flexiblefullpage - default
Currently Reading

How to Insulate a Monolithic Slab

Advertisement
billboard -

How to Insulate a Monolithic Slab

Tips for insulating slab-on-grade foundations


By By Dan Morrison July 14, 2016
concrete wall insulation
This article first appeared in the July 2016 issue of Pro Remodeler.

Concrete is an excellent thermal conductor, which is another way of saying it’s a poor insulator. Heat flows from hot to cold, so in winter, concrete pulls heat out of the house, making the heating system work harder (and making a slab-on-grade floor uncomfortably cold). In summer, the concrete pulls heat in and radiates it into the room, making the HVAC system work harder. 

This heat transfer occurs faster the larger the difference is between inside and outside temperatures. For example, heat will flow through concrete more quickly  in cold-climate winters when the difference between outside and inside temperatures may be 70 degrees or more (0O F outside, say, and 70O F inside). The summer differential is smaller, so heat transfers more slowly. 

Perimeter problem

Putting insulation under slabs may be required for some energy certifications, but in typical homes, the temperature differential is pretty small. It’s better to spend money here only after you’ve significantly improved air sealing, attic and wall insulation (R-100 and R-40 respectively), and boosted the windows to triple-glazed R-5 units.

That said, combatting heat flow through a slab boils down to putting insulation over the biggest energy leaks, and one big cold spot on a slab-on-grade house is where the edge of the slab is left exposed to the outdoors. It’s a big source of heat loss in warm climates (zones 1, 2, and 3), and creates an even bigger thermal leak in cold climates (zones 4, 5, and 6) because the temperature differential is much larger than in hot climates. 

Perimeter problem

Blocking the exposed perimeter eliminates most of the heat loss, keeping the monolithic slab warm and dry all the time. It’s no surprise, then, that insulating the edges of slabs is one of the items on the Energy Star checklist. The details shown here are not high-performance details because the underside of the footings and slab are still thermal bridges to the ground. But the sides of the slab are covered, and that’s where most heat loss occurs, so these details will be a big improvement for most homes.

The details shown here assume that the monolithic slab has been placed over a layer of poly over a layer of stone to act as a capillary break. (Otherwise you may have to treat the top of the slab to prevent moisture from wicking in (see “Building Things Right: Finished Basement”). 

You can use any insulation rated for below-grade use, but most builders and remodelers favor extruded polystyrene (XPS). In areas with a lot of termites, you will need to take extra precautions, including borate-treated foam, termite shields, and inspection gaps.

With the exception of the first step, the details are similar for both hot and cold climates, including the depth at the perimeter, which doesn’t need to be dug below the frost line in cold climates; depending where you live, 12, 14, or 16 inches deep is deep enough.

See step-by-step instructions here


written by

Dan Morrison

Dan Morrison is senior technical editor of ProTradeCraft.com, a sister site to Professional Remodeler.


Add new comment

Plain text

  • No HTML tags allowed.
  • Web page addresses and email addresses turn into links automatically.
  • Lines and paragraphs break automatically.
  • CAPTCHA

    This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.

Related Stories

3 Pro Tips for Insulating Foundation

Symbi Homes builds and remodels green from the ground up at the Model ReModel 2022 project, the Regeneration House

Using XPS to Insulate Foundation

Symbi Homes plans to achieve several green certifications for its remodel of a 19th-century Victorian, our Model ReModel 2022 project.

Siding Restoration on a Historic Home: Part 1

In order to give our Model ReModel 2022, a 19th-century Victorian, a proper facelift, the Symbi Homes team opted to repaint in a new palette. The age of the home required a specific approach to manage lead paint and historical requirements.

The New American Remodel 2022: Performance Showcase

The New American Remodel 2022 team transformed an old home into a Net Zero thoroughbred while keeping its charm.

What is a Deep Energy Retrofit?

Model ReModel 2022 will undergo a deep energy retrofit—here's how.

Estimating Three Ways

Three remodelers reveal how they estimate a project

10 Criteria to Measure the Health of Your Business

Richardson walks business owners through different areas to reveal necessary insights.

How to Solve the Deck Ledger Problem in Fat Walls

Brick veneer and rigid exterior foam raise questions about deck ledger connections. We provide some answers.

Grow Your Business Through Collaboration with Remodeling Peers

Remodeling Mastery Forums offers a unique business opportuniy for remodelers

Better Kitchen Lighting in 3 Steps

A simple lighting makeover in one kitchen dramatically improved the space. Here’s how it was done.

Advertisement
boombox1 -
Advertisement
native1 -

More in Category



Model ReModel

Siding Restoration on a Historic Home: Part 1

In order to give our Model ReModel 2022, a 19th-century Victorian, a proper facelift, the Symbi Homes team opted to repaint in a new palette. The age of the home required a specific approach to manage lead paint and historical requirements.


Advertisement
native2 -
Advertisement
halfpage1 -