Hot Water Pipes and Basement Sills – The Secret Heat Loss

by Erik North on July 1, 2013

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I’ll preface this by saying that this photo is from my basement…notice the insulated sills in the background (*strains arm patting self on the back*).

I wrote previously about low cost/high impact DIY projects for the enterprising homeowner. These are projects that are both common and if so desired, well within the means of a moderately skilled homeowner.

One of those projects was insulating the sill plate in the basement. Now insulating an uninsulated section of the building enclosure may seem pretty brainless. But the lack of insulation is not the only issue. There is an invisible one that subtly impacts a heating system distribution’s efficiency.

What’s the Problem – Insulation

Most of the rest of the country, having an actually need to cool in the summer, installs mechanical systems for all seasons. A heat pump and duct system can provide heating and cooling as needed.

But our neck of the woods heats about six months out of the year and cools none at all. Even our current summer conditions (90 F and quite humid) just calls for a fan and open windows. We much more often have hot water boiler systems with baseboard radiators in New England.

Hot water baseboard are installed along the base of exterior walls and radiant heating systems are installed in the basement ceilings. The hot water pipes are attached to the overhead joists (or drilled through them) in the basement. These pipes carry hot water and travel 6 inches away from the foundation wall and sills.

Concrete has almost no insulating value (an R-value around 1 per 8 inches) and the wooden sills are not much better. I’ve often seen heating systems with a single zone where the hot water pipes runs the house’s entire perimeter. Once it reaches the loop’s conclusion, there’s not a lot of heat left in the pipes.

What’s The Other Problem – Air Leakage

It’s a little more complex a situation that at first glance. You have a long heating loop and the wooden sill. Heat is radiating from the heating pipes and passing into the uninsulated sills. That’s one factor. Another is air leakage.

“What?” I hear you cry. We’ve only been talking about radiant heat loss and insulation. Air leakage across the sills and heating pipes subtly increases heat loss along the pipes.

Every building undergoes what is known as stack effect. Stack effect is a phenomenon which is well known in the building industry. Outside the industry we get mostly blank stares (it’s our fault…we shouldn’t keep these things to ourselves). Everyone knows that warm air is more buoyant than cool air, so it naturally floats upward.

When the outdoor temperature is colder than the indoor temperature, the warm updraft grows stronger. This updraft creates a slight but definite positive air pressure near the upper levels of your house, forcing air to escape from every nook, crack, and hole it can find.

This air convection moves heat out of the top of your house, and as an added bonus, creates a slight suction (negative pressure) in the lower part of your house. For every cubic foot of warm air escaping from the top, a cubic foot of chilly winter air seeps in the bottom.

And what is one of the largest cracks in the bottom of the house? The sill plate and band joist. Suppose there’s only 1/50th of an inch gap on average. If the house has a 40′ by 25′ footprint, the sill will be 130 linear feet. That would be a bit over a 2 1/2″ square inch hole over the length. And the stack effect will be sucking in cold winter air directly across your hot water pipes.

How to Fix an Invisible Problem

Simple enough: insulation (pipes), air sealing (sills), insulation (sills) and even more insulation (walls). Pipe insulation should be added, ensuring that the hot water more easily arrives at the final destination actually hot.

Select pipe insulation that is at least one inch thick and preferably closed cell foam. Then throw it out and find a different pipe insulation that is one inch thicker. Insulate and seal the sills and insulate the foundation, preferably all in closed cell foam insulation.

Uninsulated hot water pipes are obviously an issue. An uninsulated sill is a less obvious one that sometimes gets address. But the unsealed sill blowing cold air across your hot water pipes is a hidden problem that is begging to be fixed.

{ 6 comments… read them below or add one }

Stewart July 6, 2013 at 6:33 am

Yeah! it’s true most of the country, having an actually need to cool in the summer, installs mechanical systems for all seasons. Insulation help to make home more comfortable, it’s not good with your neck of the woods heats about six months. Thanks for sharing your experience how to fix an invisible problem.


hot water systems Perth July 8, 2013 at 4:21 am

Nice post!!! Very helpful information!!!


Eric July 15, 2013 at 10:47 am

Where does one find pipe insulation w/ 1″ thick walls? The stuff at my local hardware store is nowhere near that thick…


Erik North July 21, 2013 at 3:54 pm

I found some just Googling for it. If you’re shopping at big box stores, maybe try an industrial supply store like Redlon, Grainger or Kamco.


Eric July 22, 2013 at 5:17 pm

Thanks, I suppose I could have tried The Google. 🙂


Billy Flemings January 7, 2014 at 12:33 am

A solar hot water system is a great way to reduce energy bills and energy emissions. With a few simple precautions such a system can benefit you throughout a whole year.


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