) measures heat transfer through an average building element such as a wall, door or window. 

Some typical values of these are: 

• Uninsulated cavity wall: ~0.26 BTU/hr·ft²·°F 
• Modern insulated wall: ~0.032 BTU/hr·ft²·°F  
• Double-glazed window: ~0.25 BTU/hr·ft²·°F 
• Solid timber door: ~0.35 BTU/hr·ft²·°F 
• Floors: ~0.026 BTU/hr·ft²·°F 
• Roofs (pitched): ~0.023 BTU/hr·ft²·°F 

4. Calculate Heat Loss for Each Surface 

The formula for calculating Heat Loss (measured in BTUs per hour) is: 

U × A × ΔT 

So that’s the U-Value of the building element multiplied by the Area of the individual surface multiplied by the property’s Delta T figure. 

An example for a modern insulated wall would be: 

• U = 0.032 BTU/hr·ft²·°F 
• A = 164ft² 
• ΔT = 28°F 

Which means that the total heat loss for this wall would be: 0.032 × 164 × 28 ≈ 145–150 BTU/hr 

You should produce the heat loss figures for all the building elements of the specific space: its walls, windows, doors, ceiling and floor. 

5. Air Infiltration Heat Loss 

Air infiltration heat loss measures the air that escapes a room through joints in a property’s fabrication as well as cracks around doors and windows. 

This figure is measured in BTUs per hour and can be worked out using the following formula: 

Volume of air in the room (measured in ft³) × ΔT × ACH × 0.018 

Where: 

• ACH = Air Changes per Hour. This refers to the number of times air volume in a room or space is completely replaced in an hour. 
• 0.018 = a constant for the capacity of air to store heat 

Some example figures for a typical room would be:
Room dimensions of 25 ft length × 15 ft width × 8 ft height = 3000ft³ Total Volume 

ΔT = 28°F 

ACH = 1.5 

So, therefore the calculation would be: 

3000 × 28 × 1.5 × 0.018 = 2268 BTU/hr 

6. Total Heat Loss 

To work out the total heat loss level for a space, simply add up the individual heat loss figures for all surfaces / building elements in the room plus the total air infiltration figure of the space: 

Walls + Windows + Doors + Ceiling + Floor + Air Infiltration of the Space = Total Heat Loss in BTU/hr 

Always Work with an Experienced Professional 

It is always recommended that you work with a specialist in energy modelling to conduct a thorough heat loss assessment of a property. Those working in this field will utilize the latest technology to expose points of heat loss as well as air and moisture infiltration; identifying these areas yourself is often impossible using a visual inspection as they are hidden beneath flooring, behind walls and above ceilings. However, you can prepare for an energy audit by sealing up the obvious leaks around windows and doors and addressing those areas in your home where insulation is clearly needed. Installing good quality floor insulation is a great way to not only dramatically reduce your home’s heat loss levels but also improve the responsiveness of a new heating system. 

Will Radiant Floor Heating Provide Enough Heat for My House? 

Nothing is as comfortable as a warm floor touching cold feet in the winter and electric radiant floor heating is the ideal system for supplemental heat in a single room or for your entire home. Warmup systems are UL-approved and provide gentle, even heat across the floor surface to prevent hot and cold spots in a room whilst helping to conserve energy usage and save you money on your utility bills.  

Interested in a Warmup heating system for your house? Get instant pricing for our range of electric radiant floor heating systems or contact us today for more information on how we can help you solve your heat loss problems.