My neighbour said definitely two radiators. My builder said one is enough. I went with the builder. Here's what actually happened.
The 1 vs 2 radiator debate
This question comes up in almost every attic conversion. It sounds simple, but the answer depends on your space, your insulation, and how you plan to use the room. Here's the honest trade-off:
More even heat distribution across the room. Faster warm-up time, especially on cold mornings. If one is at each end of the room, you avoid cold spots. The standard recommendation for rooms over 20m² is two heat sources.
We went with one. A standard double-panel radiator on the gable wall (the tall, flat wall at the end of the room). It's been fine. On the coldest days, if the Velux windows have been open all day, it takes a few hours to bring the room up to 21-22°C. But in practice that rarely happens - you don't tend to leave attic windows wide open in January.
Getting the size right
Whether you go with one radiator or two, the size matters more than the quantity. An undersized radiator will leave you cold no matter how many you have. The standard way to calculate this is BTU (British Thermal Units) - the measure of a radiator's heat output.
The calculation depends on room dimensions, ceiling height, insulation quality, window size, and how exposed the room is. A well-insulated attic conversion with modern insulation (spray foam or Kingspan) and double-glazed Velux windows needs fewer BTUs than an older, draughty room.
Rule of thumb
For a well-insulated attic room, you typically need around 40-50 watts per m². A 15m² room needs roughly 600-750 watts. A 25m² room needs 1,000-1,250 watts. These are approximations - use a proper BTU calculator for your specific room.
Useful tool: The Radiator Shop Ireland - BTU Calculator
If in doubt, go slightly larger than the calculation suggests. A radiator that's too big can always be turned down. One that's too small will never heat the room properly, no matter how long you leave it on.
Source: BestHeating - BTU Calculator & Sizing Guide
Central heating vs electric radiators
The first question is whether you can extend your existing central heating system to the attic at all. In most cases you can - but it depends on your boiler and system type.
This is the preferred option. Your plumber runs new pipework from the existing system up to the attic and connects a standard radiator. It's cheaper to run long-term because gas is significantly less expensive than electricity for heating.
However, there are situations where this isn't straightforward. Older boilers may not have the capacity for an additional radiator. Some systems use an expansion tank (open-vented) that may not have enough pressure to push water up to the attic - your plumber may need to convert to a pressurised (sealed) system. Check this before assuming it's a simple job.
Our recommendation
If your existing central heating system can support an extra radiator, always go with that. It's cheaper to run and your builder/plumber can handle it as part of the conversion. Only fall back to electric if a wet radiator genuinely isn't feasible for your setup.
The zone problem
Here's something that catches people off guard. Most Dublin houses have two heating zones: upstairs and downstairs. Each zone has one thermostat - when it calls for heat, every radiator in that zone comes on.
When you add an attic radiator to the upstairs zone, its temperature is controlled by wherever the upstairs thermostat sits - usually the landing. If the landing is warm but the attic is cold (which happens, because heat rises and the landing warms up first), the heating won't kick in for the attic. Your attic radiator sits cold while the thermostat thinks the upstairs is warm enough.
There are a few ways to deal with this:
Smart thermostatic radiator valves (TRVs) give you per-room temperature control. A wireless temperature sensor in the attic tells the valve when to open or close, independent of the main zone thermostat. The attic can effectively call for heat based on its own temperature. This is what we did, and it works well for control - though it's worth noting that running the boiler for a single radiator is less efficient than heating a full zone.
Where to put the radiator
Radiator placement matters more in an attic than in a normal room, because your wall space is limited by the sloped ceilings.
Use the gable wall
The tall, flat wall at each end of the attic (the gable wall) is the best spot. Full height, no slope to work around, and it doesn’t compete with Velux windows.
Avoid the knee walls
The low walls under the sloped ceiling (typically 1.1-1.4m) are too short for most radiators and the heat rises straight into the slope rather than circulating the room.
Keep it away from the bed head
Sleeping next to a hot radiator is uncomfortable. If the radiator and bed are on the same wall, leave space between them.
Consider a vertical radiator
If horizontal wall space is tight, a tall vertical radiator takes up less width while delivering the same BTU output. They’re more expensive but can solve a tricky layout.
What to ask your builder
"Can my boiler handle an additional radiator?"
Older or smaller boilers may be at capacity. Your plumber should check the boiler output and the current system load.
"Is my system open-vented or sealed/pressurised?"
Open-vented systems with an expansion tank can struggle to push water up to the attic. A sealed system handles this better.
"What size radiator do you recommend for this room?"
They should be able to give you a BTU figure based on the room size and insulation. If they guess, that’s a concern.
"Will the attic radiator be on the existing upstairs zone?"
If yes, ask how you’ll control the attic temperature independently. A smart TRV or separate zone valve are the main options.
"Is a wet radiator feasible, or would you recommend electric?"
A good plumber will be honest about whether extending the system is practical. If they default to electric without explaining why, push back.
The verdict
For a standard Dublin semi-d attic conversion (15-25m², well insulated, Velux windows), one correctly sized radiator is enough - especially when your BER rating and energy efficiency are strong. It heats the room, it saves you wall space, and it's cheaper to install. Two radiators make sense if the space is large (40m²+), poorly insulated, or if you're splitting it into multiple zones.
The more important decision is how you connect and control it. Extend your central heating if you can - it's far cheaper to run than electric. And think about how you'll manage the temperature independently from the rest of the house, whether that's a smart valve, a separate zone, or just being comfortable adjusting the TRV manually.
Further reading: Rointe Ireland - Efficient Heating Solutions for Attic Conversions | Trade Radiators - How to Keep Your Loft Warm
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