# LDD-02 · Radiant Slab System

> **Status:** 🟡 LOCKED v2.0 — strategy locked, densities + zoning need mechanical designer to finalize.

## One-line intent

Comfort-density tubing for people; maintenance-density tubing for garages — warm where life happens, tempered where function matters.

## Why this matters

This is the doc's most fully thought-through mechanical strategy and the right philosophical framing: not every slab is a "heated slab" in the same way. Getting the two-tier strategy explicit avoids the common mistake of either over-conditioning garages (wasted energy) or installing no radiant at all and regretting cold winter slabs (no fix later).

## Locked decisions

**Two-tier philosophy**

| Tier | Spacing (planning) | Where | Goal |
|---|---|---|---|
| Comfort-density | 6"–9" OC (tighter at perimeter glazing) | Living wing, ILS living/bed/bath, downstairs baths, primary circulation | Barefoot comfort, stable winter occupancy |
| Athletic / mid | 9"–12" OC | Gym slab | Real comfort during use, not just freeze protection |
| Maintenance-density | 12"–18" OC | South garage bays, ILS garage, hybrid receiving, lift/service | Slab tempering, dry storage, usable winter garage |

**Three manifold clusters** (locked):

- **North manifold** — ILS living + bedroom + bath, north living-wing quiet, upper north bedroom baths.
- **Central manifold** *(primary distribution hub)* — main mech room. Living wing comfort zones, gym slab, potentially UCR and upper living radiant.
- **South manifold** — workshop, WC, equipment, garage bays, hybrid receiving, LOW (above), lift/service.

**Upper floor strategy**

- Lightweight hydronic radiant with gypcrete (or equivalent lightweight thermal topping) over wood floor framing.
- Compatible with locked exposed industrial ceiling philosophy below.
- Primary upper-floor radiant zones: UCR / pajama lounge, LOW, selected upstairs bathrooms.
- Upper radiant intended for calm baseline comfort — **not** primary HVAC response. Upstairs HVAC handles cooling, ventilation, humidity, responsive temperature.

**Slab assembly (locked, bottom → top)**

1. Compacted subgrade
2. Stone base as required
3. Underslab vapor barrier
4. Rigid slab insulation (continuous; slab-edge insulation also required)
5. Reinforced concrete slab
6. Hydronic tubing
7. Finished slab surface

> ⚠️ **Foundation type cascade — verify before locking radiant assembly.** The site plan shows the proposed dwelling on a **pier foundation** (FFE 252.4). Pier foundations typically support either (a) a structural slab on piles + grade beams (radiant strategy above still applies) or (b) a structural wood subfloor on piers with radiant overlay (entirely different assembly: gypcrete or thin-plate hydronic over plywood). **These two paths produce different costs, different acoustic behavior, different floor masses, and different slab-edge insulation strategies.** This LDD must reconcile with [LDD-01 §site context](01-structural-pemb.md#site-context-delaware) before tubing layout begins.

**Execution rules**

- Pressure testing before pour.
- Tubing layout photographed and mapped before pour.
- Manifold labels installed.
- Photo documentation mandatory.
- No undocumented tubing under slab.
- No giant single-zone system — every zone independently controllable.

## Open items / requires engineer review

- **Final tubing spacing** per zone — Manual J / radiant load calc by mechanical designer.
- **Slab thickness + reinforcement** — coordinate with structural and geotechnical.
- **Underslab insulation R-value** by zone — varies between comfort and maintenance zones; current LDD is directional.
- **Maintenance-zone target temperature** — "enough to keep slab and space moderated during winter" needs a number for control setpoints.
- **Gypcrete assembly:**
  - Dead-load impact on second-floor framing.
  - Drying time during construction (typically weeks; affects schedule).
  - Stair, door, threshold height coordination with the ~1.5" added floor depth.
  - Acoustic continuity with mineral-wool joist insulation below.
- **Gym slab option choice** — the LDD lists three options (no tubing / maintenance-density / selective). Recommended position is "athletic comfort 9–12" OC," which is option D and isn't explicitly named in the options list. Reconcile.

## Cross-references

- → [LDD-12 exposed ceilings](12-exposed-ceilings.md) — upstairs gypcrete must remain compatible with exposed ceilings below; weight + acoustic continuity.
- → [LDD-05 HVAC](05-hvac-system.md) — radiant is not a complete HVAC system; mini-splits/ducted handle response, ventilation, dehumidification.
- → [LDD-15 mechanical room](15-mechanical-room.md) — central manifold cluster lives here.
- ← [LDD-01 structural](01-structural-pemb.md) — slab assembly + frame anchorage detail.
- ← [LDD-11 envelope](11-exterior-envelope.md) — slab-edge insulation continuity at IMP-to-slab joint.

## Cost drivers

- **Tubing + labor**: $5–9/sqft for comfort-density installed, $3–5/sqft for maintenance-density. ~6,000 sqft slab area weighted-average ≈ $35–45K for tubing + labor alone.
- **Manifolds**: 3 clusters × $4–7K each installed ≈ $12–21K.
- **Boiler / heat source**: if served from a single hydronic heat pump or condensing boiler: $8–15K + flue. If from same heat pumps as ducted system: largely sunk.
- **Underslab and slab-edge insulation**: $1.50–2.50/sqft above grade across heated zones ≈ $9–15K.
- **Gypcrete upstairs** (≈ 2,200 sqft upper floor): $4–8/sqft material+install = $9–18K, plus tubing on top ≈ $15–25K total. **This is the biggest single radiant cost lever.** Dry climate-panel system instead = ~$5–7K but worse acoustic performance.

**Likely-case rollup: $90–125K total for the radiant system.**

## Air-gap concerns

1. **Two numbered §11 sections in the source.** The locked doc has Zoning Strategy and Upper-Floor Radiant both numbered §11 — a clue that this LDD was edited in pieces without a final pass. Worth a clean-up read by your friend before bidding.
2. **Comfort spacing in bathrooms.** The LDD says 6–9" OC at perimeter glazing, but bathrooms typically want 4–6" near showers and tubs. Not explicit; verify with mech designer that bathroom comfort isn't compromised by the more generic "comfort density."
3. **Workshop classification is wishy-washy.** The LDD says "comfort-density or intermediate-density depending on final use level." Decide now — workshop slab is poured once. Recommendation: intermediate (9–12") with comfort-density (6") at the bench wall.
4. **Gym slab strategy unresolved.** Section 10 commits to athletic-comfort tubing, then Section 12 lists three options that don't quite match. Pick one and delete the others.
5. **Upper-floor radiant in bedrooms not addressed.** The LDD discusses UCR, LOW, upstairs bathrooms — but the **bedrooms** are mentioned in zoning as ducted HVAC only. If you sleep with a cold floor in winter, that's a regret. Decide explicitly.
6. **Maintenance-zone control during shoulder seasons.** A maintenance-tempered garage in March might overshoot on a warm afternoon if controlled purely by air temperature; a slab sensor + outdoor reset curve avoids the issue. Spec it.

## Diagram

![Radiant zone keying with comfort, mid, and maintenance density](../../diagrams/07-radiant-zones.svg)

![Three manifold clusters](../../diagrams/08-manifold-clusters.svg)

## Status

🟡 **Yellow — sound philosophy, needs mech designer for spacing + final upper-floor system choice.** Pick the gypcrete vs dry system early; everything else is calc-ready.