Hi All, I’m running a bunch of energy models to assess the impact of different weatherization interventions. I’m looking at added insulation (wall, floor and ceiling/attic), upgraded windows and air sealing. I modelled all the measures separately as i did not expect significant interactive effects from weatherization measures alone, but out of curiosity i modelled them all together and found that the resultant savings was higher then if i just summed the impacts of the individual measures.

I was wondering if anyone had a good explanation why the measures would show more savings when applied together?

There 48 models of various sizes ranging from 1300 ft2 up to 3600 ft2, all in NYC area. All are typical older existing residential 2 story buildings with attics and unheated basements. The measures are all basically going from a very low performance baseline (essentially uninsulated) up to something resembling code. ACH50 was reduced by 25%. Note that infiltration was also reduced similarly in the unconditioned attic and basement. All with gas furnace heating.

Obviously its very likely that the issue is with the models, but i was wondering if anyone had any theories as to why the modelled results might be capturing some real world interaction between the measures. There is no obvious trend  of in which models (in terms of size, location, etc.) the effect is more pronounced except for those that started with lower performance baseline (i.e. those with the highest savings) show the highest difference (see attached image).

Thanks in advance for any insights.