Why is my bathroom so cold?
January 7, 2013 6:56 AM   Subscribe

Am I losing all my heat through a large glass block window?

My bathroom has a large glass block window which is north facing. Is is 7 blocks wide by 6 blocks high, so the the "window" is 56" x 48" In between the glass blocks is grout.

The room is always 10 degrees colder in the winter and 10 degrees warmer in the summer. It is not that far from the furnace. There is also a Jacuzzi tub that has its vent to the the same outside wall.

A while ago I had a friend who claimed to have HVAC experience and he said that glass block is notoriously inefficient. In particular, I recall a story he told about having a client who insisted on having a HUGE glass block window in their bathroom and it required an entirely separate A/C compressor to keep the room cool (we live in desert with summer temps approaching 110). But in researching my problem I am reading that glass brick should not be that bad of an insulator. Can it the grout?

I know anyone who wants to give a serious answer will need to qualify their answer based upon the size of the room and output of the furnace, number of registers and returns, etc.. FWIW, I just want to get an overall impression about glass block windows (should they have used grout in between blocks?) and their insulating capacity.
posted by teg4rvn to Home & Garden (8 answers total)
Do the glass blocks feel significantly colder than the wall? Then you are losing heat. Do they feel warmer or colder than the regular windows?

The glass block is both a good and a bad insulator. The air cavity makes it a good insulator, but the fact that there is a direct physical connection (the edges of the block and the mortar) between the inside and the outside means that heat will conduct from the warmer to the cooler. However, those areas are something like 4 inches thick, so that slows down the conduction pretty significantly. Glass block is certainly more efficient than a single pane window, but I don't know how it compares to a nice tight double or triple pane window.
posted by gjc at 7:17 AM on January 7, 2013

The thick air cavity inside a glass block makes it a poor insulating system as the air inside simply swirls/convects heat from the hot side to the cool side. Basically, glass block is very drafty inside.

Counterintuitively, a thinner cavity is more insulating overall, as it would reduce convective heat transfer.

That's why double and triple glazed insulating windows have just a 5-10mm gap; this is thick enough to all but eliminate conductive transfer yet thin enough to get rid of convection too.
posted by seanmpuckett at 7:25 AM on January 7, 2013 [1 favorite]

I don't know, but I did a quick Google and came up with an R-Value for glass block of around 1.7 to 2.0 (sources: eHow, Yahoo answers, GlassBlockSales). Here's a table of R-Values in general (glass block not listed): double pane windows are 1.6 to 4.0, triple pane is 2.6 to 3.2. (Single pane glass is 0.9).

So I'd take all that to mean that glass block windows aren't as good as proper double pane windows, but isn't awful either. I'd take gjc's point above about the mortar etc too, though.
posted by Nelson at 8:37 AM on January 7, 2013

FYI, heat loss is proportional to temperature differential and thermal resistance.

To determine the additional heating load presented by the window, you need to establish what temperature differential you are trying to overcome. Maintaining a small differential is easier than a large.

(First, I think you are correct... the blocks provide thermal gain and thermal loss. The gain comes from both the conduction of ambient heat from outside AND from the light streaming in. Depending on the direction it's facing, that part could be substantial. Not enough info to judge. Add more if you want more opinion.)

Were I you, and if I desired to make an on site evaluation, I'd set up a test. Get a sheet of Urea formaldehyde foam with foil surface. Double it up and temporarily nail it to the outside of the house covering the block wall. See if it improves your problem. You can tell how much rather quickly.... a few hours. I'd supplement it with measurements of ambient temp inside and out, just for fun. That's what I do, however... fun with measurements. Woot.

If you like how it feels, you may want to consider adding a thermopane window OUTSIDE the block wall. I have these on my 1898 Victorian leaded glass windows here in Vermont. Works great. Easy to add. Better than replacing the beautiful old windows for the same reason you may want to keep your block wall. It looks better. Relatively cheap, too, since you don't have to deal with opening. You can get the latest high tech windows made. Low E, argon filled or vacuum filled.

This isn't an analysis, but it is free advice. Other HVAC folks here can help faster than I can and I have to go do some work now. Good luck.
posted by FauxScot at 8:53 AM on January 7, 2013 [1 favorite]

I recommend calling your utility (gas or electric) and see if they offer a free "energy audit". They will come by your place and check everything for you. They can make recommendations, and some even have arrangements with providers who give special deals on whatever is outdated or could use a boost.
posted by Ruthless Bunny at 9:24 AM on January 7, 2013

I got a bunch of bubble wrap when workers were installing stuff at work. I made window covers to use in winter. free and reasonably effective.
posted by theora55 at 10:49 AM on January 7, 2013

Glass block is a crappy insulator. As an assembly (IE: mortor, blocks, ties, spacers) in large sizes it is worse than a single pane glass window. And it has poorer solar gain though that isn't of concern in your location.

In line with FauxScot's idea: A single sheet of glass spaced half an inch from the glass brick and then siliconed in place (i've always done this on the inside) will make a huge difference in how much heat you loose through this window. It'll also be easier to clean. And it'll be pretty cheap, a few dollars a square foot for the glass.
posted by Mitheral at 11:54 AM on January 7, 2013 [1 favorite]

How well sealed/insulated is the bathroom venting? You note the jacuzzi has a vent so you may have two to check and both could simply allow outside air into your bathroom. An improper seal is the easiest and most direct route for heat transfer and I would start with how well sealed the room is.

I would then check other gaps - is your electrical switch boxes sealed? Water fixtures?

I would then check how well insulated the space is - should there be insulation above the room, or below and remember that although it seems dumb home renos or additions can simply skip this step. If the tub is against the wall is there insulation behind that (although that seems unlikely you wouldn't have noticed the cold/hot tub).

You seem focused on the glass window/wall and it may simply need to be resealed - the bricks and grout may allow air passage.

Your state may have a program to inspect houses for energy efficiency - sometimes called a weatherization or energy audit.
posted by zenon at 3:15 PM on January 7, 2013

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