Tenants can usually claim a reduction of rent if there has been interference with their reasonable enjoyment of the rental premises. In the situation that you describe, you should try to reach an agreement with the tenant on what would be a reasonable amount of a rent reduction. If you come to an agreement with the tenant, there is not a formal document to use in this case. You can just draft something up yourself and have the tenant sign it as it is important to document it. If you cannot come to an agreement, the tenant then has the option of filing an application with the Landlord and Tenant Board for them to make the decision on this matter.