Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.
Apologies if this has been brought up previously but I can't seem to find it. SAP 9.90 calculates air changes per hour (ach) by tallying up the infiltration introduced by chimneys, flues, fans, vents, windows, doors + a degree of stuctrual & floor related infiltration. It then considers the number of sheltered sides to revise the infiltration rate (ach) and multiplies it against a wind speed factor (which is based on local wind speed divided by 4 - anyone know where the figure 4 comes from?). All this ultimately results in an adjusted infiltration rate for each month which may be totalled to give the total modelled air changes per hour. Correct?
The Building Regs however works in m3/m2/hr@50pa. Is there a BS recognised way of converting from SAP 'ach' rates to m3/m2/hr@50pa rates? If such a common calculation does exisit, would I be right to assume it would only appy to the first SAP ach infiltration figure i.e. the one that is just looking at leakage and not considering number of sheltered sides or local wind speed? This figure is essentially air changes per hour at 0pa isn't it?
The Building Regs will be talking about a pressure test done with a blower in a doorway (ie: how much air in m3 is leaked at 50Pa of pressure, expressed per square metre of floor per hour). If you look at the SAP worksheets if you've done a pressure test then you skip calculating sheltered sides, percentage of windows draft stripped, etc and just use the actual data you've measured instead of the estimated infiltration rate.
I'm thinking along the lines of comparing the two i.e. modelled using SAP vs. as measured with a blower door. Surely there's a way of converting the SAP ach figure in order to do this?
Books 
