7. Basements Archives - Hammer & Hand

7.1 BASEMENTS: NEW CONSTRUCTION

sproutbox — Thu, 26 Sep 2024 19:55:17 +0000

In addition to structural integrity, we need to address two priorities when constructing new basements: moisture protection and thermal control. With new construction, we have the luxury of installing our moisture management layers outboard of the foundation wall. Our preferred method uses a fluid-applied elastomeric membrane applied directly to the exterior face of concrete followed by a drainage mat with integrated filter fabric. Water in the soils adjacent to the foundation wall can then drain down to a drainage pipe installed in a bed of gravel next to the foundation wall footing.

A bentonite waterstop laid atop a capillary break on the footing inhibits moisture movement through the cold joint between foundation footing and foundation wall. To limit heat, air, and moisture transmission we use low or medium density spray foam insulation in a stud wall (or, alternatively, a continuous layer of EPS against the concrete wall combined with blown-in insulation in a stud wall immediately inboard of that EPS). The concrete slab sits on 2-6” of rigid insulation with a perimeter thermal break and is then topped with a soil-barrier membrane, completing the basement’s thermal and moisture isolation from the earth.

A. DRYWALL
B. 2X4 STUD WALL
C. 1- 2 1/2” GAP BETWEEN STUD WALL AND FOUNDATION WALL
D. LOW OR MEDIUM DENSITY SPRAY FOAM INSIDE AND BEHIND STUD WALL
E. FOUNDATION WALL
F. FLUID APPLIED ELASTOMERIC MEMBRANE
G. DIMPLED DRAINAGE MEMBRANE WITH GEOTEXTILE FABRIC

H. MINIMUM 4” LAYER WASHED AND CLEANED (NO FINES) CRUSHED STONE OR GRAVEL
I. 4” DRAINAGE PIPE, HOLES DOWN
J. 2-4” RIGID INSULATION
K. NOT SHOWN: IF RIGID FOAM INSULATION IS USED ON AN EXTERIOR WALL, PROTECT THE TOP WITH A COATING OR PROTECTION BOARD. BELOW GRADE, PLACE INSULATION BETWEEN DIMPLE DRAINAGE MAT AND ELASTOMERIC WATERPROOF MEMBRANE. FOAM MUST BE PROTECTED WITH DURABLE FINISH WHEN EXTENDED ABOVE GRADE.

A. HEAVY DUTY GEOTEXTILE FILTER FABRIC
B. WASHED GRAVEL FILL
C. 4” DRAINAGE PIPE, HOLES DOWN. ALWAYS LOCATE BELOW BOTTOM OF SLAB.
D. FOOTING
E. DIMPLED DRAINAGE MEMBRANE WITH GEOTEXTILE FABRIC
F. FLUID APPLIED ELASTOMERIC WATERPROOF MEMBRANE
G. EXPANDING JOINT WATERSTOP*

H. FOUNDATION WALL
I. FLUID APPLIED CAPILLARY BREAK (SEE SECTION 7.2)
J. LOW OR MEDIUM DENSITY FOAM
K. DRYWALL
L. FLOOR SLAB
M. 12 MIL REINFORCED SOIL BARRIER WITH SEALED SEAMS (SEE SECTION 7.3)
N. 2-4” RIGID INSULATION

*Waterstops should be installed at all joints below grade. Place water stop a minimum of 3” to the exterior surface of the wall.

Note: Utilize pipe cast into footings to interconnect sub-slab drainage zone.

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7.2 BASEMENTS: CAPILLARY BREAK

sproutbox — Fri, 27 Sep 2024 07:16:11 +0000

Footings and stem wall coated with capillary break/fluid applied waterproofing.

Fluid-applied elastomeric waterproof membrane.

Fluid-applied capillary break.

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7.3 BASEMENTS: SUB SLAB VAPOR/SOIL-BARRIER

sproutbox — Sat, 28 Sep 2024 16:24:17 +0000

Taped seams and sealed penetrations in the sub slab vapor and soil barrier.

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7.4 BASEMENTS: RETROFIT

sproutbox — Mon, 30 Sep 2024 04:40:23 +0000

Basement retrofits require special care and attention to moisture management, but with a unique constraint in most cases: all our efforts are limited to the inside of the structure. Our experience with basement retrofits is that there are two types of existing basements: ones that leak now and ones that will leak in the future. Therefore, our retrofit strategy is to establish a robust behind-the-wall drainage system interconnected to an active dewatering system.

First, we create an interior French Drain to capture any water that leaks through foundation walls. Second, we install a dimple drainage mat on the inside face of the foundation wall to ensure that water is directed down to that French Drain. Third, we seal up and insulate the assembly to control for vapor and thermal transfer.

Interior French Drain Installation – Step-by-Step

Remove existing concrete basement floor with concrete saw or a 75lb+ jackhammer.
Cut concrete floor at least 14” away from foundation wall (wider if footing is in the way).
Leave 16” sections of the concrete floor (contact points) every 15’-20’ to keep foundation wall stable.

After concrete has been removed, dig a trench 12” from top of concrete floor.
Use a sump pump to manage water during digging.
Always use a sump pump basin with air/vapor tight lid.
The sump basin should be surrounded by at least 6 to 8 inches of 3/4” washed gravel.
DO NOT undermine footing or foundation wall, maintain a 45 degree angle.

Install heavy duty geotextile fabric in trench.

Start from sump and lay perforated plastic pipe (holes down).
Typical pipe slope/pitch is 1/4” per 5ft.
This discharge line is typically a 1-1/2” inch PVC schedule 40 (solid) pipe.
Always place in-line check valve directly above pump.

Fill trench with clean, crushed stone (3/4” to 1-1/2”).

Run dimple drainage mat at foundation wall to trench for weep.

The cut concrete edge must be sprayed down and brushed so the new concrete has a clean surface to bond to.
Pour new concrete flush with existing floor.

Note: It is critical that dimple drainage mat is terminated before intersecting embedded wood beams and the top of the foundation wall. This prevents the wood from coming into contact with the humid air between the foundation wall and dimple mat.

Frame basement wall 1” off wall with a capillary break under bottom plate.
Install vertical, floor to ceiling fire blocking per code.

Spray foam rim joist and foundation wall.

Install Drywall.

Additional Information

Note: Warn clients against using carpet and vapor impermeable flooring on existing uninsulated slabs.

Flooring Options Going Forward:

Leave as is (highest risk).
Add vapor-tight dimple mat over existing slab and install new flooring.
Add rigid insulation over existing slab and install new flooring.
Remove existing slab and install rigid insulation, heavy duty sealed soil barrier, and new slab.

Always Provide Radon System Option:
Provide electrical near the sump pump to allow for easy addition of a radon system if necessary.

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