Quick Decision: DPM, Insulation, Or Both?
A damp proof membrane (DPM, 1200 gauge polythene) is required for internal or habitable ground floors under Part C (resistance to moisture). We also recommend it under most enclosed outbuildings to protect finishes from ground moisture. Insulation is essential for heated or habitable spaces to meet comfort and Part L (thermal efficiency). It’s usually optional for simple, unheated sheds.
Many projects need both: the DPM blocks moisture; insulation limits heat loss and prevents cold floors. Patios, driveways and external pads should not have a DPM. For extensions and conversions in Suffolk, Essex and Norfolk, your local Building Control has the final say. See our notes on domestic bases and Building Control, or ask for technical advice.
Sheds And Garden Buildings: What’s Needed?
Basic unheated shed (tool storage): compacted Type 1, sand blinding, 1200g DPM (recommended), and 100–125 mm concrete. Insulation is not usually required. Keep the slab level, allow falls outside, and plan simple saw-cut joints.
Insulated garden rooms and studios: use DPM + insulation to avoid cold surfaces and condensation. A common build-up is slab, then insulation, then screed/board. For workshops with machinery, use higher compressive strength insulation (EPS100/XPS) or place insulation above the slab with a screed to spread point loads. See our garden room slabs guide.
House Extensions: Building Control Expectations
Typical ground-bearing detail: 150 mm Type 1, sand blinding, 1200g DPM linked to the wall DPC, and a 100–150 mm slab. Insulation can go below the slab (EPS/XPS—check compressive class) or above it under a 65–75 mm screed. Above-slab PIR with UFH is popular for speed and comfort.
Target U-values are commonly around 0.18 W/m²K. Exact thickness depends on the build-up and perimeter/area ratio—get a U-value calculation if unsure. Always tape the DPM upstand to the DPC and include perimeter edge insulation. For heating layers and screeds, see UFH and screeds advice.
This image was generated with AI and may not always represent the product or service exactly.
Garage Conversions: Upgrading Cold Slabs
First, check for a DPM: look for a polythene upstand at the edges or test slab moisture. If missing or suspect, install a liquid-applied DPM or lay a new 1200g membrane over the slab. Then add insulation (often PIR) with a 65–75 mm screed—ideal for UFH—or use an insulated board subfloor for dry finishes.
Watch headroom and thresholds. You may need thinner, higher-performance insulation or minor level changes at doors. Building Control approval is required for conversion to habitable use. Plan early and confirm final floor height against skirtings and doors.
DPM Basics: Materials And Installation
A DPM is a moisture barrier. Use at least 1200 gauge (300 micron) polythene for most domestic floors. Protect it with sand blinding or smooth the sub-base to avoid punctures. Lap sheets by at least 150 mm and tape all joints.
Form upstands to meet and tape to the wall DPC. Seal around service penetrations. Repair any tears with compatible tape or patches. Where radon or ground gases are a risk, specify a certified gas-resistant membrane and follow the manufacturer’s detailing. Do not trap surface water above a membrane on external slabs.
Insulation Under Slabs: Types, Strength And Placement
PIR achieves lower thickness for the same U-value and is typically used above the slab under screed. EPS is cost-effective; use EPS100 or higher under slabs. XPS offers higher compressive strength and moisture resistance for damp sites or heavier loads.
Below-slab insulation brings thermal mass inside and can reduce thermal bridges if detailed well. Above-slab insulation is simpler for services and suits UFH. Typical ranges to reach ~0.18 W/m²K: about 100–120 mm PIR or 150–200 mm EPS (confirm with a calc). Always add perimeter edge insulation to limit cold bridging.
Typical Suffolk Build-Ups We See (With Specs)
Unheated shed on clay: 150 mm Type 1, sand blinding, 1200g DPM (recommended), 100–125 mm C25/30 with fibres, saw-cut joints. No insulation.
Heated garden room with UFH: 150 mm Type 1, sand blinding, 1200g DPM tied to wall DPC/upstand, 100–125 mm slab, 100–120 mm PIR, UFH in 65–75 mm screed. Confirm thicknesses with design. Need quantities? Try our concrete calculator.
Single-storey extension to Part L: as above, but confirm insulation by U-value calculation and with Building Control. Include perimeter edge insulation and airtight taped laps. On Suffolk clays, ensure drainage falls and avoid soft spots.
This image was generated with AI and may not always represent the product or service exactly.
When You Don’t Use A DPM
External slabs—patios, driveways, yards—and equipment pads should not have a DPM, as a membrane can trap rainwater. Instead, design adequate falls, drainage, joints and a solid sub-base.
For suspended beam-and-block floors, the DPM or vapour control layer usually sits above the deck within the screed build-up. In retrofits, liquid DPMs over existing internal slabs can be effective where a sheet membrane is impractical.
Moisture And Gases: East Anglia Context
East Anglia has plenty of clay and variable water tables. Good compaction, blinding, and careful membrane detailing keep floors dry. Maintain clear perimeter drainage paths and check thresholds.
Radon is generally low to moderate across Suffolk, Norfolk and Essex, but check the official map for your postcode. Where required, specify a certified gas membrane with sealed laps and any ventilation layers, and agree details with Building Control.
Concrete Spec, Thickness And Ordering Made Simple
Typical thickness: 100–125 mm for sheds and garden rooms; 125–150 mm for garages/workshops. Use mesh (A142/A193) or macro/micro-fibres for load and crack control. C25/30 is a versatile choice; discuss slump for placement and finish. Ask about GGBS/PFA for lower carbon options.
Eco Concrete Ltd mixes on site, so you only pay for what you lay. Flexible loads and same/next‑day delivery across Suffolk, Ipswich, Woodbridge, Bury St Edmunds, Lowestoft, Norfolk and Essex. Tight access? Our wheelbarrow service can help.
Step-By-Step Install Checklist
- Excavate; compact Type 1 in layers; level sand blinding.
- Lay and tape 1200g DPM with upstands; set formwork; seal around services.
- Place insulation (grade/thickness per design) and perimeter edge insulation; run services.
- Install UFH if used; pour concrete/screed; strike, finish and joint.
- Cure properly, protect from frost/heat, and test moisture before flooring. See concrete curing explained.
Costs, Timing And Value
Costs depend on area and thickness, reinforcement, insulation type and thickness, membrane spec, access, and finish. Weather and curing affect time on site. Good planning reduces extras and rework.
With volumetric mixing there’s no waste and accurate quantities. That means fewer overruns and flexible schedules. Use our calculator for a quick estimate, then call for a firm plan.
When To Call Building Control — And When To Call Us
Call Building Control for extensions, garage conversions, habitable garden rooms, and changes to thermal elements. They’ll want details of membranes, insulation, junctions, and U-value calculations.
To book with Eco Concrete Ltd, we need size and depth, reinforcement choice, DPM/insulation spec, access limits, pour date/time, distance to pour, wheelbarrow or pump, and finish. For fast local help, get in touch via our contact page.
FAQs
What thickness of insulation will meet Building Regulations?
It depends on the build-up and perimeter/area ratio. As a guide, ~100–120 mm PIR or 150–200 mm EPS often achieves ~0.18 W/m²K. Get a U-value calculation to confirm.
Can I put insulation under the slab instead of above?
Yes. Below-slab EPS/XPS works well if detailed to avoid cold bridges. Above-slab PIR with screed is simpler for services and UFH.
How do I link the DPM to the wall DPC?
Take the DPM up the wall as an upstand and tape it to the DPC with compatible tape. Keep laps clean, tight and fully sealed.
Do I need a DPM under a patio or driveway?
No. External slabs should not have a DPM. Provide falls, drainage and a quality sub-base instead.
What concrete mix do you recommend for a shed base?
C25/30 is a good all-round choice. Use fibres or mesh depending on load and crack control, and pour 100–125 mm thick for most sheds.
How soon can I lay flooring on a new screed?
Allow adequate drying and confirm with a moisture test to the flooring manufacturer’s limit. Conditions, thickness and mix all affect drying time.