Use of dimpled membrane on basement floor with no vapor barrier nor insulation
Refinishing Basement with no foam nor polysheet under concrete. Zone 5. High water table with sump pump. No radon. Crawlspace dirt floor is dry.
Floor is sloped from one side to other, then graded to a low spot.
Early 1900’s builders did think about events.
>Propose to modify this approach or clarify the design intent (from links):
(floor leveler, rigid foam, underlayment, flooring) or
(floor leveler, dimpled mat, rigid foam, underlayment, flooring)
https://www.greenbuildingadvisor.com/question/use-of-dimpled-membrane-on-basement-floor
https://www.greenbuildingadvisor.com/question/basement-wall-insulation-5
>Proposed modification places the dimpled mat on top, not under, the foam.
leveler, rigid foam, dimpled mat(cup up), underlayment, flooring
Wall: rigid foam, horizontal then vertically crossed furring strips, thermal barrier (as seen on This Old House)
A ~square gap is left at the wall to floor intersection. All edges of foam are tape sealed. Floor ~ square gap also acts as drainage path.
A separate 2×4 wall with gap is an option. Underlayment options crossed ½” plywood, tongue groove OSB, or glass fabric cement board (needs level floor).
Design intent: dimple mat provides limited air path to wall-floor gap, then up the wall gap to room. Floor drain with sump pump is a dedicated, non-perforated (sewer, but no sewage) basin-provides hot water tank relief. For any event, dimpled mat provides appropriate path to low side.
Optional: Bath fan(s) on ceiling (low floor side) with AirCycler SmartExhaust Timer Switch provides periodic ventilation or manual with delay. The fan exhaust would be directed to the Ceiling (high floor side) to flow down and across the dimpled mat to aid drying. Could be be inhibited with >60% humidity too.
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Dimpled mat -- and its cousin rainscreen -- is used when you have a surface that is waterproof but not impermeable and you want to make sure that hydrostatic pressure can't build up and force water through the surface. On a basement floor the direction you don't want water to flow is up and there isn't going to be up pressure. What you do want is for your covering to be impermeable, a flat sheet is going to be easier to tape at the joints to seal.
I think the biggest thing I have learned from this site is that every part of your house has to be clearly either inside or outside the building envelope -- there is no sort of or kind of, which is how crawlspaces and attics in older houses were often treated. If it's part of the building envelope it has to be airtight, watertight and vaportight. If it's not, the boundary between it and the building envelope has to be all of those things.
"building envelope it has to be airtight, watertight and vaportight" Yes, but the design goal is heat loss and floor drainage. The heat loss from the basement floor is substantial and R-28 is recommended vs concrete.
In the recommended approach for a dry basement, sealing the dimpled mat and rigid foam board, it prevents drainage from above the flooring (water tank or washer leak, outside rain storm...). Circulating air at the concrete floor would substantially reduce the R value (like soffit air insulation washing).
Do you have any suggestions for the floor drain if the dimple mat is on the floor? Thanks!
If you are worried about plumbing appliance leaks, put them in a pan with a drain. It's now code for above-ground installations, if you have a finished basement the logic is the same.
Yes, could raise each and add a condensate pump. Was trying to connect potential leaks directly to the floors "airtight sump crock" per the article linked below.
The Dranjer Sump and/or Floor Drain should work instead of raising the appliances.
Added Miridrain and Miridry to home many years ago--same approach but outside-it really worked! Missed a spot (now corrected) and the mold and mildew followed.
Many reasons this house must be done inside. Thanks again :)
The dimple mat should be on the floor. On the walls too, if there is any chance of leakage through the walls. The dimple mat should be taped at the seams and air, vapor and water-tight. No air circulation is necessary, as there is nothing below the surface of the dimple mat that cares if it is wet or dry. No square gap at the wall/floor intersection. The dimple mat takes care of drainage around the corner. There should never be water on top of the floor.
You have no more need to drain water from your finished basement floor than you do your finished kitchen floor. If water gets on top of the floor, you have a problem in either case. Something has failed and you've got a cleanup on your hands. In your case, I would rig the sump pit lid to be reasonably air, vapor and watertight, but removable for servicing. Worst case of a flood from a plumbing failure, you could pull the lid to allow bulk water to enter the sump that way. Both the topside and bottomside of the dimple membrane should be able to drain into the sump. If you want to, include an access port in your floor to get to the floor drain as well. This port can also be pulled to allow bulk water to drain.
FWIW, you can buy tapered insulation from roofing insulation manufacturers. The insulation is made custom for your profile. This would allow you to leave the floor pitch in place to drain to the sump and/or floor drain, while having a level floor above.
Thanks. Appreciate replies :) Excellent points.
--"dimple mat should be on floor, taped at the seams air, vapor and water-tight"
Yes, appreciating the logic. Dedicated floor drain/sump tied below this complete barrier. (cannot be tied to the existing, necessary sump system)
--"dimple mat on the walls too"
Adds cost when a gap exist between thermal barrier and other side of rigid board. Best addressed outside - it works!
-- " Something has failed and you've got a cleanup on your hands" .
Do not want subfloor damaged--concrete board with level floor best solution. Met owner who sadly had burst hose onto compressed board flooring.
-- "include an access port in your floor to get to the floor drain, reasonably tight"
Yes. Important to test pump. Design is to cut hole through mat and board sealing the basin and the board. Basin top has sealed ports and is the access port.
-- "No square gap at the wall/floor intersection"
Building on your comment, perhaps that ‘square’ channel can serve another purpose.
A) seal mat+insulation board to the floor( not wall) leaving ~2" gap before wall
B) Add tape/membrane from on top of A), along floor, and back up the wall ~6"
C) Wall insulation begins about 1" higher than the floor rigid board
The square channel now acts as a reservoir for any event above floor grade, especially a broken laundry hose while maintaining the complete seal.
More importantly, add a Dranjer Sump Drain to the cover lid to drain the dimpled mat or 'square reservoir. Include an air admittance valve to limit any air entry. Comments?
-- "tapered insulation" Yes for a level floor! Here, floor is already sloped from one side to another, then graded to low spot, 'square channel' could act as reservoir. A Dranjer floor drain would be the equivalent of "pulling the plug" to reach basin. Excellent.
Here's a good article that covers a lot of these issues:
https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations#P08