diff --git a/grizli/prep.py b/grizli/prep.py
index cde8a72e..b639a0b2 100644
--- a/grizli/prep.py
+++ b/grizli/prep.py
@@ -6196,19 +6196,6 @@ def get_jwst_wfssbkg_file(file, valid_flat=[0.6, 1.3], make_figure=False):
     from jwst.wfss_contam import WfssContamStep
     from jwst.flatfield import FlatFieldStep
 
-    
-    with pyfits.open(file) as im:
-        if im[0].header['INSTRUME'] == 'NIRISS':
-            # Test local file
-            bkg_file = "nis-{0}-{1}_skyflat.fits"
-            bkg_file = os.path.join(GRIZLI_PATH, 'CONF',
-                                    bkg_file.format(im[0].header['PUPIL'],
-                                                    im[0].header['FILTER']))
-            bkg_file = bkg_file.lower()
-            
-            if os.path.exists(bkg_file):
-                return bkg_file
-                    
     bkg_file = WfssContamStep().get_reference_file(file, 'wfssbkg')
 
     # Not a FITS file?  e.g., N/A for imaging exposures
@@ -6222,8 +6209,25 @@ def get_jwst_wfssbkg_file(file, valid_flat=[0.6, 1.3], make_figure=False):
         _im.close()
         return bkg_file
 
+    h = pyfits.getheader(file,0)
+    if h['INSTRUME'] == 'NIRISS':
+        
+        # Test local file
+        local_bkg_file = "nis-{0}-{1}_skyflat.fits"
+        local_bkg_file = local_bkg_file.format(h['PUPIL'],h['FILTER'])
+        local_bkg_file = local_bkg_file.lower()
+        local_bkg_file = os.path.join(GRIZLI_PATH, 'CONF',local_bkg_file)
+        
+        # If local file exists and is up-to-date, use it
+        if os.path.exists(local_bkg_file) and pyfits.getval(local_bkg_file,'DATE',0) == h['DATE']:
+            return local_bkg_file
+                    
     flat_file = FlatFieldStep().get_reference_file(file, 'flat')
     
+    # If we don't have write access copy to local file
+    if (not os.access(bkg_file,os.W_OK)) or (('CRDS_READONLY_CACHE' in os.environ) and (os.environ['CRDS_READONLY_CACHE'] == '1')):
+        shutil.copy(bkg_file,local_bkg_file)
+        bkg_file = local_bkg_file
     wf = pyfits.open(bkg_file, mode='update')
     key = f"{wf[0].header['FILTER']} {wf[0].header['PUPIL']}"