DP Rating or Design Pressure

Design pressure or dp rating is a measure of the strength of a window.  It will tell you how well a product will stand up to high winds.  The higher the dp rating the stronger the window.  It does not relate to energy efficiency, but dp rating is still an important factor to consider.

What’s a good dp rating?

Typically you’ll see residential replacement windows with ratings from 15 to 50.  Higher is better.  It makes more sense to focus on dp rating when you’re in a situation where wind is of particular concern.  For example we if you live on the water or up high on a hill where you’re exposed to the elements this can be important.  In those situations I’d suggest going with at least a dp 30 and a dp 50 might be an even better choice.  There’s really no need to go any stronger than that unless you’re in a high rise or a hurricane zone.

How can understanding dp ratings help me?

A common sales ploy is to show folks a cutaway view of a thick window next to a thin window and tell them that the thick windows is “strong” and the thin window is “weak”.  This is intuitive which makes it an effective way to sell the thicker windows.

We can tell if a potential customer has seen this maneuver when they ask how many chambers are in a specific window or when they ask to see a cutaway.

As you’re considering these options keep in mind that everything in life has tradeoffs and everything in the window business can be measured so you don’t need to use a gut feeling.  The thicker window will have a lower vt rating because that thick frame blocks a substantial amount of light, and the thinner window, if well built, could easily have a higher dp rating meaning it’s actually stronger.

So how do you avoid getting sold on something that you don’t necessarily need?  

Just like always, avoid taking someone’s word for it and ask about the dp rating.  If it’s higher it’s better.  The window doesn’t need to be thick to have a higher rating and a thick window without a higher rating is blocking light for no purpose other than to say it’s “better”.

Remember that the cutaway sample really doesn’t show you anything.  It gives you an idea, but the rating tells you for sure.  There’s no need to guess when it’s so easy to be sure.

If you’re looking for a window company right now, the best advice we have is to join Angie’s List.  For just a couple bucks you can get a 1-month membership and it’ll be worth much more than a caramel macchiato in the long run.  You can find the best pricing for Angie’s List on the internet right here

If you’re already a member of Angie’s List or if you’re just not going to join (despite my ringing endorsement), you can find our suggestions for the best companies in your area right here.

Air Infiltration or Air Leakage Rating Explained

Air infiltration or air leakage ratings are an important factor to consider when comparing energy efficient replacement windows.  In this post we’ll look into how these ratings are often misinterpreted, what is considered to be a pretty good rating and what to watch out for as you’re comparing the options.

To understand the rating you’ll first need to understand what the number actually represents.  The unit of measure for air infiltration is cubic feet per minute per square foot.  That is how many cubic feet of air can pass through the window in a minute at a constant 25 mph wind divided by the total window area.  I know, the unit itself is a little tricky.

best replacement windows of 2015

How are air infiltration and air leakage ratings misinterpreted?

Manufacturers are always looking for a way to tell folks that their products are “better” than another.  Focusing on air infiltration ratings is an increasingly common way to do that.

For example, the manufacturer Soft-lite uses a chart in their Elements window brochure that some customers could easily misinterpret.  It tries to put air infiltration in terms of how many milk jugs or pop cans of air can get through the widow.  Unfortunately, it clearly leaves out the size and the style of the window being tested.

Since the unit of measure for these ratings has the total window area in the denominator the size of the window affects the rating.  There are also several options offered by manufacturers that do change the ratings.  For one company to say that they’re rating is XX is oversimplifying the situation.

It’s also worthwhile to consider how often your home is hit with a constant 25 mph wind.  I’m not much of a sailor, but I’m aware of the wind enough to know that doesn’t happen by my house very often.

Manufacturers do this to try to separate themselves from their competition and there’s noting wrong with that, but trying to say a 0.01 difference in anything will make or break a project is pretty silly.

So what is a good air leakage or air infiltration rating?

Obviously the rating of how much air can get through the window is important.  The American Architectural Manufacturers Association (AAMA) says that any residential window that achieves a rating below 0.30 cfm/sqft meets the requirements for their gold rating.  This is the best rating they offer.  This is also the minimum air infiltration rating for the energy star program.  In fact, many of us in this business expected this minimum rating to be reduced with the 2015 energy star program, but it was not.  I expect we will see changes to this standard in the years to come.

High quality vinyl replacement windows consistently achieve ratings MUCH lower than this standard.  For a vinyl double hung window anything below 0.05 is pretty darn good.  Casement windows and picture windows are typically even lower while sliding windows are often the worst of the bunch.

On the other hand, wood, fiberglass and composite windows typically cannot achieve as tight of a seal.  These products typically have much higher air infiltration ratings.  The fact that wood, fiberglass and composite windows typically have substantially worse air infiltration or air leakage ratings should be weighed when considering those options.

What should I watch out for when looking at air infiltration ratings?

Typically it is wise to avoid putting too much weight into statements from folks who are drastically over simplifying things.  For example if a salesperson tells you his windows achieve a rating of 0.01 and that everything else is worse and any other window will make your house drafty he’s not telling you the truth (he’s probably the guy using the chart above).

Remember for vinyl windows anything below 0.05 is pretty darn good any you’ll likely never know the difference.  In fact, the National Fenestration Rating Council or NFRC doesn’t even give ratings below 0.10 because they don’t believe the air infiltration measurements are accurate below that point.  If the major independent testing body in our industry doesn’t recognize ratings below 0.10 that’s probably a good sign that you don’t need to worry about it below that point.

Important points to remember:

  • Air infiltration or air leakage is just one factor to consider as you sort through window ratings
  • It’s pretty easy to compare the differences between models
  • Lower is better, but below a certain point there is not much difference
  • Wood, composite and fiberglass options will typically have higher ratings than high quality vinyl
  • All types of windows are available below the 0.30 threshold
  • Not all types are available below the 0.10 level

If you’re looking for a window company right now, the best advice we have is to join Angie’s List.  For just a couple bucks you can get a 1-month membership and it’ll be worth much more than a caramel macchiato in the long run.  You can find the best pricing for Angie’s List on the internet right here

If you’re already a member of Angie’s List or if you’re just not going to join (despite my ringing endorsement), you can find our suggestions for the best companies in your area right here.

Visible Transmittance

So you picked out your new windows after doing all of your research.  You ordered them then waited a few weeks for delivery and installation.  The morning after the big day you’re walking around drinking a warm cup of coffee in your slippers admiring your fancy new windows.  Then you notice that it feels a little darker in here than it used to.  Is it cloudy outside?  No, that’s not it.  It turns out the glass area of your new windows is quite a bit smaller than your old windows and it looks like those fancy triple pane windows are also a little bit tinted.  You hadn’t expected that, but there isn’t much that can be done about it now.  This happens all the time and it’s exactly why visible transmittance is an important rating to consider.

best replacement windows of 2015

The visible transmittance (or VT as it’s known) is a measure of how much light gets through the window.  This is typically determined by the thickness of the frame and sash, but the coating or tint on the glass and any grids or muntins will also affect the ratings as they block some of the light.

Take a look at this picture of two relatively popular window models.  These 2 windows are the exact same size and when they’re ordered with the same options they have the same efficiency ratings.

pic of a window with a good vt rating along with one that is not as good.
Notice the difference in visible transmittance in these two replacement windows and consider how it will affect the look and feel of your home.

One of them sure does block a lot of light and I’d bet many of the folks ordering that product aren’t aware that they had another choice.

Why then would a company promote that thicker model if it’s not more efficient?  As always is the case in life the answer isn’t as simple as you’d hope.  That thicker window does allow for heavier triple pane glass where the thinner model can only offer double pane.  For the customer who wants triple pane glass that represents a pretty good option.  For the customer who wants double pane it makes no sense at all.

What’s the tricky sales tactic?

Unfortunately when there’s a good reason for something there’s typically also a bad reason.  Some salespeople will use a cutaway piece of that thicker window to sell you on “strength” they’ll show you how it has 37 air chambers in the frame and talk about those “other” windows that use thinner weaker frames.

As you’ve probably guessed by now there is absolutely no reason that you’d need to take their word on something like this.  The “strength” of a product is measured by the design pressure which will be covered in another post.  Does this thicker window have a better/higher design pressure than the thinner one?  Nope.  They’re exactly the same.

So how do I compare the options?

In order to compare you’ll need to know what you’re comparing.  Options like types of glass or grids will affect the ratings.  One company might give you the rating with grids between the glass and another might give you the rating without.  If you don’t know what you’re comparing you’ll be likely to misinterpret the information.

To make things relatively simple (everything is relative right?) I’d suggest comparing options without grids and with the glass option that you’re leaning towards.  Taking the grids out of the equation will give you the info based on the frame size and the glass coating alone.  If you will be ordering your windows with grids they will typically be almost exactly the same from one manufacturer to another so they won’t make much (if any) difference.

So what’s a good or a great rating?

Assuming you’re comparing energy star rated double pane windows a pretty good option will be in the range of 0.54 or better.  Great options will be as high as 0.56.

Triple pane options will reduce the VT rating as the light needs to pass through 3 panes of glass and typically 2 low-e coatings.  They will also often have thicker frames as shown in the pic above.  A pretty good triple pane window will offer a VT of 0.42 or better, but some will go as low 0.30 depending on the window and the glass option.  Remember a lower number means a darker living room so be sure you understand what you’re picking.

Getting a more efficient window does sometimes result in a lower VT so that’s a trade off you’ll need to consider.  For me it’s a big deal as I love the sunlight, but other folks don’t care.  It’s your house not mine; get whatever makes you happy!

If you’re looking for a window company right now, the best advice we have is to join Angie’s List.  For just a couple bucks you can get a 1-month membership and it’ll be worth much more than a caramel macchiato in the long run.  You can find the best pricing for Angie’s List on the internet right here

If you’re already a member of Angie’s List or if you’re just not going to join (despite my ringing endorsement), you can find our suggestions for the best companies in your area right here.